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rev. 0.6 september 2012 www.aosmd.com page 1 of 13 AOZ1282DI ezbuck? 1.2a simple buck regulator general description the AOZ1282DI is a high efficiency, simple to use, 1.2a buck regulator flexible enough to be optimized for a variety of applications. the AOZ1282DI works from a 4.5v to 36v input voltage range, and provides up to 1.2a of continuous output current. the output voltage is adjustable down to 0.8v. the fixed switching frequency of 450khz pwm operation reduces inductor size. features ? 4.5v to 36v operating input voltage range ? 240m ? internal nmos ? up to 95% efficiency ? internal compensation ? 1.2a continuous output current ? fixed 450khz pwm operation ? internal soft start ? output voltage adjustable down to 0.8v ? cycle-by-cycle current limit ? short-circuit protection ? thermal shutdown ? small size dfn2x2-8l applications ? point of load dc/dc conversion ? set top boxes and cable modems ? dvd drives and hdds ? lcd monitors & tvs ? telecom/networking/datacom equipment typical application figure 1. 1.2a buck regulator vin en lx bs gnd fb AOZ1282DI 22h l1 c1 4.7f c3 r1 r2 c2 10f vin vout
AOZ1282DI rev. 0.6 september 2012 www.aosmd.com page 2 of 13 ordering information aos green products use reduced levels of halogens, and are also rohs compliant. please visit www.aosmd.com/media/aosgreenpolicy.pdf for additional information. pin configuration pin description part number ambient temperature range package environmental AOZ1282DI -40 c to +85 c dfn2x2-8l green product pin number pin name pin function 1 lx pwm output. connect to inductor. 2, 3 vin supply voltage input. range from 4.5v to 36v. when vin rises above the uvlo threshold the device starts up. 4 en enable pin. the enable pin is active high. connect en pin to vin through current limiting resistor. do not leave the en pin floating. 5 fb feedback input. it is regulated to 0.8v. th e fb pin is used to determine the pwm output voltage via a resistor divider between the output and gnd. 6, 7 gnd ground. 8 bst bootstrap voltage input. high side driver supply. connected to 100nf capacitor between bst and lx. exposed pad epad thermal exposed pad. pad can be connected to gnd if necessary for improved thermal performance. dfn 2x2-8l (top view) 1 2 3 4 8 7 6 5 bst gnd gnd fb lx vin vin en epad
AOZ1282DI rev. 0.6 september 2012 www.aosmd.com page 3 of 13 absolute maximum ratings exceeding the absolute maxi mum ratings may damage the device. note: 1. devices are inherently esd s ensitive, handling precautions are required. human body model rating: 1.5k ? in series with 100pf. recommended operating conditions the device is not guaranteed to operate beyond the recommended operating conditions. note: 2. the value of ? ja is measured with the device mounted on a 1-in 2 fr-4 board with 2 oz. copper, in a still air environment with t a = 25 c. the value in any given application depends on the user?s specific board design. electrical characteristics t a = 25 c, v in = v en = 12v, unless otherwise spec ified. specifications in bold indicate a temperature range of -40c to +85c. these specifications are guaranteed by design. parameter rating supply voltage (v in ) 40v lx to gnd -0.7v to v vin + 0.3v en to gnd -0.3v to 40v fb to gnd -0.3v to 6v bst to gnd v lx + 6v junction temperature (t j ) +150c storage temperature (t s ) -65c to +150c esd rating (1) 2kv parameter rating supply voltage (v in ) 4.5v to 36v output voltage (v out ) 0.8v to v vin ambient temperature (t a ) -40c to +85c package thermal resistance ( ? ja ) (2) dfn 2x2-8l 55c/w symbol parameter conditions min. typ. max. units v in supply voltage 4.5 36 v v uvlo input under-voltage lockout threshold v in rising v in falling 2.3 2.9 v v uvlo hysteresis 260 mv i in supply current (quiescent) i out = 0, v fb = 1v, v en > 1.2v 11.5 ma i off shutdown supply current v en = 0v 8 ? a v fb feedback voltage t a = 25oc 784 800 816 mv v fb_load load regulation 120ma < load < 1.08a 0.5 % v fb_line line regulation load = 600ma 0.03 %/v i fb feedback voltage input current v fb = 800mv 500 na enable v en_off v en_on en input threshold off threshold on threshold 1.2 0.4 v v v en_hys en input hysteresis 200 mv i en enable input current 3 ? a modulator f o frequency 360 450 540 khz d max maximum duty cycle 87 % t on_min minimum on time 150 ns i lim current limit 1.5 1.9 a over-temperature shutdown limit t j rising t j falling 150 110 c c t ss soft start interval 1.5 ms power state output i leakage nmos leakage v en = 0v, v lx = 0v 10 ? a r ds(on) nmos on-resistance v in = 12v 420 m ?
AOZ1282DI rev. 0.6 september 2012 www.aosmd.com page 4 of 13 block diagram pwm logic enable detect softstart regulator fb en 0.8v error amplifier driver bst lx vin ramp generator osc pwm comparator gnd current sense bst ldo oc clk
AOZ1282DI rev. 0.6 september 2012 www.aosmd.com page 5 of 13 typical performance characteristics circuit of figure 1. t a = 25c, v in = v en = 12v, v out = 3.3 v, unless otherwise specified. in voltage (5v/div) out current (1a/div) out voltage (2v/div) out voltage (100mv/div) out current (1a/div) 200s /div 5ms /div start up to full load load transient full load operation 2s/ div 2s /div in out voltage (100mv/div) load current (1a/div) lx voltage (10v/div) voltage (500mv/div) in out voltage (100mv/div) load current (1a/div) lx voltage (10v/div) voltage (1v/div) light load operation out voltage (2v/div) lx voltage (10v/div) load current (1a/div) lx out voltage (2v/div) load current (1a/div) voltage (10v/div) 2ms /div 2ms /div short circuit protection short circuit recovery
AOZ1282DI rev. 0.6 september 2012 www.aosmd.com page 6 of 13 typical performance characteristics (continued) 0.8 efficiency (vo=5v) vs. load current 50 55 60 65 70 75 85 80 90 95 100 0.4 0.6 0.2 load current (a) efficiency (%) 1.2 01.0 efficiency (vo=3.3v) vs. load current load current (a) efficiency (%) 0 50 55 60 65 70 75 85 80 90 95 100 0.8 0.4 0.6 0.2 1.2 1.0 24v?5v 12v?5v 18v?5v 25 29 current limit vs. input voltage (vo=3.3v) 0.6 0.8 1.0 1.2 1.6 1.4 1.8 2.0 17 21 913 input voltage (v) current limit (a) 37 533 5v?3.3v 18v?3.3v 24v?3.3v 12v?3.3v
AOZ1282DI rev. 0.6 september 2012 www.aosmd.com page 7 of 13 detailed description the AOZ1282DI is a current-mode step down regulator with integrated high side nmos switch. it operates from a 4.5v to 36v input voltage range and supplies up to 1.2a of load current. features include enable control, under voltage lock-out, internal soft-start, output over- voltage protection, over-current protection and thermal shut down. the AOZ1282DI is available in dfn2x2-8l package. enable and soft start the AOZ1282DI has internal soft start feature to limit in- rush current and ensure the output voltage ramps up smoothly to regulation voltage. a soft start process begins when the input voltage rises to the voltage higher than uvlo and voltage on en pin is high. in soft start process, the output voltage is ramped to regulation voltage in typically 400s. the 400s soft start time is set internally. the en pin of the AOZ1282DI is active high. connect the en pin to vin if enable function is not used. pull it to ground will disable the aoz128 2di. do not le ave it open. the voltage on en pin must be above 1.2 v to enable the AOZ1282DI. when voltage on en pin falls below 0.4v, the AOZ1282DI is disabled. steady-state operation under steady-state conditions, the converter operates in fixed frequency and continuous-conduction mode (ccm). the AOZ1282DI integrates an internal nmos as the high-side switch. inductor current is sensed by amplifying the voltage drop across the drain to source of the high side power mosfet. output voltage is divided down by the external voltage divider at the fb pin. the difference of the fb pin voltage and reference is amplified by the internal transconductance error amplifier. the error voltage is compared against the current signal, which is sum of inductor current signal and ramp compensation signal, at pwm comparator input. if the current signal is less than the error voltage, the internal high-side switch is on. the inductor current flows from the input through the inductor to the output. when the current signal exceeds the error voltage, the high-side switch is off. the inductor current is freewheeling through the external schottky diode to output. switching frequency the AOZ1282DI switching frequency is fixed and set by an internal oscillator. the switching frequency is set internally 450khz. output voltage programming output voltage can be set by feeding back the output to the fb pin with a resistor divider network. in the application circuit shown in figure 1. the resistor divider network includes r1 and r2. us ually, a design is started by picking a fixed r2 value and calculating the required r1 with equation below. some standard values of r1 and r2 for the most commonly used output voltage values are listed in ta b l e 1 . table 1. the combination of r1 and r2 should be large enough to avoid drawing excessive current from the output, which will cause power loss. protection features the AOZ1282DI has multiple protection features to prevent system circuit damage under abnormal conditions. over current protection (ocp) the sensed inductor current signal is also used for over current protection. the cycle by cycle current li mit threshold is set normal value of 1.9a. when the load current reaches the current limit threshold, the cycle by cyc le current limit circuit turns off the high side switch immediately to terminate the current duty cycle. the inductor current stop rising. the cycle by cycle current limit protection directly limits inductor peak current. the average inductor current is also limited due to the limitation on peak inductor current. when cycle by cycle current limit circuit is triggered, the output voltage drops as the duty cycle decreasing. vo (v) r1 (k ? ) r2 (k ? ) 1.8 80.6 64.2 2.5 49.9 23.4 3.3 49.9 15.8 5.0 49.9 9.53 v o 0.8 1 r 1 r 2 ------ - + ?? ?? ?? ? =
AOZ1282DI rev. 0.6 september 2012 www.aosmd.com page 8 of 13 the AOZ1282DI has internal short circuit protection to protect itself from catastrophic failure under output short circuit conditions. the fb pin voltage is proportional to the output voltage. whenever fb pin voltage is below 0.2v, the short circuit protection circuit is triggered. as a result, the converter is shut down and hiccups. the converter will start up via a so ft start once the short circuit condition disappears. in short circuit protection mode, the inductor average current is greatly reduced. under voltage lock out (uvlo) an uvlo circuit monitors the input voltage. when the input voltage exceeds 2.9v, the converter starts operation. when input voltage falls below 2.3v, the converter will st op switching. thermal protection an internal temperature sensor monitors the junction temperature. it shuts down the internal control circuit and high side nmos if the junction temperature exceeds 150oc. the regulato r will restart automat ically under the control of soft-start circuit when the junction temperature decreases to 110c. application information the basic AOZ1282DI application circuit is shown in figure 1. component selection is explained below. input capacitor the input capacitor must be connected to the vin pin and pgnd pin of the AOZ1282DI to maintain steady input voltage and filter out the pulsing input current. the voltage rating of input capacitor must be greater than maximum input voltage plus ripple voltage. the input ripple voltage can be approximated by equation below: since the input current is discontinuous in a buck converter, the current stress on the input capacitor is another concern when selecting the capacitor. for a buck circuit, the rms value of i nput capacitor current can be calculated by: if we let m equal the conversion ratio: the relationship between the input capacitor rms current and voltage conversion ratio is calculated and shown in figure 2. it can be seen that when v o is half of v in , c in is under the worst current stress. the worst current stress on c in is 0.5 x i o . figure 2. i cin vs. voltage conversion ratio for reliable operation and best performance, the input capacitors must have curr ent rating higher than i cin-rms at worst operating conditions. ceramic capacitors are preferred for input capacitors because of their low esr and high ripple current rating. depending on the application circuits, other low esr tantalum capacitor or aluminum electrolytic capacitor may also be used. when selecting ceramic capacitors, x5r or x7r type dielectric ceramic capacitors are preferred for their better temperature and voltage characteristics. note that the ripple current rating from capacitor manufactures is based on certain amount of life time. further de-rating may be necessary for practical design requirement. inductor the inductor is used to supply constant current to output when it is driven by a switching voltage. for given input and output voltage, inductance and switching frequency together decide the inductor ripple current, which is: the peak inductor current is: high inductance gives low inductor ripple current but requires larger size inductor to avoid saturation. low ripple current reduces inductor core losses. it also reduces rms current through inductor and switches, which results in less conduction loss. ? v in i o fc in ? ----------------- 1 v o v in -------- - ? ?? ?? ?? v o v in -------- - ? ? = i cin_rms i o v o v in -------- - 1 v o v in -------- - ? ?? ?? ?? ? = v o v in -------- - m = 0 0.1 0.2 0.3 0.4 0.5 0 0.5 1 m i cin_rms (m) i o ? i l v o fl ? ---------- - 1 v o v in -------- - ? ?? ?? ?? ? = i lpeak i o ? i l 2 -------- + =
AOZ1282DI rev. 0.6 september 2012 www.aosmd.com page 9 of 13 when selecting the inductor, make sure it is able to handle the peak current without saturation even at the highest operating temperature. the inductor takes the highest current in a buck circuit. the conduction loss on inductor needs to be checked for thermal and efficiency requirements. surface mount inductors in different shape and styles are available from coilcraft, elytone and murata. shielded inductors are small and radiate less emi noise. but they cost more than unshielded inductors. the choice depends on emi requirement, price and size. output capacitor the output capacitor is selected based on the dc output voltage rating, output ripple voltage specification and ripple current rating. the selected output capacitor must have a higher rated voltage specification than the maximum desired output voltage including ripple. de-rating needs to be considered for long term reliability. output ripple voltage specification is another important factor for selecting the output capacitor. in a buck converter circuit, output ripple voltage is determined by inductor value, switching frequency, output capacitor value and esr. it can be calculated by the equation below: where, c o is output capacitor value, and esr co is the equivalent series resistance of the output capacitor. when low esr ceramic capacitor is used as output capacitor, the impedance of the capacitor at the switching frequency dominates. output ripple is mainly caused by capacitor value and inductor ripple current. the output ripple voltage calculation can be simplified to: if the impedance of esr at switching frequency dominates, the output ripple voltage is mainly decided by capacitor esr and inductor ripple current. the output ripple voltage calculation can be further simplified to: for lower output ripple voltage across the entire operating temperature range, x5r or x7r dielectric type of ceramic, or other low esr tantalum capacitor or aluminum electrolytic capa citor may also be used as output capacitors. in a buck converter, output capacitor current is continuous. the rms current of output capacitor is decided by the peak to peak inductor ripple current. it can be calculated by: usually, the ripple current rating of the output capacitor is a smaller issue because of the low current stress. when the buck inductor is selected to be very small and inductor ripple current is high, output capacitor could be overstressed. schottky diode selection the external freewheeling diode supplies the current to the inductor when the high side nmos switch is off. to reduce the losses due to the forward voltage drop and recovery of diode, schottky diode is recommended to use. the maximum reverse voltage rating of the chosen schottky diode should be greater than the maximum input voltage, and the current rating should be greater than the maximum load current. thermal management and layout consideration in the AOZ1282DI buck regulator circuit, high pulsing current flows through two circuit loops. the first loop starts from the input capacitors, to the vin pin, to the lx pins, to the filter inductor, to the output capacitor and load, and then return to the input capacitor through ground. current flows in the first loop when the high side switch is on. the second loop starts from inductor, to the output capacitors and load, to the anode of schottky diode, to the cathode of schottky diode. current flows in the second loop when the low side diode is on. in pcb layout, minimizing the two loops area reduces the noise of this circuit and im proves efficiency. a ground plane is strongly recommended to connect input capacitor, output capacitor, and pgnd pin of the AOZ1282DI. in the AOZ1282DI buck regulator circuit, the major power dissipating components are the AOZ1282DI, the schottky diode and output inductor. the total power dissipation of converter circui t can be measured by input power minus output power. ? v o ? i l esr co 1 8 fc o ? ? ------------------------- + ?? ?? ? = ? v o ? i l 1 8 fc o ? ? ------------------------- ?? ?? ? = ? v o ? i l esr co ? = i co_rms ? i l 12 ---------- = p total_loss v in i in ? ?? v o v in ? ?? ? =
AOZ1282DI rev. 0.6 september 2012 www.aosmd.com page 10 of 13 the power dissipation in schottky can be approximated as: where, v fw_schottky is the schottky diode forward voltage drop. the power dissipation of inductor can be approximately calculated by output curr ent and dcr of inductor. the actual junction temperature can be calculated with power dissipation in the AOZ1282DI and thermal impedance from junction to ambient. the maximum junction temperature of AOZ1282DI is 150oc, which limits the maximum load current capability. the thermal performance of the AOZ1282DI is strongly affected by the pcb layout. extra care should be taken by users during design process to ensure that the ic will operate under the recommended environmental conditions. several layout tips are listed below for the best electric and thermal performance. 1. the input capacitor should be connected as close as possible to the vin pin and the gnd pin. 2. the inductor should be placed as close as possible to the lx pin and the output capacitor. 3. keep the connection of the schottky diode between the lx pin and the gnd pin as short and wide as possible. 4. place the feedback resistors and compensation components as close to the chip as possible. 5. keep sensitive signal traces away from the lx pin. 6. pour a maximized copper area to the vin pin, the lx pin and especially the gnd pin to help thermal dissipation. 7. pour a copper plane on all unused board area and connect the plane to stable dc nodes, like vin, gnd or vout. p diode_loss i o 1 d ? ?? v fw_schottky ? ? = p inductor_loss i o 2 r inductor 1.1 ? ? = t junction p total_loss p diode_loss ? p inductor_loss ? ?? ? ja t + ambient -------------------------------------------------------------------------------------------------------------------------- =
AOZ1282DI rev. 0.6 september 2012 www.aosmd.com page 11 of 13 package dimensions, dfn2x2-8l d e 1 8 2x aaa c aaa c 2x a b top view ccc c ddd c c a a1 c seating plan side view bottom view bottom view 8 1 pin#1 identification option 2 chamfer 0.2x45 pin#1 identification option 1 d1 1 8 l e1 r e b bbb ab c 0.50 0.25 0.25 1.50 0.30 1.70 0.85 0.90 unit: mm recommended land pattern notes: 1. dimensioning and tolerancing conform to asme y14.5m-1994. 2. controlling dimension is in millimeter, converted inch dimensions are not necessarily exact. 3. dimension b applies to matellized terminal and is measured between 0.10mm and 0.30mm from the terminal tip. if the terminal has the optional radius on the other end of the terminal, the dimension b should not be measured in that radius area. 4. coplanarity ddd applies to the terminals and all other bottom surface metallization. symbols a a1 b c d d1 e e1 e l r aaa bbb ccc ddd dimensions in millimeters min. 0.70 0.00 0.18 1.90 1.35 1.90 0.75 0.20 nom. 0.75 0.02 0.25 0.20 ref 2.00 1.50 2.00 0.90 0.50 bsc 0.30 0.20 0.15 0.10 0.10 0.08 max. 0.80 0.05 0.30 2.10 1.60 2.10 1.00 0.40 symbols a a1 b c d d1 e e1 e l r aaa bbb ccc ddd dimensions in inches min. 0.028 0.000 0.007 0.075 0.053 0.075 0.030 0.008 nom. 0.030 0.001 0.010 0.008 ref 0.079 0.059 0.079 0.035 0.020 bsc 0.012 0.008 0.006 0.004 0.004 0.003 max. 0.031 0.002 0.012 0.083 0.063 0.083 0.039 0.016
AOZ1282DI rev. 0.6 september 2012 www.aosmd.com page 12 of 13 carrier tape reel leader / trailer & orientation tape size 8mm reel size ?178 m ?178.0 1.0 w 9.0 0.5 package dfn 2x2 (8mm) a0 2.30 0.20 b0 2.30 0.20 k0 1.00 0.20 d0 1.00 min. d1 1.50 +0.10/-0 e 8.00 +0.30/-0.10 e1 1.75 0.10 e2 3.50 0.05 p0 4.00 0.20 p1 4.00 0.20 p2 2.00 0.05 t 0.30 0.05 n 70.5 1.0 trailer tape 300mm min. or 75 empty pockets components tape orientation in pocket leader tape 500mm min. or 125 empty pockets unit: mm unit: mm w1 11.8 1.1 h ?13.0 +0.5/-0.2 k 10.25 0.1 s 2.4 0.1 g ?9.8 r n/a v n/a k0 d1 feeding direction t b0 a0 d0 p0 p1 p2 e1 e2 e g v r m w w1 n s h k tape and reel dimensions, dfn2x2-8l
AOZ1282DI rev. 0.6 september 2012 www.aosmd.com page 13 of 13 part marking af1a 9b12 part number code year code assembly lot code option code assembly location code week code AOZ1282DI (2x2 dfn) as used herein: 1. life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body or (b) support or sustain life, and (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury of the user. 2. a critical component in any component of a life support, device, or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. this data sheet contains preliminary data; supplementary data may be published at a later date. alpha & omega semiconductor reserves the right to make changes at any time without notice. life support policy alpha & omega semiconductor products are not authorized for use as critical components in life support devices or systems.

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