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rev. 1.0 june 2011 www.aosmd.com page 1 of 12 aoz1281 ezbuck? 1.8 a simple buck regulator general description the aoz1281 is a high efficiency, simple to use, 1.8 a buck regulator flexible enough to be optimized for a variety of applications. the aoz1281 works from a 3 v to 26 v input voltage range, and provides up to 1.8 a of continuous output current. the output voltage is adjustable down to 0.8 v. the fixed switching frequency of 1.5 mhz pwm operation reduces inductor size. features z 3 v to 26 v operating input voltage range z 240 m ? internal nmos z high efficiency: up to 95 % z internal compensation z 1.8 a continuous output current z fixed 1.5 mhz pwm operation z internal soft start z output voltage adjustable down to 0.8 v z cycle-by-cycle current limit z short-circuit protection z thermal shutdown z small size: dfn 2x2, 8l applications z point of load dc/dc conversion z set top boxes z dvd drives and hdd z lcd monitors & tvs z cable modems z telecom/networking/datacom equipment typical application figure 1. 1.8 a buck regulator lx bst vin vout fb en gnd c2 10f c3 r1 r2 c1 4.7f l1 2.2h aoz1281
aoz1281 rev. 1.0 june 2011 www.aosmd.com page 2 of 12 ordering information aos green products use reduced levels of halogens, and are also rohs compliant. please visit www.aosmd.com/web/qualit y/rohs_compliant.jsp for additional information. pin configuration pin description part number ambient temperature range package environmental AOZ1281DI -40 c to +85 c dfn 2 x 2, 8l green product pin number pin name pin function 1 lx pwm output connection to inductor. 2, 3 vin supply voltage input. range from 3 v to 26 v. when vin rises above the uvlo threshold the device starts up. if vin is lower than 4.5 v, an external 5 v is needed to add through the external diode for bst. 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 regula ted to 0.8 v. the 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 10 nf capacitor between bst and lx. exposed pad thermal exposed pad. pad can be connec ted 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 exposed pad
aoz1281 rev. 1.0 june 2011 www.aosmd.com page 3 of 12 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: 1.5 k ? in series with 100 pf. 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 vin = v en = 12 v. specifications in bold indicate a temperature range of -40 c to +85 c. these specifications are guaranteed by design. parameter rating supply voltage (v in ) 30 v lx to gnd -0.7 v to v vin +2 v en to gnd -0.3 v to 26 v fb to gnd -0.3 v to 6 v bst to gnd v lx + 6 v junction temperature (t j ) +150 c storage temperature (t s ) -65 c to +150 c esd rating (1) 2 kv parameter rating supply voltage (v vin ) 3.0 v to 26 v output voltage range 0.8 v to 0.85 x v vin ambient temperature (t a ) -40 c to +85 c package thermal resistance dfn 2x2, 8l ( ja ) (2) 55 c/w symbol parameter conditions min. typ. max. units v vin supply voltage 326 v v uvlo input under-voltage lockout threshold v vin rising v vin falling 2.3 2.9 v v uvlo hysteresis 200 mv i vin supply current (quiescent) i out = 0, v fb = 1 v, v en > 1.2 v 11.5 ma i off shutdown supply current v en = 0 v 8 a v fb feedback voltage t a = 25 oc 784 800 816 mv v fb_load load regulation 120 ma < load < 1.08 a 0.5 % v fb_line line regulation load = 600 ma 0.03 % / v i fb feedback voltage input current v fb = 800 mv 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 1.2 1.5 1.8 mhz d max maximum duty cycle 87 % t on_min minimum on time 100 ns i lim current limit 2.2 2.6 a over-temperature shutdown limit t j rising t j falling 150 110 c c t ss soft start interval 400 s power state output r ds(on) nmos on-resistance v in = 12 v 240 m ? v in = 3.3 v 380 m ? i leakage nmos leakage v en = 0 v, v lx = 0 v 10 a
aoz1281 rev. 1.0 june 2011 www.aosmd.com page 4 of 12 block diagram gnd vin en fb regulator enable detect softstart pwm logic error amplifier ? + 0.8v ? + pwm comparator osc clk + ramp generator oc current sense driver bst ldo lx bst
aoz1281 rev. 1.0 june 2011 www.aosmd.com page 5 of 12 typical performance characteristics circuit of figure 1. v in = 12 v, v out = 3.3 v, l = 4.7 h, c1 = 10 f, c2 = 22 f, t a = 25 c, unless otherwise specified. light load operaiton full load operation startup to full load short circuit protection 50% to 100% load transient short circuit recovery 500ns/div 500ns/div 2ms/div 2ms/div 200 s/div 2ms/div vin 50mv/div vlx 5v/div vo 1v/div ll 1a/div il 500ma/div vlx 5v/div vo 20mv/div vin 200mv/div il 1a/div vlx 5v/div vo 20mv/div vin 5v/div iin 1a/div vo 1v/div vin 5v/div vo 50mv/div io 1a/div vo 1v/div ll 1a/div vlx 5v/div vin 5v/div
aoz1281 rev. 1.0 june 2011 www.aosmd.com page 6 of 12 efficiency detailed description the aoz1281 is a current-mode step down regulator with integrated high side nm os switch. it operates from a 3 v to 26 v input voltage range and supplies up to 1.8 a of load current. features include: enable control, under voltage lock-out, internal soft-start, output over-voltage protection, over-current pr otection, and thermal shut down. the aoz1281 is available in a dfn 2x2, 8l package. enable and soft start the aoz1281 has an 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 a voltage higher than uvlo and the voltage level on the en pin is high. in the soft start process, the output voltage is typically ramped to regulation voltage in 400 s. the 400 s soft start time is set internally. the en pin of the aoz1281 is active high. connect the en pin to vin if the enable function is not used. pulling en to ground will disable the aoz1281. do not leave en open. the voltage on the en pin must be above 1.2 v to enable the aoz1281. when voltage on the en pin falls below 0.4 v, the aoz1281 is disabled. steady-state operation under steady-state conditions, the converter operates in fixed frequency and continuous-conduction mode (ccm). the aoz1281 integrates an internal nmos as the high-side switch. inductor curren t is sensed by amplifying the voltage drop across the drain to the source of the high-side power mosfet. output voltage is divided down by the external voltag e divider at the fb pin. the difference of the fb pin voltage and reference voltage is amplified by the internal transconductance error amplifier. the error voltage is compared against the current signal, which is su m of inductor current signal plus ramp compensation signal, at the 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 aoz1281 switching frequency is fixed and set by an internal oscillator. the s witching frequency is set internally 1.5 mhz. efficiency (v in = 12v) vs. load current 95 90 85 80 75 70 65 60 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 load current (a) efficiency (%) 5.0v output 3.3v output 1.8v output 1.2v output
aoz1281 rev. 1.0 june 2011 www.aosmd.com page 7 of 12 output voltage programming output voltage can be set by feeding back the output to the fb pin with a resistor di vider network. refer to the application circuit shown in figure 1. the resistor divider network includes r 1 and r 2 . usually, a design is started by picking a fixed r 2 value and calculating the required r 1 with equation below. some standard values of r 1 and r 2 for the most commonly used output voltage values are listed in ta b l e 1 . table 1. the combination of r 1 and r 2 should be large enough to avoid drawing excessive current from the output, which will cause power loss. protection features the aoz1281 has multiple protection features to prevent system circuit damage under abnormal conditions. over current protection (ocp) the sensed inductor current si gnal is also used for over current protection. the cycle-by-cycle current limit threshold is set to 2 a. when the load current reaches the current limit threshold, the cycle-by-cyc le current limit circuit immediately turns off the high-side switch to terminate the current duty cycle. the indu ctor 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 limitat ion on peak inductor current. when cycle-by-cycle current limit circuit is triggered, the output voltage drops as the duty cycle decreases. the aoz1281 has internal short circuit protection to protect itself from catastroph ic failure under output short circuit conditions. the fb pin voltage is proportional to the output voltage. whenever the fb pin voltage is below 0.2 v, the short circuit protection circuit is triggered. as a result, the converter is shut down and hiccups. the converter will start up via a soft start once the short circuit condition is resolved. in the 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 2.9 v, the converter starts operation. when input voltage falls below 2.3 v, the converter will stop switching. thermal protection an internal temperature sensor monitors the junction temperature. the sensor shuts down the internal control circuit and high side nmos if the junction temperature exceeds 150 c. the regulator will restart automatically under the control of the soft-start circuit when the junction temperature decreases to 100 c. application information the basic aoz1281 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 the gnd pins of the aoz1281 to maintain steady input voltage and filter out the pulsing input current. the voltage rating of the input capacitor must be greater than maximum input voltage plus ripple voltage. the input ripple voltage can be approximated by the 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 input capacitor current can be calculated by: if we let m equal the conversion ratio: v o (v) r 1 (k ? ) r 2 (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 ------ - + ?? ?? ?? = 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 =
aoz1281 rev. 1.0 june 2011 www.aosmd.com page 8 of 12 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 a current rating higher than i cin_rms at the worst operating conditions. ceramic capacitors are preferred for use as input capacitors because of their low esr and high ripple current rating. depending on the applicatio n circuits, other low esr tantalum capacitors or aluminum electrolytic capacitors may 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 manufacturers is based on a fixed life time. further de-rating may be necessary for practical design requirements. inductor the inductor is used to supply constant current to output when it is driven by a swit ching 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 provides a low inductor ripple current but requires larger size inductor to avoid saturation. low ripple current reduces inductor core losses and also reduces rms current through inductor and switches. this results in less conduction loss. when selecting the inductor, confirm it is able to handle the peak current without saturation 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 cost more than unshielded inductors. the choice depends on emi requirement, price and size. output capacitor the output capacitor is select ed based on the dc output voltage rating, output ripple voltage specification and ripple current rating. the selected output capacito r 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 specif ication 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 a low esr ceramic capacitor is used as the 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: 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 -------- + = v o i l esr co 1 8 fc o ------------------------- + ?? ?? = v o i l 1 8 fc o ------------------------- ?? ?? = v o i l esr co =
aoz1281 rev. 1.0 june 2011 www.aosmd.com page 9 of 12 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 capacitor may also be used as output capacitors. in a buck converter, out put capacitor current is continuous. the rms current of the 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 , the 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 the diode, a schottky diode is recommended. the maximum reverse voltage rating of the 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 aoz1281 buck regulator circuit, high pulsing current flows through two circuit loops. the first loop starts from the input capacito rs, to the vin pins, to the lx pin, to the filter inductor, to the output capacitor and load, and then returns to the input capacitor through ground. current flows in the first loop when the high side switch is on. the second loop starts from the 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 area of the two loops will reduce the noise of this circuit and improves efficiency. a ground plane is strongly recommended to connect the input capacitor, the output capacitor, and the gnd pin of the aoz1281. in the aoz1281 buck regulator circuit, the major power dissipating components are the aoz1281, the schottky diode and the output inductor. the total power dissipation of the converter circuit can be measured by input power minus output power. the power dissipation in the schottky diode can be approximated as: where, v fw_schottky is the schottky diode forward voltage drop. the power dissipation of the inductor can be approximately calculated by output current and dcr of the inductor. the actual junction temperature can be calculated with power dissipation in the aoz1281 and thermal impedance from junction to ambient. the maximum junction tem perature of aoz1281 is 150 oc, which limits the maximu m load current capability. the thermal performance of the aoz1281 is strongly affected by the pcb layout. extra care should be taken during the design process to ensure that the ic will operate under the recommended environmental conditions. several layout tips are listed below for the best electrical and thermal performance. 1. the input capacitor should be connected as close as possible to the vin pins 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 pins, the lx pin and especially the gnd pin to help thermal dissipation. 7. pour a copper plane on all unused board areas and connect the plane to stable dc nodes, like vin, gnd or vout. i co_rms i l 12 ---------- = p total_loss v in i in () v o v in () ? = 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 amb + =
aoz1281 rev. 1.0 june 2011 www.aosmd.com page 10 of 12 package dimensions, dfn 2x2, 8l top view bottom view side view recommended land pattern notes: 1. dimensions and tolerances conform to asme y14.5m-1994. 2. controlling dimension is millimeter, converted inch dimensions are not necessarily exact. 3. dimension b applied to metallized 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, 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 1.35 0.75 0.20 nom. 0.75 0.02 0.25 0.20 ref. 2.00 bsc 1.50 2.00 bsc 0.90 0.50 bsc 0.30 0.20 0.15 0.10 0.10 0.08 max. 0.80 0.05 0.30 1.60 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 0.053 0.030 0.008 nom. 0.030 0.001 0.010 0.008 ref. 0.079 bsc 0.059 0.079 bsc 0.035 0.020 bsc 0.012 0.008 0.006 0.004 0.004 0.003 max. 0.031 0.002 0.012 0.063 0.039 0.016 unit: mm e d b e l d1 chamfer 0.2 x 45 a1 seating plane a 0.50 0.25 1.70 1.50 0.30 0.90 0.85 c e1 r bottom view pin #1 id option 2 pin #1 id option 1
aoz1281 rev. 1.0 june 2011 www.aosmd.com page 11 of 12 tape and reel dime nsions, dfn 2x2, 8l carrier tape reel leader / trailer & orientation tape size 8mm reel size ?180 m ?180.00 0.50 package dfn 2x2 a0 2.25 0.05 b0 2.25 0.05 k0 1.00 0.05 d0 1.50 +0.1/-0 d1 1.00 +0.25/-0 e 8.00 +0.30/-0.10 e1 1.75 0.10 e2 3.50 0.05 p0 4.00 0.10 p1 4.00 0.10 p2 2.00 0.10 t 0.254 0.02 n 60.0 0.50 w1 r m s k h n trailer tape 300mm min. components tape orientation in pocket leader tape 500mm min. p1 p0 unit: mm unit: mm a0 b0 t k0 p2 e e2 e1 d0 d1 feeding direction w1 8.4 +1.5/-0.0 h 13.0 0.20 s 1.5 min. k 13.5 min. r 3.0 0.50
aoz1281 rev. 1.0 june 2011 www.aosmd.com page 12 of 12 part marking an1a 9b12 part number code underscore denotes green product year code assembly lot code option code assembly location code week code AOZ1281DI (2 x 2 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 prov ided 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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