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1 for more information www.linear.com/ltc4081 typical application features description 500ma li-ion charger with ntc input and 300ma synchronous buck the lt c ? 4081 is a complete constant-current /constant- voltage linear battery charger for a single-cell 4.2v lithium-ion/ polymer battery with an integrated 300ma synchronous buck converter . a 3mm 3 mm dfn pack- age and low external component count make the ltc4081 especially suitable for portable applications . furthermore, the ltc4081 is specifically designed to work within usb power specifications. the chrg pin indicates when charge current has dropped to ten percent of its programmed value (c/10). an internal 4.5 -hour timer terminates the charge cycle. the full- featured ltc4081 battery charger also includes trickle charge, automatic recharge , soft-start (to limit inrush current) and an ntc thermistor input used to monitor battery temperature . the ltc4081 integrates a synchronous buck converter that is powered from the bat pin . it has an adjustable output voltage and can deliver up to 300ma of load cur - rent. the buck converter also features low current high efficiency burst mode operation that can be selected by the mode pin. the ltc4081 is available in a 10-lead, low profile (0.75mm) 3mm 3mm dfn package. applications battery charger : n constant-current/constant-voltage operation with thermal feedback to maximize charge rate without?risk of overheating n internal 4.5-hour safety timer for termination n charge current programmable up to 500ma with 5% accuracy n ntc thermistor input for temperature qualifed charging n c/10 charge current detection output n 5a supply current in shutdown mode switching regulator: n high effciency synchronous buck converter n 300ma output current (constant-frequency mode) n 2.7v to 4.5v input range (powered from bat pin) n 0.8v to v bat output range n mode pin selects fixed (2.25mhz) constant-frequency pwm mode or low i cc (23a) burst mode ? operation n 2a bat current in shutdown mode n 10-lead, low profle (0.75 mm) 3mm 3mm dfn package n wireless headsets n bluetooth applications n portable mp3 players n multifunction wristwatches l, lt , lt c , lt m , linear technology , the linear logo and burst mode are registered trademarks and thinsot and powerpath are trademarks of linear technology corporation . all other trademarks are the property of their respective owners. protected by u.s. patents, including 6522118. li-ion battery charger with 1.8v buck regulator load current (ma) 0.01 40 efficiency (%) power loss (mw) 60 80 0.1 10 100 1 1000 20 0 100 1 10 100 0.1 0.01 1000 4081 ta01b v bat = 3.8v v out = 1.8v l = 10h c = 4.7f efficiency (burst) power loss (burst) efficiency (pwm) power loss (pwm) buck efficiency vs load current (v out = 1.8v) 500ma 4.2v li-ion/ polymer battery 4.7f 806 510 806k 4.7f 4081 ta01a 10pf 1m ltc4081 ntc en_chrg mode fb prog v cc chrg bat 100k 100k v out (1.8v/300ma) 1oh gnd 4.7f v cc (3.75v to 5.5v) + t sw en_buck ltc 4081 4081fa
2 for more information www.linear.com/ltc4081 the l denotes specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c , v cc = 5 v , v b at = 3.8v , v en_ chrg = 0 v , v ntc = 0 v , v en_ buck = v b at , v mode? =?0v . ( note 2) pin configuration absolute maximum ratings v cc , t < 1 ms and duty cycle < 1% ............... C0.3 v to 7 v v cc steady state .......................................... C0.3 v to 6 v bat , chrg ................................................... C0.3 v to 6 v en _ chrg , prog , ntc .................... C0.3 v to v cc + 0.3 v mode , en _ buck ........................... C0.3 v to v bat + 0.3 v fb ................................................................ C0.3 v to 2 v (note 1) order information top view dd package 10-lead (3mm 3mm) plastic dfn 10 9 6 7 8 4 5 3 2 1 bat v cc en_chrg prog ntc sw en_buck mode fb chrg 11 t jmax = 110c, ja = 43c/w (note 3) exposed pad (pin 11) is gnd, must be soldered to pcb lead free finish tape and reel part marking * package description temperature range ltc4081edd#pbf ltc4081edd#trpbf ldbx 10-lead (3mm 3mm) dfn 0c to 70c consult ltc marketing for parts specified with wider operating temperature ranges . consult ltc marketing for information on non-standard lead based finish parts . for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ symbol parameter conditions min typ max units v cc battery charger supply voltage (note 4) l 3.75 5 5.5 v v bat input voltage for the switching regulator (note 5) l 2.7 3.8 4.5 v i cc quiescent supply current (charger on, switching regulator off) v bat = 4.5v ( forces i bat and i prog = 0), v en_ buck = 0 l 110 300 a i cc_sd supply current in shutdown ( both battery charger and switching regulator off) v en_chrg = 5v, v en_buck = 0, v cc > v bat v en_chrg = 4v, v en_buck = 0, v cc (3.5v) < v bat (4v) l 5 2 10 a a i bat_sd battery current in shutdown ( both battery charger and switching regulator off) v en_chrg = 5v, v en_buck = 0, v cc > v bat v en_chrg = 4v, v en_buck = 0, v cc (3.5v) < v bat (4v) l 0.6 2 5 a a electrical characteristics bat short - circuit duration ........................... continuous bat pin current .................................................. 800 ma prog pin current .................................................... 2 ma junction temperature ........................................... .125 c operating temperature range ( note 2) .... C40 c to 85 c storage temperature range .................. C65 c to 125 c ltc 4081 4081fa
3 for more information www.linear.com/ltc4081 the l denotes specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c , v cc = 5 v , v b at = 3.8v , v en_ chrg = 0 v , v ntc = 0 v , v en_ buck = v b at , v mode? =?0v . ( note 2) electrical characteristics symbol parameter conditions min typ max units battery charger v float v bat regulated output voltage i bat = 2ma i bat = 2ma, 4.3v < v cc < 5.5v l 4.179 4.158 4.2 4.2 4.221 4.242 v v i bat current mode charge current r prog = 4k; current mode; v en_buck = 0 r prog = 0.8k; current mode; v en_buck = 0 l l 90 475 100 500 110 525 ma ma v uvlo_chrg v cc undervoltage lockout voltage v cc rising v cc falling l l 3.5 2.8 3.6 3.0 3.7 3.2 v v v prog prog pin servo voltage 0.8k r prog 4k l 0.98 1.0 1.02 v v asd automatic shutdown threshold voltage (v cc C v bat ), v cc low to high (v cc C v bat ), v cc high to low 60 15 82 32 100 45 mv mv t ss_chrg battery charger soft-start time 180 s i trkl trickle charge current v bat = 2v, r prog = 0.8k 35 50 65 ma v trkl trickle charge threshold voltage v bat rising l 2.75 2.9 3.05 v v trhys trickle charge threshold voltage hysteresis 100 150 350 mv dv rechrg recharge battery threshold voltage v float C v bat , 0c < t a < 85c 70 100 130 mv dv uvcl1, dv uvcl2 (v cc C v bat ) undervoltage current limit threshold voltage i bat = 0.9 i chg i bat = 0.1 i chg 180 90 300 130 mv mv t timer charge termination timer l 3 4.5 6 hrs recharge time l 1.5 2.25 3 hrs low-battery charge time v bat = 2.5v l 0.75 1.125 1.5 hrs i c/10 end of charge indication current level r prog = 2k (note 6) l 0.085 0.1 0.115 ma/ma t lim junction temperature in constant- temperature mode 115 c r on_chrg power fet on-resistance (between v cc and bat ) i bat = 350ma, v cc = 4v 700 mw f badbat defective battery detection chrg pulse frequency v bat = 2v 2 hz d badbat defective battery detection chrg pulse frequency duty ratio v bat = 2v 75 % i ntc ntc pin current v ntc = 2.5v 1 a v cold cold temperature fault threshold voltage rising voltage threshold hysteresis 0.76 ? v cc 0.015 ? v cc v v v hot hot temperature fault threshold voltage falling voltage threshold hysteresis 0.35 ? v cc 0.017 ? v cc v v v dis ntc disable threshold voltage falling threshold; v cc = 5v hysteresis 82 50 mv mv f ntc fault temperature chrg pulse frequency 2 hz d ntc fault temperature chrg pulse frequency duty ratio 25 % ltc 4081 4081fa
4 for more information www.linear.com/ltc4081 note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: the ltc4081 is guaranteed to meet performance specifications from 0c to 85c. specifications over the C40c to 85c operating temperature range are assured by design, characterization and correlation with statistical process controls. note 3: failure to solder the exposed backside of the package to the pc board ground plane will result in a thermal resistance much higher than 43c/w. note 4: although the ltc4081 charger functions properly at 3.75v, full charge current requires an input voltage greater than the desired final battery voltage per d v uvcl1 specification. note 5: the 2.8 v maximum buck undervoltage lockout (v uvlo_buck ) exit threshold must first be exceeded before the minimum v bat specification applies. note 6: i c/10 is expressed as a fraction of measured full charge current with indicated prog resistor. symbol parameter conditions min typ max units buck converter v fb fb servo voltage l 0.78 0.80 0.82 v i fb fb pin input current v fb = 0.85v C50 50 na f osc switching frequency l 1.8 2.25 2.75 mhz i bat_nl_cf no-load battery current (continuous frequency mode) no-load for regulator, v en_chrg = 5v, l = 10h, c = 4.7f 1.9 ma i bat_nl_bm no-load battery current (burst mode operation) no-load for regulator, v en_chrg = 5v, mode = v bat , l = 10h, c = 4.7f 23 a i bat_slp battery current in sleep mode v en_chrg = 5v, mode = v bat , v out > regulation voltage l 10 15 20 a v uvlo_buck buck undervoltage lockout voltage v bat rising v bat falling l l 2.6 2.4 2.7 2.5 2.8 2.6 v v r on_p pmos switch on-resistance 0.95 w r on_n nmos switch on-resistance 0.85 w i lim_p pmos switch current limit 375 520 700 ma i lim_n nmos switch current limit 700 ma i zero_cf nmos zero current in normal mode 15 ma i peak peak current in burst mode operation mode = v bat 50 100 150 ma i zero_bm zero current in burst mode operation mode = v bat 20 35 50 ma t ss_buck buck soft-start time from the rising edge of en_buck to 90% of buck regulated output 400 s logic v ih input high voltage en_chrg, en_buck, mode pin low to high l 1.2 v v il input low voltage en_chrg, en_buck, mode pin high to low l 0.4 v v ol output low voltage (chrg) i sink = 5ma l 60 105 mv i ih input current high en_buck, mode pins at 5.5v, v bat = 5v l C1 1 a i il input current low en_chrg, en_buck, mode pins at gnd l C1 1 a r en_chrg en_chrg pin input resistance v en_chrg = 5v 1 1.45 3.3 mw i chrg chrg pin leakage current v bat = 4.5v, v en_chrg = 5v l 1 a electrical characteristics the l denotes specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c , v cc = 5 v , v b at = 3.8v , v en_ chrg = 0 v , v ntc = 0 v , v en_ buck = v b at , v mode? =?0v . ( note 2) ltc 4081 4081fa
5 for more information www.linear.com/ltc4081 v cc supply voltage (v) 4 float voltage (v) 4.20 5.5 4.05 3.95 4.5 5 3.90 3.85 4.25 4.15 4.10 4.00 6 4081 g03 charge current (ma) 0 4.17 4.18 4.21 4.20 150 4081 g01 4.16 4.15 50 100 200 250 4.14 4.13 4.19 float voltage (v) r prog = 2k temperature (c) ? 50 float voltage (v) 4.195 10 4.180 4.170 ? 30 ? 10 30 4.165 4.160 4.210 4.205 4.200 4.190 4.185 4.175 50 70 90 4081 g02 temperature (c) ?50 200 250 100 150 100 0?25 50 125 25 75 50 0 charge current (ma) 4081 g04 v cc = 6v v bat = 3v r prog = 2k thermal control loop in operation charge current (ma) 0 v prog (v) 0.6 0.8 1.0 175 0.4 0.2 0 25 75 125 50 100 150 200 4081 g05 r prog = 2k temperature (c) ? 50 r ds(on) () 0.7 10 0.4 0.2 ? 30 ?10 30 0.1 0 0.9 0.8 0.6 0.5 0.3 50 70 90 4081 g06 v cc = 4v i bat = 350ma temperature (c) ?50 0.50 threshold voltage (v) 0.55 0.65 0.70 0.75 30 0.95 0.60 ?10 ?30 50 70 10 90 0.80 0.85 0.90 4081 g07 falling rising temperature (c) 1.0 1.1 1.3 1.4 1.5 1.7 1.2 1.6 4081 g08 ?50 pulldown resistance (m) 30 ?10 ?30 50 70 10 90 battery regulation (float) voltage vs charge current battery regulation (float) voltage vs temperature battery regulation (float) voltage vs v cc supply voltage charge current vs temperature with thermal regulation (constant-current mode) prog pin voltage vs charge current charger fet on-resistance vs temperature en_chrg, en_buck and mode pin threshold voltage vs temperature en_chrg pin pull-down resistance vs temperature typical performance characteristics (t a = 25c, v cc = 5v, v b at = 3.8v, unless otherwise specified) ltc 4081 4081fa
6 for more information www.linear.com/ltc4081 20 25 35 15 10 5 0 30 4081 g17 battery voltage (v) 2.5 buck input current (a) 4.5 3.0 3.5 4.0 i out = 1ma v out = 1.8v l = 10h temperature (c) 0.80 normalized timer period 0.90 1.05 0.85 1.00 0.95 4081 g10 ?50 30 ?10 ?30 50 70 10 90 battery voltage (v) 2.5 2.26 2.27 2.28 4.0 4081 g11 2.25 2.24 3.0 3.5 4.5 2.23 2.22 frequency (mhz) temperature (c) ?60 1.8 frequency (mhz) 1.9 2.0 2.1 2.2 ?20 20 60 100 2.3 2.4 ?40 0 40 80 4081 g12 v bat = 2.7v v bat = 4.5v v bat = 3.8v load current (ma) 0.01 40 efficiency (%) power loss (mw) 60 80 0.1 10 100 1 1000 20 0 100 1 10 100 0.1 0.01 1000 4081 g13 v bat = 3.8v v out = 1.8v l = 10h c = 4.7f efficiency (burst) power loss (burst) efficiency (pwm) power loss (pwm) battery voltage (v) 2.5 1.780 buck output voltage (v) 1.785 1.790 1.795 1.800 1.805 1.810 3.0 3.5 4.0 4.5 4081 g15 pwm mode i out = 1ma v out set for 1.8v burst mode operation temperature (c) ?50 buck output voltage (v) 1.800 1.805 1.810 10 50 1.795 1.790 ?30 ?10 30 70 90 1.785 1.780 4081 g16 pwm mode burst mode operation i out = 1ma v out set for 1.8v no-load buck input current (burst mode operation) vs battery voltage normalized charge termination time vs temperature buck oscillator frequency vs battery voltage buck oscillator frequency vs temperature buck efficiency vs load current (v out = 1.8v) buck output voltage vs battery voltage buck output voltage vs temperature voltage (mv) 70 40 20 10 0 80 60 50 30 4081 g09 i chrg = 5ma temperature (c) ?50 30 ?10 ?30 50 70 10 90 chrg pin output low voltage vs temperature typical performance characteristics buck efficiency vs load current (v out = 1.5v) load current (ma) 0.01 40 efficiency (%) power loss (mw) 60 80 0.1 10 100 1 1000 20 0 100 1 10 100 0.1 0.01 1000 4081 g14 v bat = 3.8v v out = 1.5v l = 10h c = 4.7f efficiency (pwm) power loss (pwm) efficiency (burst) power loss (burst) (t a = 25c, v cc = 5v, v b at = 3.8v, unless otherwise specified) ltc 4081 4081fa
7 for more information www.linear.com/ltc4081 2.7 4.2 3 3.6 3.3 3.9 4.5 4081 g24 battery voltage (v) maximum output current (ma) 40 50 60 30 20 0 10 80 70 l = 10h v out set for 1.8v battery voltage (v) maximum output current (ma) 300 200 100 500 400 2.7 4.2 3 3.6 3.3 3.9 4.5 4081 g23 l = 10h v out set for 1.8v 20 25 35 15 10 5 0 30 no load input current (a) l = 10h c = 4.7f v out = 1.8v temperature (c) ?50 10 50 ?30 ?10 30 70 90 4081 g18 v bat = 4.2v v bat = 3.8v v bat = 2.7v no-load buck input current (burst mode operation) vs temperature buck main switch (pmos) on-resistance vs battery voltage buck main switch (pmos) on-resistance vs temperature buck synchronous switch (nmos) on-resistance vs battery voltage buck synchronous switch (nmos) on-resistance vs temperature maximum output current ( pwm mode) vs battery voltage maximum output current (burst mode operation) vs battery voltage 1.0 1.2 0.8 0.6 0.4 0.2 0 4081 g19 battery voltage (v) 2.5 on-resistance () 4.5 5.0 3.0 3.5 4.0 temperature (c) ?50 10 50 ?30 ?10 30 70 90 4081 g20 1.0 1.2 0.8 0.6 0.4 0.2 0 on-resistance () 4081 g21 battery voltage (v) 2.5 on-resistance () 0.8 1.0 1.2 4.5 5.0 0.6 0.4 0 3.0 3.5 4.0 0.2 4081 g22 temperature (c) ?50 10 50 ?30 ?10 30 70 90 on-resistance () 0 0.8 1.0 1.2 0.6 0.4 0.2 typical performance characteristics (t a = 25c, v cc = 5v, v b at = 3.8v, unless otherwise specified) ltc 4081 4081fa
8 for more information www.linear.com/ltc4081 200s/div v out 1v/div v en _ buck 5v/div 4081 g28 0v 0v 50s/div v out 20mv/div ac coupled i load 250ma/div 0ma 4081 g25 output voltage transient step response (burst mode operation) charger v prog soft-start output voltage transient step response ( pwm mode) output voltage waveform when switching between burst and pwm mode (i load = 10ma) buck v out soft-start (i load = 50ma) 50s/div v out 50mv/div ac coupled v mode 5v/div 4081 g26 0v 50s/div v out 20mv/div ac coupled i load 50ma/div 4081 g27 0ma 50s/div v prog 200mv/div 4081 g29 0v typical performance characteristics (t a = 25c, v cc = 5v, v b at = 3.8v, unless otherwise specified) ltc 4081 4081fa
9 for more information www.linear.com/ltc4081 bat (pin 1): charge current output and buck regulator input. provides charge current to the battery and regulates the final float voltage to 4.2v. an internal precision resistor divider from this pin sets the float voltage and is disconnected in charger shutdown mode . this pin must be decoupled with a low esr capacitor for low noise buck operation. v cc ( pin 2): positive input supply voltage . this pin provides power to the battery charger . v cc can range from 3.75v to 5.5v. this pin should be bypassed with at least a 1f capacitor. when v cc is less than 32 mv above the bat pin voltage, the battery charger enters shutdown mode. en _ chrg ( pin 3): enable input pin for the battery charger . pulling this pin above the manual shutdown threshold (v ih ) puts the ltc4081 charger in shutdown mode , thus stopping the charge cycle . in battery charger shutdown mode, the ltc4081 has less than 10a supply current and less than 5 a battery drain current provided the regula - tor is not running . enable is the default state , but the pin should be tied to gnd if unused. prog (pin 4): charge current program and charge cur - rent monitor pin. connecting a 1% resistor, r prog , to ground programs the charge current . when charging in constant-current mode , this pin servos to 1 v . in all modes , the voltage on this pin can be used to measure the charge current using the following formula: i bat = v prog r prog ?400 ntc (pin 5): input to the ntc ( negative temperature coef- ficient) thermistor temperature monitoring circuit . for normal operation, connect a thermistor from the ntc pin to ground and a resistor of equal value from the ntc pin to v cc . when the voltage at this pin drops below 0.35 ? v cc at hot temperatures or rises above 0.76 ? v cc at cold, charging is suspended , the internal timer is frozen and the chrg pin output will start to pulse at 2hz. pulling this pin below 0.016 ? v cc disables the ntc feature . there is approximately 3c of temperature hysteresis associated with each of the input comparators thresholds. chrg (pin 6): open-drain charge status output . the charge status indicator pin has three states : pull-down, high impedance state, and pulsing at 2hz. this output can be used as a logic interface or as an led driver . when the battery is being charged , the chrg pin is pulled low by an internal n-channel mosfet . when the charge current drops to 10% of the full-scale current , the chrg pin is forced to a high impedance state . when the battery volt - age remains below 2.9 v for one quarter of the full charge time, the battery is considered defective , and the chrg pin pulses at a frequency of 2hz with 75% duty cycle. when the ntc pin voltage rises above 0.76 ? v cc or drops below 0.35 ? v cc , the chrg pin pulses at a frequency of 2hz (25% duty cycle). fb (pin 7): feedback pin for the buck regulator . a resistor divider from the regulator s output to the fb pin programs the output voltage. servo value for this pin is 0.8v. mode (pin 8): burst mode enable pin. tie this pin high to force the ltc4081 regulator into burst mode operation for all load conditions. tie this pin low to force constant- frequency mode operation for all load conditions . do not float this pin. en_buck (pin 9): enable input pin for the buck regulator. pull this pin high to enable the regulator , pull low to shut down. do not float this pin. sw (pin 10): switch pin for the buck regulator . minimize the length of the metal trace connected to this pin . place the inductor as close to this pin as possible. gnd (pin 11): ground. this pin is the back of the exposed pad package and must be soldered to the pcb for electrical connection and rated thermal performance . pin functions ltc 4081 4081fa
10 for more information www.linear.com/ltc4081 block diagram ? + ? + 6 mp4 mp3 mp1 x1 x400 v cc r1 r2 charger enable chrg prog 11 gnd 4081 bd 1v 0.1v prog c1 0.1v d3 d2 d1 1.22v ? + ca ma ? + va 1 10 7 2 bat sw 0.8v l1 v out c out r8 c pl fb counter logic charger oscillator charge control + ? 2.9v bat badbat uvlo suspend ? + c3 c2 en_chrg r en 0.82v charger shutdown 3 ? + c6 0.82v enable buck en_buck mode 9 ? + c7 0.82v 8 pwm control and drive ? + 2.25mhz buck oscillator error amp linear battery charger synchronous buck converter mn1 r7 mp2 r prog + ? 115c t die ta pulse logic 4 r nom r ntc v cc v cc ? + ? + ? + too cold too hot ntc_en r9 c8 c9 c10 r10 r11 r12 5 ntc ? + ? + c4 c5 v bat + 80mv v cc 3.6v t ltc 4081 4081fa
11 for more information www.linear.com/ltc4081 operation the ltc4081 is a full-featured linear battery charger with an integrated synchronous buck converter designed pri - marily for handheld applications . the battery charger is capable of charging single-cell 4.2 v li-ion batteries. the buck converter is powered from the bat pin and has a programmable output voltage providing a maximum load current of 300ma. the converter and the battery charger can run simultaneously or independently of each other. battery charger operation featuring an internal p-channel power mosfet , mp1, the battery charger uses a constant-current /constant- voltage charge algorithm with programmable current . charge current can be programmed up to 500ma with a final float voltage of 4.2v 0.5%. the chrg open-drain status output indicates when c /10 has been reached. no blocking diode or external sense resistor is required; thus, the basic charger circuit requires only two external components. an internal charge termination timer adheres to battery manufacturer safety guidelines . furthermore, the ltc4081 battery charger is capable of operating from a usb power source . a charge cycle begins when the voltage at the v cc pin rises above 3.6v and approximately 82 mv above the bat pin voltage, a 1% program resistor is connected from the prog pin to ground , and the en_chrg pin is pulled below the shutdown threshold (v il ). when the bat pin approaches the final float voltage of 4.2v , the battery charger enters constant-voltage mode and the charge current begins to decrease. when the current drops to 10% of the full-scale charge current , an internal comparator turns off the n-channel mosfet driving the chrg pin, and the pin becomes high impedance. an internal thermal limit reduces the programmed charge current if the die temperature attempts to rise above a preset value of approximately 115c. this feature protects the ltc4081 from excessive temperature and allows the user to push the limits of the power handling capability of a given circuit board without the risk of damaging the ltc4081 or external components . another benefit of the thermal limit is that charge current can be set according to typical, rather than worst-case , ambient temperatures for a given application with the assurance that the battery charger will automatically reduce the current in worst-case conditions. an internal timer sets the total charge time , t timer (typi- cally 4.5 hours). when this time elapses , the charge cycle terminates and the chrg pin assumes a high impedance state even if c /10 has not yet been reached . to restart the charge cycle, remove the input voltage and reapply it or momentarily force the en_chrg pin above v ih . a new charge cycle will automatically restart if the bat pin volt - age falls below v rechrg (typically 4.1v). constant-current / constant-voltage / constant-temperature the ltc4081 battery charger uses a unique architecture to charge a battery in a constant-current , constant-voltage and constant-temperature fashion. three of the amplifier feedback loops shown control the constant-current , ca, constant-voltage , va , and constant-temperature , ta modes ( see block diagram ). a fourth amplifier feedback loop , ma , is used to increase the output impedance of the current source pair , mp1 and mp3 (note that mp1 is the internal p-channel power mosfet). it ensures that the drain cur - rent of mp1 is exactly 400 times the drain current of mp 3. amplifiers ca and va are used in separate feedback loops to force the charger into constant-current or constant- voltage mode , respectively. diodes d1 and d2 provide priority to either the constant-current or constant-voltage loop, whichever is trying to reduce the charge current the most . the output of the other amplifier saturates low which effectively removes its loop from the system . when in constant-current mode , ca servos the voltage at the prog pin to be precisely 1v. va servos its non-inverting input to 1.22v when in constant-voltage mode and the internal resistor divider made up of r 1 and r2 ensures that the battery voltage is maintained at 4.2v. the prog pin voltage gives an indication of the charge current any - time in the charge cycle , as discussed in programming charge current in the applications information section. if the die temperature starts to creep up above 115c due to internal power dissipation, the transconductance amplifier, ta , limits the die temperature to approximately 115 c by reducing the charge current. diode d 3 ensures that ta does not affect the charge current when the die ltc 4081 4081fa
12 for more information www.linear.com/ltc4081 temperature is below 115c. in thermal regulation, the prog pin voltage continues to give an indication of the charge current. in typical operation, the charge cycle begins in constant- current mode with the current delivered to the battery equal to 400v/r prog . if the power dissipation of the ltc4081 results in the junction temperature approaching 115 c , the amplifier (ta ) will begin decreasing the charge current to limit the die temperature to approximately 115c. as the battery voltage rises , the ltc4081 either returns to full constant-current mode or enters constant-voltage mode straight from constant-temperature mode. battery charger undervoltage lockout (uvlo) an internal undervoltage lockout circuit monitors the v cc input voltage and keeps the battery charger off until v cc rises above 3.6v and approximately 82 mv above the bat pin voltage. the 3.6 v uvlo circuit has a built-in hysteresis of approximately 0.6v, and the 82mv automatic shutdown threshold has a built-in hysteresis of approximately 50mv. during undervoltage lockout conditions , maximum battery drain current is 5 a and maximum supply current is 10a. undervoltage charge current limiting (uvcl) the battery charger in the ltc4081 includes undervoltage charge current limiting that prevents full charge current until the input supply voltage reaches approximately 300 mv above the battery voltage ( d v uvcl 1 ). this feature is particu - larly useful if the ltc4081 is powered from a supply with long leads (or any relatively high output impedance ). see applications information section for further details. trickle charge and defective battery detection at the beginning of a charge cycle , if the battery volt - age is below 2.9v, the battery charger goes into trickle charge mode, reducing the charge current to 10% of the programmed current. if the low battery voltage persists for one quarter of the total time (1.125 hr), the battery is assumed to be defective , the charge cycle terminates and the chrg pin output pulses at a frequency of 2 hz with a 75% duty cycle . if, for any reason, the battery voltage rises above 2.9v, the charge cycle will be restarted . to restart the charge cycle (i.e., when the dead battery is replaced with a discharged battery less than 2.9v), the charger must be reset by removing the input voltage and reapplying it or temporarily pulling the en_ chrg pin above the shutdown threshold. battery charger shutdown mode the ltc4081 s battery charger can be disabled by pulling the en_chrg pin above the shutdown threshold (v ih ). in shutdown mode, the battery drain current is reduced to about 2a and the v cc supply current to about 5a provided the regulator is off. when the input voltage is not present, the battery charger is in shutdown and the battery drain current is less than 5a. chrg status output pin the charge status indicator pin has three states : pull-down , pulsing at 2hz ( see trickle charge and defective battery detection and battery temperature monitoring ) and high impedance. the pull-down state indicates that the bat - tery charger is in a charge cycle . a high impedance state indicates that the charge current has dropped below 10% of the full-scale current or the battery charger is disabled . when the timer runs out (4.5 hrs), the chrg pin is also forced to the high impedance state . if the battery charger is not in constant-voltage mode when the charge current is forced to drop below 10% of the full-scale current by uvcl, chrg will stay in the strong pull-down state. charge current soft-start the ltc4081 s battery charger includes a soft-start circuit to minimize the inrush current at the start of a charge cycle. when a charge cycle is initiated, the charge current ramps from zero to full-scale current over a period of ap - proximately 180s. this has the effect of minimizing the transient current load on the power supply during start-up . timer and recharge the ltc4081 s battery charger has an internal charge termination timer that starts when the input voltage is greater than the undervoltage lockout threshold and at least 82 mv above bat, and the battery charger is leaving shutdown. operation ltc 4081 4081fa
13 for more information www.linear.com/ltc4081 at power-up or when exiting shutdown , the charge time is set to 4.5 hours. once the charge cycle terminates , the battery charger continuously monitors the bat pin voltage using a comparator with a 2ms filter time . when the aver - age battery voltage falls below 4.1v (which corresponds to 80% C 90% battery capacity ), a new charge cycle is initiated and a 2.25 hour timer begins . this ensures that the battery is kept at , or near, a fully charged condition and eliminates the need for periodic charge cycle initiations . the chrg output assumes a strong pull-down state dur - ing recharge cycles until c /10 is reached or the recharge cycle terminates. battery temperature monitoring via ntc the battery temperature is measured by placing a nega - tive temperature coefficient (ntc) thermistor close to the battery pack . the ntc circuitry is shown in figure 1. to use this feature , connect the ntc thermistor , r ntc , be- tween the ntc pin and ground and a resistor , r nom , from the ntc pin to v cc . r nom should be a 1% resistor with a value equal to the value of the chosen ntc thermistor at 25 c (this value is 10k for a vishay nths 0603 no1n1002 j thermistor). the ltc4081 goes into hold mode when the value of the ntc thermistor drops to 0.53 times the value of r nom , which corresponds to approximately 40c, and when the value of the ntc thermistor increases to 3.26 times the value of r nom , which corresponds to approxi- mately 0c. hold mode freezes the timer and stops the charge cycle until the thermistor indicates a return to a valid temperature. for a vishay nths 0603no1n1002j thermistor, this value is 32.6k which corresponds to approximately 0c. the hot and cold comparators each have approximately 3 c of hysteresis to prevent oscillation about the trip point. when the charger is in hold mode ( battery temperature is either too hot or too cold ) the chrg pin pulses in a 2hz, 25% duty cycle frequency unless the charge task is finished or the battery is assumed to be defective . if the ntc pin is grounded, the ntc function will be disabled. switching regulator operation the switching buck regulator in the ltc 4081 can be turned on by pulling the en_buck pin above v ih . it has two user- selectable modes of operation : constant-frequency ( pwm ) mode and burst mode operation . the constant-frequency mode operation offers low noise at the expense of efficiency whereas the burst mode operation offers higher efficiency at light loads at the cost of increased noise , higher output voltage ripple, and less output current. a detailed descrip- tion of different operating modes and different aspects of operation follow. operations can best be understood by referring to the block diagram. 4081 f01 r nom r ntc v cc ? + ? + ? + too cold too hot ntc_enable 0.76 v cc 0.35 v cc 0.016 v cc 6 ntc t figure 1. ntc circuit information operation ltc 4081 4081fa
14 for more information www.linear.com/ltc4081 constant-frequency ( pwm ) mode operation the switching regulator operates in constant-frequency ( pwm ) mode when the mode pin is pulled below v il . in this mode, it uses a current mode architecture including an oscillator, an error amplifier , and a pwm comparator for excellent line and load regulation . the main switch mp2 (p-channel mosfet ) turns on to charge the inductor at the beginning of each clock cycle if the fb pin voltage is less than the 0.8v reference voltage . the current into the inductor (and the load) increases until it reaches the peak current demanded by the error amp . at this point, the main switch turns off and the synchronous switch mn1 (n-channel mosfet) turns on allowing the inductor current to flow from ground to the load until either the next clock cycle begins or the current reduces to the zero current (i zero ) level. oscillator: in constant-frequency mode, the switching regulator uses a dedicated oscillator which runs at a fixed frequency of 2.25mhz. this frequency is chosen to minimize possible interference with the am radio band . error amplifier: the error amplifier is an internally com - pensated transconductance (g m ) amplifier with a g m of 65? mhos . the internal 0.8 v reference voltage is compared to the voltage at the fb pin to generate a current signal at the output of the error amplifier . this current signal represents the peak inductor current required to achieve regulation. pwm comparator : lossless current sensing converts the pmos switch current signal to a voltage which is summed with the internal slope compensation signal . the pwm comparator compares this summed signal to determine when to turn off the main switch . the switch current sensing is blanked for ~12ns at the beginning of each clock cycle to prevent false switch turn-off. burst mode operation burst mode operation can be selected by pulling the mode pin above v ih . in this mode, the internal oscil- lator is disabled, the error amplifier is converted into a comparator monitoring the fb voltage , and the inductor current swings between a fixed i peak (~100ma) and i zero (35ma) irrespective of the load current as long as the fb pin voltage is less than or equal to the reference voltage of 0.8v. once v fb is greater than 0.8v, the control logic shuts off both switches along with most of the circuitry and the regulator is said to enter into sleep mode. in sleep mode, the regulator only draws about 20 a from the bat pin provided that the battery charger is turned off. when the output voltage droops about 1% from its nominal value, the regulator wakes up and the inductor current resumes swinging between i peak and i zero . the output capacitor recharges and causes the regulator to re-enter the sleep state if the output load remains light enough. the frequency of this intermittent burst operation depends on the load current . that is , as the load current drops further, the regulator turns on less frequently . thus burst mode operation increases the efficiency at light loads by minimizing the switching and quiescent losses . however, the output voltage ripple increases to about 2%. to minimize ripple in the output voltage , the current limits for both switches in burst mode operation are reduced to about 20% of their values in the constant-frequency mode . also the zero current of the synchronous switch is changed to about 35ma thereby preventing reverse conduction through the inductor . consequently , the regulator can only deliver approximately 67ma of load current while in burst mode operation. any attempt to draw more load cur rent will cause the output voltage to drop out of regulation. current limit to prevent inductor current runaway , there are absolute current limits (i lim ) on both the pmos main switch and the nmos synchronous switch. these limits are internally set at 520 ma and 700 ma respectively for pwm mode. if the peak inductor current demanded by the error amplifier ever exceeds the pmos i lim , the error amplifier will be ignored and the inductor current will be limited to pmos i lim . in burst mode operation , the pmos current limit is reduced to 100ma to minimize output voltage ripple. zero current comparator the zero or reverse current comparator monitors the induc - tor current to the output and shuts off the synchronous rectifier when this current reduces to a predetermined value ( i zero ). in fixed frequency mode, this is set to operation ltc 4081 4081fa
15 for more information www.linear.com/ltc4081 negative 15 ma meaning that the regulator allows the inductor current to flow in the reverse direction ( from the output to ground through the synchronous rectifier ) to a maximum value of 15ma. this is done to ensure that the regulator is able to regulate at very light loads without skipping any cycles thereby keeping output voltage ripple and noise low at the cost of efficiency. however, in burst mode operation , i zero is set to positive 35ma meaning that the synchronous switch is turned off as soon as the current through the inductor to the output decreases to 35ma in the discharge cycle . this preserves the charge on the output capacitor and increases the overall efficiency at light loads. soft-start the ltc4081 switching regulator provides soft-start in both modes of operation by slowly charging an internal capacitor. the voltage on this capacitor , in turn, slowly ramps the current limits of both switches from a low value to their respective maximum values over a period of about 400 s. the soft-start capacitor is discharged completely whenever the regulator is disabled. short-circuit protection in the event of a short circuit at the output or during start-up, v out will be near zero volts . since the downward slope of the inductor current is ~v out /l, the inductor current may not get a chance to discharge enough to avoid a runaway situation. because the current sensing is blanked for ~12ns at the beginning of each clock cycle , inductor current can build up to a dangerously high level over a number of cycles even if there is a hard current operation limit on the main pmos switch. this is why the switching regulator in the ltc4081 also monitors current through the synchronous nmos switch and imposes a hard limit on it. if the inductor current through the nmos switch at the end of a discharge cycle is not below this limit , the regulator skips the next charging cycle thereby preventing inductor current runaway. switching regulator undervoltage lockout whenever v bat is less than 2.7v, an undervoltage lock- out circuit keeps the regulator off , preventing unreliable operation. however, if the regulator is already running and the battery voltage is dropping , the undervoltage comparator does not shut down the regulator until v bat drops below 2.5v. dropout operation when the bat pin voltage approaches v out , the duty cycle of the switching regulator approaches 100%. when v bat is approximately equal to v out , the regulator is said to be in dropout. in dropout, the main switch (mp2) stays on continuously with the output voltage being equal to the battery voltage minus the voltage drops across the main switch and the inductor. global thermal shutdown the ltc4081 includes a global thermal shutdown which shuts off the entire device ( battery charger and switch - ing regulator) if the die temperature exceeds 160c. the ltc4081 resumes normal operation once the temperature drops approximately 14c. ltc 4081 4081fa
16 for more information www.linear.com/ltc4081 battery charger programming charge current the battery charge current is programmed using a single resistor from the prog pin to ground. the charge current is 400 times the current out of the prog pin . the program resistor and the charge current are calculated using the following equations: r prog = 400 ? 1v i bat , i bat = 400 ? 1v r prog the charge current out of the bat pin can be determined at any time by monitoring the prog pin voltage and using the following equation: i bat = v prog r prog ?400 stability considerations the ltc4081 battery charger contains two control loops : constant-voltage and constant-current . the constant- voltage loop is stable without any compensation when a battery is connected with low impedance leads . excessive lead length, however, may add enough series inductance to require a bypass capacitor of at least 1 f from bat to gnd. furthermore, a 4.7f capacitor with a 0.2w to 1w series resistor from bat to gnd is required to keep ripple voltage low when the battery is disconnected . in constant-current mode, the prog pin voltage is in the feedback loop, not the battery voltage . because of the additional pole created by prog pin capacitance , capacitance on this pin must be kept to a minimum . with no additional capacitance on the prog pin , the battery charger is stable with program resistor values as high as 25k. however, additional capacitance on this node reduces the maximum allowed program resistor. the pole frequency at the prog pin should be kept above 100khz. therefore, if the prog pin is loaded with a capacitance , c prog , the following equation should be used to calculate the maximum resistance value for r prog : r prog 1 2 ?100khz ?c prog figure 2. isolating capacitive load on prog pin and filtering 4081 f02 c filter charge current monitor circuitry r prog ltc4081 prog gnd 10k average, rather than instantaneous , battery current may be of interest to the user. for example, when the switching regulator operating in low current mode is connected in parallel with the battery , the average current being pulled out of the bat pin is typically of more interest than the instantaneous current pulses . in such a case , a simple rc filter can be used on the prog pin to measure the average battery current as shown in figure 2. a 10 k resistor has been added between the prog pin and the filter capacitor to ensure stability. undervoltage charge current limiting (uvcl) usb powered systems tend to have highly variable source impedances (due primarily to cable quality and length ). a transient load combined with such impedance can easily trip the uvlo threshold and turn the battery charger off un - less undervoltage charge current limiting is implemented . consider a situation where the ltc4081 is operating under normal conditions and the input supply voltage begins to sag (e.g. an external load drags the input supply down ). if the input voltage reaches v uvcl (approximately 300mv above the battery voltage , dv uvcl ), undervoltage charge current limiting will begin to reduce the charge current in an attempt to maintain dv uvcl between v cc and bat . the ltc4081 will continue to operate at the reduced charge current until the input supply voltage is increased or volt- age mode reduces the charge current further. operation from current limited wall adapter by using a current limited wall adapter as the input sup - ply, the ltc4081 can dissipate significantly less power when programmed for a current higher than the limit of the wall adapter. applications information ltc 4081 4081fa
17 for more information www.linear.com/ltc4081 v cc mp1 mn1 1k 4k 1k 1 i chg 2 d1 4 li-ion battery system load 4081 f03 ltc4081 bat usb power (100ma) 5v wall adapter (500ma) prog + figure 3. combining wall adapter and usb power consider a situation where an application requires a 200 ma charge current for a discharged 800 mah li-ion battery. if a typical 5v (non-current limited) input supply is avail - able then the peak power dissipation inside the part can exceed 300mw. now consider the same scenario , but with a 5v input sup - ply with a 200ma current limit . to take advantage of the supply, it is necessary to program the ltc4081 to charge at a current greater than 200 ma. assume that the ltc 4081 charger is programmed for 300ma (i.e., r prog = 1.33kw) to ensure that part tolerances maintain a programmed current higher than 200ma. since the battery charger will demand a charge current higher than the current limit of the input supply, the supply voltage will collapse to the battery voltage plus 200 ma times the on-resistance of the internal pfet. the on-resistance of the battery charger power device is approximately 0.7w with a 5v supply. the actual on-resistance will be slightly higher due to the fact that the input supply will have collapsed to less than 5v. the power dissipated during this phase of charging is approximately 30mw. that is a ten times improvement over the non-current limited supply power dissipation. usb and wall adapter power although the ltc4081 allows charging from a usb port, a wall adapter can also be used to charge li-ion batter - ies. figure 3 shows an example of how to combine wall adapter and usb power inputs . a p-channel mosfet, mp1, is used to prevent back conducting into the usb port when a wall adapter is present and schottky diode , d1, is used to prevent usb power loss through the 1k pull-down resistor. typically a wall adapter can supply significantly more current than the current-limited usb port . therefore, an n-channel mosfet, mn1, and an extra program resistor can be used to increase the charge current when the wall adapter is present. power dissipation the conditions that cause the ltc4081 battery charger to reduce charge current through thermal feedback can be approximated by considering the total power dissipated in the ic. for high charge currents, the ltc4081 power dissipation is approximately: p d = v cc ? v bat ( ) ?i bat + p d _ buck where p d is the total power dissipated within the ic , v cc is the input supply voltage, v bat is the battery voltage , i bat is the charge current and p d_buck is the power dissipation due to the regulator. p d_buck can be calculated as: p d _ buck = v out ?i out 1 ? 1 ? ? ? ? ? ? ? ? where v out is the regulated output of the switching regulator, i out is the regulator load and is the regulator efficiency at that particular load. it is not necessary to perform worst-case power dissipa - tion scenarios because the ltc4081 will automatically reduce the charge current to maintain the die temperature at approximately 115c. however, the approximate ambi - ent temperature at which the thermal feedback begins to protect the ic is: t a = 115c C p d ja t a = 115c C (v cc C v bat ) ? i bat ? ja if the regulator is off. example: consider the extreme case when an ltc4081 is operating from a 6 v supply providing 250 ma to a 3 v li-ion battery and the regulator is off . the ambient temperature above which the ltc4081 will begin to reduce the 250ma charge current is approximately: t a = 115c C (6v C 3v) ? (250ma) ? 43c/w t a = 115c C 0.75w ? 43c/w = 115c C 32.25c t a = 82.75c applications information ltc 4081 4081fa
18 for more information www.linear.com/ltc4081 if there is more power dissipation due to the regulator , the thermal regulation will begin at a somewhat lower temperature. in the above circumstances , the ltc4081 can be used above 82.75c, but the charge current will be reduced from 250ma. the approximate current at a given ambient temperature can be calculated: i bat = 115 c ? t a v cc ? v bat ( ) ? ja using the previous example with an ambient temperature of 85c, the charge current will be reduced to approximately : i bat = 115 c ? 85 c 6v ? 3v ( ) ? 43 c/w = 30 c 129 c/a = 232.6ma furthermore, the voltage at the prog pin will change proportionally with the charge current as discussed in the programming charge current section. v cc bypass capacitor many types of capacitors can be used for input bypassing ; however, caution must be exercised when using multi-layer ceramic capacitors. because of the self-resonant and high q characteristics of some types of ceramic capacitors , high voltage transients can be generated under some start-up conditions, such as connecting the battery charger input to a live power source . adding a 1 w series resistor in series with an x5r ceramic capacitor will minimize start-up voltage transients. for more information, refer to application note 88. thermistors the ltc4081 ntc trip points are designed to work with therm - istors whose resistance-temperature characteristics follow vishay dale s r-t curve 1. the vishay nths0603no1 n 1002j is an example of such a thermistor . however, vishay dale has many thermistor products that follow the r-t curve 1 characteristic in a variety of sizes . furthermore, any thermis- tor whose ratio of r cold to r hot is about 5 will also work ( vishay dale r-t curve 1 shows a ratio of r cold to r hot of 3.266/0.5325 = 6.13). power conscious designs may want to use thermistors whose room temperature value is greater than 10k. vishay dale has a number of values of thermistor from 10k to 100k that follow the r-t curve 1. using different r-t curves, such as vishay dale r-t curve 2, is also possible . this curve , combined with ltc4081 internal thresholds, gives temperature trip points of approximately 0c (falling) and 40c (rising), a delta of 40c. this delta in temperature can be moved in either direction by changing the value of r nom with respect to r ntc . increasing r nom will move both trip points to higher temperatures . to calculate r nom for a shift to lower temperature for example , use the following equation: r nom = r cold 3.266 ? r ntc at 25 c where r cold is the resistance ratio of r ntc at the desired cold temperature trip point. if you want to shift the trip points to higher temperatures use the following equation: r nom = r hot 0.5325 ? r ntc at 25 c where r hot is the resistance ratio of r ntc at the desired hot temperature trip point. here is an example using a 100 k r-t curve 2 thermistor from vishay dale. the difference between the trip points is 40c, from before , and we want the cold trip point to be 0c, which would put the hot trip point at 40c. the r nom needed is calculated as follows: r nom = r cold 3.266 ? r ntc at 25 c = 2.816 3.266 ? 10k = 8.62k the nearest 1% value for r nom is 8.66k. this is the value used to bias the ntc thermistor to get cold and hot trip points of approximately 0 c and 40 c respectively. to extend the delta applications information ltc 4081 4081fa
19 for more information www.linear.com/ltc4081 4081 f04 r nom 8.87k r ntc 10k v cc ? + ? + ? + too cold too hot ntc_enable r1 604 0.76 v cc 0.35 v cc 0.016 v cc 6 ntc t figure 4. ntc circuits between the cold and hot trip points , a resistor , r1, can be added in series with r ntc (see figure 4). the values of the resistors are calculated as follows: r nom = r cold ? r hot 3.266 ? 0.5325 r 1 = 0.5325 3.266 ? 0.5325 ? ? ? ? ? ? ? ? ? r cold ? r hot ( ) ? r hot where r nom is the value of the bias resistor and r hot and r cold are the values of r ntc at the desired temperature trip points. continuing the example from before with a desired trip point of 50c: r nom = r cold ? r hot 3.266 ? 0.5325 = 10k ? 2.816 ? 0.4086 ( ) 3.266 ? 0.5325 = 8.8k, 8.87k is the nearest 1% value. r 1 = 10k ? 0.5325 3.266 ? 0.5325 ? ? ? ? ? ? ? ? ? 2.816 ? 0.4086 ( ) ? 0.4086 = 604 w , 604 is the nearest 1% value. ntc trip point error when a 1% resistor is used for r hot , the major error in the 40 c trip point is determined by the tolerance of the ntc thermistor . a typical 100 k ntc thermistor has 10% tolerance . by looking up the temperature coefficient of the thermistor at 40c , the tolerance error can be calculated in degrees centigrade. consider the vishay nths 0603n 01n 1003j thermistor, which has a temperature coefficient of C4%/ c at 40c . dividing the tolerance by the temperature coefficient , 5%/(4%/c ) = 1.25c , gives the temperature error of the hot trip point. the cold trip point error depends on the tolerance of the ntc thermistor and the degree to which the ratio of its value at 0c and its value at 40c varies from 6.14 to 1. therefore, the cold trip point error can be calculated using the tolerance , tol, the temperature coefficient of the thermistor at 0c, tc (in %/c), the value of the thermistor at 0c, r cold , and the value of the thermistor at 40c, r hot . the formula is: temperature error( c) = 1 + tol 6.14 ? r cold r hot ? 1 ? ? ? ? ? ? ? ? ? ? ? 100 tc for example, the vishay nths 0603n01n1003j thermistor with a tolerance of 5%, tc of C5%/c and r cold /r hot of 6.13, has a cold trip point error of: temperature error( c) = 1 + 0.05 6.14 ? 6.13 ? 1 ? ? ? ? ? ? ? ? ? 100 ? 5 = ? 0.95 c, 1.05 c switching regulator setting the buck converter output voltage the ltc4081 regulator compares the fb pin voltage with an internal 0.8v reference to generate an error signal at the output of the error amplifier . a voltage divider from applications information ltc 4081 4081fa
20 for more information www.linear.com/ltc4081 v out to ground (as shown in the block diagram ) programs the output voltage via fb using the formula: v out = 0.8v ? 1 + r7 r8 ? ? ? ? ? ? ? ? keeping the current low (<5a) in these resistors maxi - mizes efficiency, but making them too low may allow stray capacitance to cause noise problems and reduce the phase margin of the error amp loop . to improve the frequency response, a phase-lead capacitor (c pl ) of approximately 10pf can be used . great care should be taken to route the fb line away from noise sources , such as the inductor or the sw line. inductor selection the value of the inductor primarily determines the cur - rent ripple in the inductor . the inductor ripple current di l decreases with higher inductance and increases with higher v in or v out : d i l = v out f osc ?l ? 1 ? v out v in ? ? ? ? ? ? ? ? ? ? accepting larger values of d i l allows the use of low inductances, but results in higher output voltage ripple , greater core losses, and lower output current capability . a reasonable starting point for setting ripple current is d i l = 0.3 ? i lim , where i lim is the peak switch current limit. the largest ripple current occurs at the maximum input voltage. to guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation: l v out f 0 ? d i l ? 1 ? v out v in max ( ) ? ? ? ? ? ? ? ? ? ? ? ? for applications with v out = 1.8v, the above equation suggests that an inductor of at least 6.8h should be used for proper operation. many different sizes and shapes of inductors are available from numerous manufacturers. to maximize efficiency, choose an inductor with a low dc resistance . keep in mind that most inductors that are very thin or have a very small volume typically have much higher core and dcr losses and will not give the best efficiency . also choose an induc - tor with a dc current rating at least 1.5? times larger than the peak inductor current limit to ensure that the inductor does not saturate during normal operation . to minimize radiated noise use a toroid or shielded pot core inductor in ferrite or permalloy materials . table 1 shows a list of several inductor manufacturers. table 1. recommended surface mount inductor manufacturers coilcraft www.coilcraft.com sumida www.sumida.com murata www.murata.com toko www.tokoam.com input and output capacitor selection since the input current waveform to a buck converter is a square wave, it contains very high frequency components . it is strongly recommended that a low equivalent series resistance (esr) multilayer ceramic capacitor be used to bypass the bat pin which is the input for the converter . tantalum and aluminum capacitors are not recommended because of their high esr . the value of the capacitor on bat directly controls the amount of input voltage ripple for a given load current. increasing the size of this capacitor will reduce the input ripple. to prevent large v out voltage steps during transient load conditions, it is also recommended that a ceramic capacitor be used to bypass v out . a typical value for this capacitor is 4.7f. multilayer ceramic chip capacitors (mlcc) typically have exceptional esr performance . mlccs combined with a carefully laid out board with an unbroken ground plane will yield very good performance and low emi emissions . applications information ltc 4081 4081fa
21 for more information www.linear.com/ltc4081 there are several types of ceramic capacitors with consider - ably different characteristics . y 5 v ceramic capacitors have apparently higher packing density but poor performance over their rated voltage or temperature ranges . under given voltage and temperature conditions , x5r and x7r ceramic capacitors should be compared directly by case size rather than specified value for a desired minimum capacitance. some manufacturers provide excellent data on their websites about achievable capacitance . table 2 shows a list of several ceramic capacitor manufacturers . table 2. recommended ceramic capacitor manufacturers taiyo yuden www.t-yuden.com avx www.avxcorp.com murata www.murata.com tdk www.tdk.com board layout considerations to be able to deliver maximum charge current under all conditions, it is critical that the exposed metal pad on the backside of the ltc4081s package has a good thermal contact to the pc board ground. correctly soldered to a 2500mm 2 double-sided 1 oz. copper board, the ltc4081 has a thermal resistance of approximately 43c/w. failure to make thermal contact between the exposed pad on the backside of the package and the copper board will result in thermal resistance far greater than 43c/w. furthermore due to its high frequency switching circuitry , it is imperative that the input capacitor , bat pin capaci - tor, inductor, and the output capacitor be as close to the ltc4081 as possible and that there is an unbroken ground plane under the ltc4081 and all of its high frequency components. applications information ltc 4081 4081fa
22 for more information www.linear.com/ltc4081 3.00 0.10 (4 sides) note: 1. drawing to be made a jedec package outline m0-229 variation of (weed-2). check the ltc website data sheet for current status of variation assignment 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.15mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package 0.40 0.10 bottom view?exposed pad 1.65 0.10 (2 sides) 0.75 0.05 r = 0.125 typ 2.38 0.10 (2 sides) 1 5 10 6 pin 1 top mark (see note 6) 0.200 ref 0.00 ? 0.05 (dd) dfn rev c 0310 0.25 0.05 2.38 0.05 (2 sides) recommended solder pad pitch and dimensions 1.65 0.05 (2 sides) 2.15 0.05 0.50 bsc 0.70 0.05 3.55 0.05 package outline 0.25 0.05 0.50 bsc dd package 10-lead plastic dfn (3mm 3mm) (reference ltc dwg # 05-08-1699 rev c) pin 1 notch r = 0.20 or 0.35 45 chamfer package description please refer to http://www .linear.com/designtools/packaging/ for the most recent package drawings. ltc 4081 4081fa
23 for more information www.linear.com/ltc4081 information furnished by linear technology corporation is believed to be accurate and reliable . however, no responsibility is assumed for its use. linear technology corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights . revision history rev date description page number a 07/15 modified typical application diagrams 1, 24 ltc 4081 4081fa
24 for more information www.linear.com/ltc4081 ? linear technology corporation 2007 lt 0715 rev a ? printed in usa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com/ltc4081 related parts typical application part number description comments battery chargers ltc3550 dual input usb/ ac adapter li-ion battery charger with adjustable output 600ma buck converter synchronous buck converter, efficiency: 93%, adjustable output: 600ma, charge current: 950ma programmable, usb compatible, automatic input power detection and selection ltc3550-1 dual input usb/ ac adapter li-ion battery charger with 600ma buck converter synchronous buck converter, efficiency: 93%, output: 1.875v at 600ma, charge current: 950ma programmable, usb compatible, automatic input power detection and selection ltc4054-4.2 standalone linear li-ion battery charger with integrated pass transistor in thinsot tm thermal regulation prevents overheating, c/10 termination ltc4061 standalone li-ion charger with thermistor interface 4.2v, 0.35% float voltage, up to 1a charge current, 3mm 3mm dfn package ltc4061-4.4 standalone li-ion charger with thermistor interface 4.4v (max), 0.4% float voltage, up to 1a charge current, 3mm 3mm dfn package ltc4062 standalone linear li-ion battery charger with micropower comparator up to 1a charge current, charges from usb port, thermal regulation 3mm 3mm dfn package ltc4063 li-ion charger with linear regulator up to 1a charge current, 100ma, 125mv ldo, 3mm 3mm dfn package ltc4080 standalone 500ma charger with 300ma synchronous buck for 1-cell li-ion/polymer batteries; trickle charge; timer termination +c/10; thermal regulation, buck output: 0.8v to v bat , buck input: 2.7v to 5.5v, 3mm 3mm dfn-10 package power management ltc3405/ ltc3405a 300ma (i out ), 1.5mhz, synchronous step- down dc/dc converter 95% efficiency, v in : 2.7v to 6v, v out = 0.8v, i q = 20a, i sd < 1a, thinsot package ltc3406/ ltc3406a 600ma (i out ), 1.5mhz, synchronous step- down dc/dc converter 95% efficiency, v in : 2.5v to 5.5v, v out = 0.6v, i q = 20a, i sd < 1a, thinsot package ltc3411 1.25a (i out ), 4mhz, synchronous step-down dc/dc converter 95% efficiency, v in : 2.5v to 5.5v, v out = 0.8v, i q = 60a, i sd < 1a, ms package ltc3440 600ma (i out ), 2mhz, synchronous buck- boost dc/dc converter 95% efficiency, v in : 2.5v to 5.5v, v out = 2.5v, i q = 25a, i sd < 1a, ms package ltc4411/ltc4412 low loss powerpath tm controller in thinsot automatic switching between dc sources , load sharing, replaces oring diodes ltc4413 dual ideal diode in dfn 2-channel ideal diode oring, low forward on-resistance, low regulated forward voltage , 2.5v v in 5.5v li-ion battery charger with 1.5v buck regulator buck efficiency vs load current (v out = 1.5v) load current (ma) 0.01 40 efficiency (%) power loss (mw) 60 80 0.1 10 100 1 1000 20 0 100 1 10 100 0.1 0.01 1000 4081 ta02b v bat = 3.8v v out = 1.5v l = 10h c = 4.7f efficiency (burst) power loss (burst) efficiency (pwm) power loss (pwm) c bat 4.7f r prog 806 d1 r2 806k c out 4.7f 4081 ta02a c pl 10pf r1 715k r nom 100k r ntc 100k v out (1.5v/300ma) l1 1oh c in 4.7f 500ma r3 510 ltc4081 ntc en_chrg mode fb prog v cc chrg bat gnd v cc (3.75v to 5.5v) + t sw en_buck 4.2v li-ion/ polymer battery ltc 4081 4081fa

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