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general description the max8540/max8541 pulse-width-modulated (pwm) controllers for forward/flyback isolated dc-to-dc convert- ers provide maximum flexibility to power-supply design- ers and reduce external component count. these controllers allow common pc board layout for current- mode (max8540) and voltage-mode (max8541) designs. both controllers feature adjustable switching frequency and external synchronization from 200khz to 1mhz. active-high and active-low enable, undervoltage pro- tection (uvp), and overvoltage protection (ovp) reduce external component count. maximum duty cycle is adjustable, and the feed-forward function scales the maximum duty cycle with input volt- age to limit the maximum volt-seconds applied to the transformer primary. the max8540 allows the user to select the value of slope compensation to further opti- mize magnetics design. the max8541 features user- adjustable ramp magnitude for the pwm comparator. a cycle-by-cycle current-limit function controls the peak primary current during overload and short circuit. both controllers can be set to latch off or to hiccup when a short circuit is detected. the number of current-limited cycles to initiate the hiccup mode and number of cycles skipped are user selectable to allow startup with high-capacitance loads and protect the converter against sustained short circuits. the max8540/max8541 feature 9 ? internal gate dri- vers for low-power applications and are compatible with external gate drivers for high-power applications. both devices are available in the space-saving 16-pin qsop package. the max8541 ev kit evaluates the max8540 and the max8541 to speed designs. design examples are also available in application notes: 50w voltage-mode forward converter design with the max8541 and 50w current-mode forward converter design with the max8540. applications isolated dc-to-dc modules (bricks) cellular base stations telecom and network systems high-performance off-line ac/dc converters features ? 200khz to 1mhz adjustable switching frequency ? synchronization to external clock ? programmable, constant maximum volt-second simplifies transformer design ? programmable hiccup/skip cycles or latch-mode protection ? clean startups with high-capacitance loads ? programmable under/overvoltage protection ? current-mode, adjustable slope compensation (max8540) ? voltage-mode, adjustable ramp magnitude (max8541) ? 10ma, 5v regulator ? 75mv to 1.25mv adjustable current limit reduces external components ? internal gate driver for low-power applications ? external gate driver for high-power applications max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies ________________________________________________________________ maxim integrated products 1 ordering information 16 15 14 13 12 11 10 9 1 2 3 4 5 6 7 8 ref v cc drv gnd skton cs sktoff ilim opto top view max8540eee qsop ov uv maxdty freq/sync ss en scomp pin configurations 19-2830; rev 0; 4/03 for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. part temp range pin-package max8540 eee -40 c to +85 c 16 qsop max8541 eee -40 c to +85 c 16 qsop functional diagrams and typical operating circuit appear at end of data sheet. pin configurations continued at end of data sheet.
max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (v vcc = 12v, c ref = 1?, v uv = v ov = v opto , v ilim = 2v, r freq/sync = 32.4k ? , c ss = 0.1?, r maxdty = 97.6k ? , v en = 0, r scomp = 68k ? , r pramp = 25k ? , c skton = 0.01?, c sktoff = 0.1?, v cs = 0.6v, c drv = 10pf, t a = 0? to +85? , unless otherwise noted. typical values are at t a = +25?.) stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. v cc to gnd ............................................................-0.3v to +20v opto, uv, ov, en , pramp, ilim, ref to gnd ......-0.3v to +6v scomp, freq, cs, skton, sktoff, ss, maxdty to gnd .....................................-0.3v to v ref + 0.3v drv to gnd ................................................-0.3v to v vcc + 0.3v continuous power dissipation (t a = +70?) 16-pin qsop (derate 8.3mw/? above +70?)...........667mw operating temperature range ...........................-40 c to +85? junction temperature ......................................................+150? storage temperature range .............................-65 c to +150? lead temperature (soldering, 10s) .................................+300? parameter conditions min typ max units v cc operating voltage range (note 1) 7.6 18.0 v v cc rising 7.6 8.0 8.4 undervoltage lockout v cc falling 6.6 7.0 7.4 v undervoltage lockout hysteresis 0.9 1.0 v no switching 3.5 4.2 supply current v ref is active, i ref = 0 switching with no load 5 6.3 ma shutdown current v en = 3v 0.5 0.9 ma ref output voltage i ref = 0 4.9 5.0 5.1 v load regulation i ref = 0 to 10 ma 20 50 mv line regulation v vcc = 8.4v to 18v 5 20 mv pulldown resistor in shutdown v en = 3v 10 30 ? undervoltage/overvoltage (uv/ov) v uv rising, 120mv typical hysteresis 1.200 1.250 1.300 uvlo threshold v uv falling 1.083 1.128 1.173 v input bias current -0.5 +0.01 +0.5 a v ov rising, 130mv typical hysteresis 2.9 3.021 3.1 ovlo threshold v ov falling 2.778 2.894 3.010 v en input low threshold v il 1.2 v input high threshold v ih 2.6 v pullup current v en = 2.6v 5 17 a current limit v ilim range 0.075 1.250 v cs current-limit to drv turn-off delay includes leading-edge blanking time 110 150 ns accuracy when v ilim = 0.075v ?0 % v cs cycle-by-cycle threshold voltage v ilim = 1.250v 1.175 1.250 1.330 v
max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies _______________________________________________________________________________________ 3 electrical characteristics (continued) (v vcc = 12v, c ref = 1?, v uv = v ov = v opto , v ilim = 2v, r freq/sync = 32.4k ? , c ss = 0.1?, r maxdty = 97.6k ? , v en = 0, r scomp = 68k ? , r pramp = 25k ? , c skton = 0.01?, c sktoff = 0.1?, v cs = 0.6v, c drv = 10pf, t a = 0? to +85? , unless otherwise noted. typical values are at t a = +25?.) parameter conditions min typ max units ilim leakage current -1 +1 ? hiccup skip off-time c sktoff = 0.1? (note 3) 90 ms hiccup skip off-charging 0.9 1.5 2.1 ? hiccup skip on-time c skton = 0.01? (note 3) 9 ms hiccup skip on-charging current 1.5 2.5 3.5 ? hiccup sktoff voltage threshold 2.3 2.5 2.7 v opto input bias current v opto = 3.5v 30 ? minimum input voltage minimum voltage for duty-cycle control 0.6 v maximum input voltage maximum voltage for duty-cycle control 2.6 v duty cycle r maxdty = 24.3k ? , v uv = 1.3v 15 r maxdty = 130k ? , v uv = 1.3v 80 v uv = 1.3v, r maxdty = 97.6k ? 60 maximum duty cycle internally programmed limit for maximum duty cycle at all frequencies 79 80 82 % soft-start source current v ss = 1v 4.5 5.5 6.5 ? pulldown resistance in shutdown i ss = 10ma, v e n = 3v 10 30 ? time 440 ms/? frequency initial accuracy r freq = 32.4k ? ? ? scomp (max8540) r scomp = 14k ? 1.5 slope compensation r scomp = 204k ? 0.1 v pramp (max8541) r pramp = 12.4k ? 2.5 r pramp = 21k ? 1 ramp voltage amplitude r pramp = 42k ? 0.5 v drv (mosfet driver) c drv = 500pf, 10% to 90% of v vcc 30 rise time c drv = 2000pf, 10% to 90% of v vcc 57 ns
max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies 4 _______________________________________________________________________________________ electrical characteristics (continued) (v vcc = 12v, c ref = 1?, v uv = v ov = v opto , v ilim = 2v, r freq/sync = 32.4k ? , c ss = 0.1?, r maxdty = 97.6k ? , v en = 0, r scomp = 68k ? , r pramp = 25k ? , c skton = 0.01?, c sktoff = 0.1?, v cs = 0.6v, c drv = 10pf, t a = 0? to +85? , unless otherwise noted. typical values are at t a = +25?.) parameter conditions min typ max units c drv = 500pf, 90% to 10% of v vcc 27 fall time c drv = 2000pf, 90% to 10% of v vcc 53 ns on-resistance pullup 9 ? on-resistance pulldown 7 ? sync (external frequency synchronization) frequency range frequency syncs 30% of frequency set by external r on freq pin 0.2 1.0 mhz freq input threshold (note 4) 2 3 v thermal shutdown thermal shutdown t a rising, 20 c hysteresis 170 c electrical characteristics (v vcc = 12v, c ref = 1?, v uv = v ov = 2v, r freq/sync = 32.4k ? , c ss = 0.1?, r maxdty = 97.6k ? , v en = 0, r scomp = 68k ? , r pramp = 25k ? , c skton = 0.01?, c sktoff = 0.1?, v opto = 2v, v ilim = 4v, v cs = 0.6v, c drv = 10pf, t a = -40? to +85? , unless otherwise noted.) (note 5) parameter conditions min typ max units v cc operating voltage range (note 1) 7.6 18.0 v v cc rising 7.6 8.4 undervoltage lockout v cc falling 6.6 7.4 v undervoltage lockout hysteresis 0.9 v no switching 4.2 supply current v ref = is active, i ref = 0 switching with no load 6.3 ma shutdown current v en = 3v 0.9 ma ref output voltage i ref = 0 4.9 5.1 v load regulation i ref = 0 to 10ma 50 mv line regulation v vcc = 8.4v to 18v 20 mv shutdown resistor v en = 3v 30 ? undervoltage/overvoltage (uv/ov) v uv falling, 120mv typical hysteresis 1.200 1.300 uvlo threshold v uv rising 1.083 1.173 v input bias current -0.5 +0.5 a v ov falling, 130 mv typical hysteresis 2.901 3.142 ovlo threshold v ov rising 2.778 3.010 v
max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies _______________________________________________________________________________________ 5 electrical characteristics (continued) (v vcc = 12v, c ref = 1?, v uv = v ov = 2v, r freq/sync = 32.4k ? , c ss = 0.1?, r maxdty = 97.6k ? , v en = 0, r scomp = 68k ? , r pramp = 25k ? , c skton = 0.01?, c sktoff = 0.1?, v opto = 2v, v ilim = 4v, v cs = 0.6v, c drv = 10pf, t a = -40? to +85? , unless otherwise noted.) (note 5) parameter conditions min typ max units en v il 1.2 input threshold v ih 2.6 v pullup current v en = 2.6v 5 a current limit v ilim range 0.075 1.25 v cs current limit to drv delay includes leading-edge blanking time 150 ns accuracy when v ilim = 0.075v ?0 % v cs c ycl e- b y- c ycl e thr eshol d v ol tag e v ilim = 1.250v 1.130 1.375 v ilim leakage current -1 +1 ? hiccup skip off-charging current 0.9 2.1 a hiccup skip on-charging current 1.5 3.5 a hiccup sktoff voltage threshold 2.3 2.7 v duty cycle duty-cycle range v uv = 1.3v 15 80 % maximum duty cycle internal programmed max duty at all frequencies 79 82 % soft-start source current v ss = 1v 4.5 6.5 ? pulldown resistance in shutdown i ss = 10ma, v en = 3v 30 ? frequency initial accuracy r freq = 32.4k ? 280 320 khz sync (external frequency synchronization) frequency range frequency syncs 30% of frequency set by external r on freq pin 0.2 1.0 m h z freq input threshold (note 4) 2 3 v note 1: this is the v cc operating range after clearing the rising uvlo. note 2: guaranteed by design. note 3: see the skton and sktoff section. note 4: the minimum freq pulse must be 200ns in duration with a minimum magnitude of 3v plus v f of the external diode and have a maximum duty cycle of 55%. note 5: specifications to -40? are guaranteed by design and not production tested.
max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies 6 _______________________________________________________________________________________ typical operating characteristics (v cc = 12v, t a = +25?, unless otherwise specified.) v cc no-load switching current vs. temperature max8540 toc01 t a ( c) i vcc (ma) 80 60 20 40 0 -20 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 4.0 -40 100 v en = 0 shutdown v cc current vs. temperature max8540 toc02 t a ( c) i vcc ( a) 80 60 20 40 0 -20 420 440 460 480 500 520 540 560 580 600 400 -40 100 v en = 5v reference voltage vs. temperature max8540 toc03 t a ( c) v ref (v) 80 60 40 20 0 -20 4.95 5.00 5.05 5.10 4.90 -40 100 maximum duty cycle vs. temperature max8540 toc04 t a ( c) d max (%) 80 60 -20 0 20 40 55 60 65 70 75 80 85 90 50 -40 100 r freq = 36.5k ? r freq = none frequency vs. temperature max8540 toc05 t a ( c) frequency (khz) 80 60 -20 0 20 40 200 400 600 800 1000 0 -40 100 r freq = 10k ? r freq = 20k ? r freq = 50k ? current-sense threshold vs. temperature max8540 toc06 t a ( c) v cs (mv) 105 110 115 120 125 130 100 80 60 -20 0 20 40 -40 100 current-limit delay time vs. temperature max8540 toc07 t a ( c) delay time (ns) 80 60 20 40 0 -20 110 120 130 140 150 160 170 180 190 200 100 -40 100 ov/uv threshold voltage vs. temperature max8540 toc08 t a ( c) ov/uv voltage (v) 80 60 -20 0 20 40 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 -40 100 v uv v ov
max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies _______________________________________________________________________________________ 7 frequency vs. input voltage max8540 toc10 input voltage (v) frequency (khz) 15.9 13.8 11.8 9.7 200 400 600 800 1000 0 7.6 18.0 r freq = 10k ? r freq = 20k ? r freq = 50k ? skton/sktoff time vs. capacitance max8540 toc11 capacitance ( f) time (ms) 0.8 0.2 0.4 0.6 200 400 600 800 1000 0 01.0 programable latch-off delay max8540 toc12 4ms/div v drv v cs 2v/div c skton = 0.01 f reference voltage vs. load current max8540 toc13 load current (ma) v ref (v) 10 8 6 4 2 4.92 4.94 4.96 4.98 5.00 5.02 5.04 5.06 5.08 5.10 4.90 012 reference voltage vs. v cc max8540 toc14 v cc (v) v ref (v) 15.9 13.8 11.8 9.7 4.96 4.98 5.00 5.02 5.04 5.06 4.94 7.6 18.0 typical operating characteristics (continued) (v cc = 12v, t a = +25?, unless otherwise specified.) latch-off overcurrent waveforms max8540 toc15 10ms/div v drv v skton v sktoff 5v 1v/div c skton = 0.01 f hiccup-mode overcurrent waveforms max8540 toc16 4ms/div v drv v skton v sktoff 1v/div 1v/div c skton , c sktfof = 0.01 f
max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies 8 _______________________________________________________________________________________ pin description pin max8540 max8541 name function 1 1 ref 5v reference voltage output. bypass to ground with a 1? capacitor. ref is pulled to gnd during shutdown. 22ov system input overvoltage monitoring. the ov threshold is 3.021v with 130mv hysteresis. connect to the center of a resistor-divider from the system input to gnd to set the overvoltage trip point (see the overvoltage threshold section). 33uv system input undervoltage monitoring and feed-forward input. the uv threshold is 1.25v with 120mv hysteresis. connect to the center of a resistor-divider from the system input to gnd to set the undervoltage trip point (see the undervoltage threshold section). this input can be used as a disable input by pulling it below 1.2v. uv remains active during shutdown. 44 freq/ sync p r og r am m ab l e fr eq uency and s ync inp ut. c onnect a r esi stor i n p ar al l el w i th rc ( 10k ? , 0.01?) fr om fre q/s y n c to gn d to set the sw i tchi ng fr eq uency or ap p l y an exter nal cl ock si g nal thr oug h a d i od e to synchr oni ze to an exter nal fr eq uency. 55sss oft- s tar t c ap aci tor c onnecti on. c onnect a cap aci tor fr om s s to gn d to set the soft- star t ti m e p er i od . 6 6 maxdty programmable maximum duty-cycle input. connect a resistor from maxdty to gnd to set the maximum duty cycle for minimum system input voltages. the maximum duty cycle is inversely proportional to the system input voltage. the higher the input voltage, the less the maximum duty cycle. the maximum duty cycle is internally limited to 80% at any switching frequency. maxdty is high impedance during shutdown. bypass maxdty with a 2200pf capacitor. 77 en active-low enable input. drive en to gnd to enable the ic. drive high to place the ic in shutdown mode. 8 scomp programmable slope compensation input. connect a resistor from scomp to gnd to set the slope compensation. 8 pramp programmable voltage-mode control-ramp input. connect a resistor from pramp to gnd to set the magnitude of the voltage ramp. 9 9 opto isol ated feed b ack inp ut. c onnect to an op tocoup l er for an i sol ated p ow er conver ter or connect to the outp ut of a vol tag e- er r or am p l i fi er for a noni sol ated p ow er conver ter for a feed b ack- er r or si g nal . 10 10 ilim current-limit threshold input. connect to the center of a resistor-divider from ref to gnd to set the current-limit threshold voltage. 11 11 sktoff c ur r ent- li m i t o ff- ti m e inp ut. c onnect a cap aci tor fr om s ktoff to gn d to set the cur r ent- l i m i t off- ti m e. p ul l s kto ff to re f thr oug h a 10k ? r esi stor to l atch off the ic after an over cur r ent event. 12 12 cs current-sense input. provides current-sense feedback for cycle-by-cycle current limit and is also the current-mode input for the max8540. connect a current-sense resistor from cs to gnd. 13 13 skton current-limit on-time input. connect a capacitor to ground to set the current-limit on-time. 14 14 gnd ground 15 15 drv gate drive for external n-channel power mosfet. connect to the gate of an external mosfet for low-power applications. connect to the input of an external gate driver for high-power applications. 16 16 v cc voltage supply for the ic. operating input voltage range is 7.6v to 18v. bypass v cc with a ceramic capacitor to gnd.
detailed description overvoltage threshold the max8540/max8541 include an overvoltage protec- tion (ovp) feature that turns off the external mosfet when the input voltage exceeds the user-set threshold. connect a resistor-divider from the system input to gnd with ov connected to the center to set the ovp trip point. the threshold voltage for ov is 3.021v (typ): where v ov is the ov threshold, v in(max) is the overvolt- age trip point, r1 is the resistor from the system input to ov, and r2 is the resistor from ov to gnd. undervoltage threshold the max8540/max8541 also include an undervoltage (uv) sensing input. the ic holds the external mosfet low until uv reaches its threshold (1.25v typ). once the threshold has been reached, the circuit enters soft-start and brings the output into regulation. connect a resistor- divider from the system input to gnd with uv at the cen- ter to set the undervoltage protection (uvp) trip point. where v uv is the uv threshold, v in(min) is the uv trip point, r3 is the resistor from the system input to uv, and r4 is the resistor from uv to gnd. an alternate method used in the application circuits for setting the ov and uv trip points is demonstrated in figure 1. use 36.5k ? for the bottom resistor (rc). ra and rb are calculated as follows: where v in(min) is the uv trip point, v in(max) is the ov trip point, v uv is the uv threshold (1.25v typ), and v ov is the ov threshold (3.021v typ). ra should consist of two equal-value resistors in series to protect against single-point failure. switching frequency and synchronization the max8540/max8541 oscillator operates in two modes: stand-alone or synchronized (sync). a single input, freq/sync, doubles as the attachment point for the frequency-programming resistor and as the syn- chronization input. the mode recognition is automatic, based on the signal applied to freq/sync. in stand-alone mode, an external resistor connected from freq/sync to gnd sets the operating frequency. a 1.25v source is internally applied to freq/sync and the oscillator frequency is proportional to the current out of freq/sync through the programming resistor. ra rc v v rb rc in min uv = ?? () rb rc vv vv ov in min uv in max = ? ? ? ? ? ? ? () () 1 v rr r v in min uv () = + ? ? ? ? ? ? 34 4 v rr r v in max ov () = + ? ? ? ? ? ? 12 2 max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies _______________________________________________________________________________________ 9 table 1. typical specifications of uv/ov and the actual on/off hysteresis of input voltage uv voltage specifications (v) v in off window (ltp) (v) v in on window (utp) (v) min 1.083 29.76 typ 1.128 31.00 max 1.173 32.24 min 1.200 32.97 typ 1.250 34.34 max 1.300 36.00 ov voltage specifications (v) v in off window (utp) (v) v in on window (ltp) (v) min 2.901 79.70 typ 3.021 83.00 max 3.142 86.32 min 2.778 76.32 typ 2.894 79.50 max 3.010 82.68 assume that 1.250v at uv/ov is scaled to 34.34v for a 48v bus. r freq/sync (k ? ) switching frequency (khz) 48.7 200 19.1 500 15.8 600 11.0 800 8.87 1000 table 2. switching frequency selection
max8540/max8541 the max8540/max8541 also synchronize with an external oscillator. drive freq/sync with a square wave through a series diode with a positive pulse width of at least 200ns and a minimum pulse amplitude of 3v, plus the v f of the external diode. alternatively, a 1.5v level-shifted external clock can be applied without the series diode. the maximum duty cycle of the external signal allowed is 55%. the max8540/max8541 syn- chronize to frequencies between 200khz and 1mhz; however, the signal must be within 30% of the fre- quency set by the external resistor at freq/sync. maximum duty cycle set the maximum duty cycle at the minimum system input voltage (v in(min) ) connecting a resistor from maxdty to gnd. the maximum duty cycle is inversely proportional to the voltage at uv. as the voltage on uv increases, the duty cycle decreases. the maximum duty cycle is internally limited to 80% at all switching frequencies. the maxdty resistor is determined as: where d max is the desired maximum duty cycle. the range of valid resistor values for r maxdty is from 24.3k ? to 130k ? . n-channel mosfet driver the drv output drives an n-channel mosfet in low- power applications. in high-power applications, the gate driver internal to the max8540/max8541 may not be capable of driving the external mosfet efficiently and an external gate driver may be required. in this situation, connect drv to the input of the external gate driver. slope compensation (max8540) the max8540 is a current-mode device and requires slope compensation for proper operation. to provide slope compensation, connect a resistor from scomp to gnd (r scomp ). the value of r scomp is determined as follows. for applications using a synchronous rectifier in the output, set the slope compensation equal to the nega- tive slope of the output inductor. r scomp is equal to: where dv ramp /dt = 2.5v(f s ), r sum = 25k ? , n is the turns ratio of the primary to secondary, l1 is the output inductance, v out is the output voltage, f s is the switch- ing frequency, and r cs is the current-sense resistance. for applications where a diode is used in the output instead of the synchronous rectifier, the slope compen- sation resistor is then equal to: where dv ramp /dt = 2.5v(f s ),r sum = 25k ? , n is the turns ratio of the primary to secondary, l1 is the output induc- tance, v out is the output voltage, v f is the diode voltage, f s is the switching frequency, and r cs is the current- sense resistance. voltage-ramp amplitude (max8541) the max8541 is a voltage-mode device and features adjustable voltage ramp. connect a resistor from pramp to gnd (r pramp ) to set the voltage-ramp magnitude, v m . vv k r m pramp = 125 25 . ? r dv dt r scf scomp ramp sum = 2 scf n vv l r out f cs = + 1 r dv dt r scf scomp ramp sum = 2 scf n v l r out cs = 1 r d maxdty max = (. ) 97 6 10 60 3 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies 10 ______________________________________________________________________________________ c1 c2 c3 ra rb rc v in 36v to 76v max8541 ov uv figure 1. a method to set uv and ov thresholds
where r pramp is in k ? . the ramp-voltage magnitude is independent of frequency. the range of values for r pramp is from 12.4k ? to 42k ? . soft-start the soft-start feature allows converters built using the max8540/max8541 to apply power to the load in a controllable soft ramp, thus reducing startup surges and stresses. it also determines power-up sequencing when several converters are used. upon power turn-on, the soft-start pin acts as a current sink to discharge any capacitance connected to it. once the voltage at v cc has exceeded its lockout value, soft- start then charges the external capacitor (c ss ), allowing the converter output voltage to ramp up. full output volt- age is reached in approximately 440ms/?. current limit the max8540/max8541 utilize two current-limit schemes: cycle-by-cycle current limit and short-circuit current limit. set the current-limit threshold using a resistor-divider from ref to gnd with ilim connected to the center. the current-limit threshold is determined as: where r5 is the resistor from ilim to gnd and r6 is the resistor from ref to ilim. use 240k ? for r16 and vary r23 to change the threshold. the cs signal provides feedback on the current ramp through the main external mosfet. the voltage on cs is monitored by the ic. the cycle-by-cycle current limit abbreviates the on-time of the external mosfet in the event that the voltage at cs is greater than the thresh- old voltage set by ilim. the current-limit feature protects against a hard short or overcurrent fault at the output by one of two selected protection modes: by latching off the output, or pulsing the output to reduce the average output current (hiccup mode). to select latched mode, connect sktoff to ref. in this mode, if the hard short exists for the time period set by the capacitance at skton, the output is latched off. to unlatch the output, toggle en or cycle the input power to v cc . to select hiccup mode, connect capacitors to skton and sktoff to program the hiccup mode on- and off- times. when a cycle-by-cycle event is detected, the ic charges the capacitor at skton. the capacitor contin- ues to charge as long as the cs voltage is greater than the ilim threshold voltage. once the voltage on skton reaches its threshold voltage, the max8540/max8541 begin skipping switching cycles for a time determined by the capacitance connected to sktoff. once this time period has elapsed, the ic begins to switch for the time period set by the capacitance connected to skton. this process continues until the output short is removed. see the skton and sktoff section for details on setting the hiccup-mode periods. connect sktoff to ref and skton to gnd to disable the latched-mode and hiccup-mode protection, and operate continuously in cycle-by-cycle current-limit. pmw comparator the pwm comparator of the max8540 transforms the optocoupled error voltage (v opto ) into a duty cycle by comparing the opto feedback-error voltage with a summed voltage. the summed voltage is the sum of the programmable slope compensation and the cur- rent-sense voltages. when the summed voltage exceeds the opto feedback error voltage, the gate- drive logic turns off the external mosfet. the pwm comparator in the max8541 compares v opto with the programmable voltage ramp. when the voltage ramp exceeds v opto , the gate-driver logic turns off the external mosfet. skton and sktoff the capacitance, c skton , determines the time period allowed before the short-circuit current limit initiates. once the cs voltage exceeds the ilim threshold, the capacitor at skton begins to charge. the capacitor continues to charge until the skton threshold voltage is reached or the overcurrent event is removed. this feature allows for the higher currents required during startup with high-capacitance loads. set c skton to allow sufficient time for startup. the required capaci- tance at skton is determined as: c skton = t on / 10 3 where t on is in ms and c skton is in ?. the allowable range for c skton is 100pf to 0.01?. the capacitance at sktoff determines the time period that the external mosfet is turned off during an over- current event. once the skton time period is exceeded, the sktoff capacitor charges. once v sktoff reaches its threshold, the ic begins to switch again. c sktoff is determined as: c sktoff = t off / 10 3 where t off is in ms and c sktoff is in ?. the allow- able range for c sktoff is 1000pf to 1?. v r rr v ilim ref = + 5 56 max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies ______________________________________________________________________________________________________ 11
max8540/max8541 pull v sktoff to v ref through a 10k ? pullup resistor to enable the latch-off feature. in this mode, once the skton time has elapsed, the ic is latched off. the cir- cuit remains off until en is toggled, or the input power is toggled. soft-start capacitor selection during startup, the capacitor at soft-start is charged using a 5? current source. once the voltage at soft- start reaches the threshold voltage (2v typ), the ic switches normally. use a low-esr ceramic capacitor placed as close as possible to the ic at soft-start. the value is determined as follows: where t ss is the desired soft-start period in ms and c ss is in ?. applications information refer to the following application notes for the max8540/max8541 application circuits and applica- tions information: 50w current-mode forward converter design with the max8540 and 50w voltage- mode forward converter design with the max8541. c t ss ss = 530 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies 12 ______________________________________________________________________________________ top view 16 15 14 13 12 11 10 9 1 2 3 4 5 6 7 8 ref v cc drv gnd skton cs sktoff ilim opto max8541eee qsop ov uv maxdty freq/sync ss en pramp pin configurations (continued) chip information transistor count: 2704 process: bicmos
max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies ______________________________________________________________________________________ 13 temp ok v cc 8v 3.02v ov uv en 1.25v 1.2v sktoff skton clk short-circuit timer enable ss_reset enable 5v reference drive logic cs slope compensation v cc uvlo comparator overvoltage comparator undervoltage comparator cs ilim ss ss_reset 5 a osc decoder ramp ref freq/sync max ton clk 80% max pwm comparator maxdty gnd opto scomp drv ref v cc max8540 functional diagrams figure 2. max8540 functional diagram
max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies 14 ______________________________________________________________________________________ temp ok v cc 8v 3.02v ov uv en 1.25v 1.2v sktoff skton clk short-circuit timer enable ss_reset enable 5v reference drive logic clk voltage ramp v cc uvlo comparator overvoltage comparator undervoltage comparator cs ilim ss ss_reset 5 a osc decoder ramp ref freq/sync max ton clk 80% max pwm comparator maxdty gnd opto pramp drv ref v cc max8541 functional diagrams (continued) figure 3. max8541 functional diagram
max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies ______________________________________________________________________________________ 15 max8515 in pgnd fb out gnd vin output 2.5v at 20a input 36v to 76v ov uv freq/sync ss ref ilim scomp opto en v cc drv cs maxdty sktoff skton gnd max8540 u1 off on optional synchronization typical operating circuit
max8540/max8541 synchronizable, high-frequency current- and voltage-mode pwm controllers for isolated supplies maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. 16 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2003 maxim integrated products printed usa is a registered trademark of maxim integrated products. package information (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation, go to www.maxim-ic.com/packages . qsop.eps f 1 1 21-0055 package outline, qsop .150", .025" lead pitch

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