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  oct. 31. 2011 / rev0.0 1/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 ADT7351 data sheet
oct. 31. 2011 / rev0.0 2/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 revision history rev. number date rev. contents 0.0 2011. 10. 31 initial version
oct. 31. 2011 / rev0.0 3/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 the ADT7351 is a step-down converter with integrated switching mosfet. it operates wide input supply voltage range from 4.5v to 28v with 3a continuous output current. it includes current limiting protection and thermal shutdown. it reduces design complexity and external component count. the ADT7351 is available in small outline sop8-pp( with exposed pad) package. general description features typical application circuit applications - distributed power systems - set-top boxes (stb) - surveillance camera modules - pre-regulator for linear regulators - cigarette lighter powered devices - battery chargers ? current mode buck regulator with 925khz fixed frequency ? input voltage range : 4.5v to 28v ? adjustable output range : 0.92v to 21v ? continuous output current : 3a ? up to 92% efficiency ? integrated power mosfet switch : 80m ? ?10 ? shutdown mode ? thermal shutdown & current limit protection ? under voltage lockout figure 1. typical application circuit (3.3v output) en l1 c2 r2 r1 vout c5 d1 c6 option c4 u1 gnd fb vin en sw bst 1 7 2 3 5 ss comp 6 4 8 vin c1 c3 r3 package outline of the ADT7351
oct. 31. 2011 / rev0.0 4/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 part list component type value (model) manufacturer u1 ic ADT7351 adtech d1 schottky barrier diode b330a diodes l1 chip inductor 4.7uh / 3.6a tdk c1 mlcc 10 ? / 50v - c2 mlcc 47 ? / 6.3v - c3 mlcc 10 ? - c4 mlcc 100 ? - c5 mlcc 10 ? - r1 chip resistor 28 ? / 1% - r2 chip resistor 11 ? / 1% - r3 chip resistor 7.5 ? / 5% - pin description pin no. name description 1bst high-side gate drive boost input. this pin acts as the power supply of high-side gate driving blocks. connect a 10nf or greater capacitor between sw and bst pin. 2vin power supply input. bypass vin to gnd with a suitably large capacitor to eliminate noise on the input to the ic. 3 sw switching node. the free-wheeling diode is connected between sw and gnd. 4 gnd ground. connect the exposed pad on backside to gnd. 5 fb feedback voltage input. the regulated fb voltage is 0.92v typically. 6 comp compensation node. comp is used to compensate the regulation control loop. 7 en chip enable input. also this pin functions uvlo input. 8 ss soft start control node. this pin controls the soft start period. 1 2 3 45 6 7 8 bst vin sw gnd fb comp en ss exposed pad * connect to gnd
oct. 31. 2011 / rev0.0 5/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 functional block diagram absolute maximum ratings (note1) note1. stresses beyond those listed under ?absolute maxi mum ratings? may cause permanent damage to the device. note2. derate 17mw/ above +25 . this is recommended to operate under this power dissipation specification. note3. measured on jesd51-7, 4-layer pcb parameter symbol min. typ. max. unit power supply voltage v in -0.3 - 30 v sw pin voltage v sw -0.5 - v in + 0.3 v bst pin voltage v bst -0.3 - v sw + 6 v all other pins - -0.3 - +6 v max. power dissipation (ta=25 ) (note2) p d - - 2.08 w thermal resistance (note3) ja -60- /w storage temperature t stg -65 - +150 junction temperature t j.max --+150 operating ratings parameter symbol min. typ. max. unit power supply voltage v in 4.5 12.0 28.0 v output voltage v out 0.92 - 21 v operating temperature t opr -40 - +85 junction temperature t j --+125 figure 2. functional block diagram en fb voltage reference internal regulator error amplifier 7 5 comparator control logic + - + - driver gnd 4 shutdown sw 3 bst 1 vin 2 osc regulator + - r s current sense amplifier current limit comp 6 ss 8 driver
oct. 31. 2011 / rev0.0 6/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 electrical characteristics (ta=25 , v in =12v, unless otherwise noted) parameters symbol condition min. typ. max. unit supply current (shutdown) i off v en = 0v - 10 19 ? supply current (quiescent) i q v en = 3v, v fb = 1.4v - 0.7 1.0 ? feedback voltage v fb 4.5v v in 28v, v comp < 2v 0.890 0.920 0.950 v error amplifier voltage gain a ea - - 750 - v/v error amplifier transconductance g ea i comp = 10 ? - 750 - ? /v high-side switch on resistance (note4) r on.h - - 80 - m? low-side switch on resistance (note4) r on.l - - 10 - ? high-side switch leakage current v en = 0v , v sw = 0v - 0.1 10 ? current limit (note4) --5.5- a current sense to comp transconductance g cs --6- a/v oscillator frequency f sw - - 925 - ? fold-back frequency v fb = 0v - 125 - ? maximum duty cycle d max v fb = 0.8v 76 81 99 % minimum on time t on - - 100 - ? uvlo rising threshold v en rising 2.00 2.35 2.70 v uvlo threshold hysteresis - - 250 - ? en threshold voltage - 0.8 1.1 1.4 v enable pull-up current v en = 0v 0.5 2.0 3.5 ? soft-start period c4 = 100 ? , l1=4.7uh c2=47uf, io=3a (cc) -7.5- ? thermal shutdown (note4) - - 148 - note4. guaranteed by design.
oct. 31. 2011 / rev0.0 7/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 typical operating characteristics vin=12v, c1=10uf, c2=2 x 22uf, l1=10uh and ta=25 , unless otherwise noted 0.89 0.90 0.91 0.92 0.93 0.94 0.95 -40 -20 0 20 40 60 80 100 120 vfb vs die temperature temperature ( ) feedback voltage (v) 60% 65% 70% 75% 80% 85% 90% 95% 100% 0.0 0.5 1.0 1.5 2.0 2.5 3.0 v out =5v v out =3.3v v in =12v efficiency load current (a) efficiency steady state operation i out =3a, v out =3.3v v out 10mv/div v en 2v/div v sw 10v/div i ind 2a/div 1 ? /div
oct. 31. 2011 / rev0.0 8/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 typical operating characteristics (continued) shutdown through enable i out =0a, v out =3.3v v out 2v/div v en 2v/div v sw 10v/div i ind 1a/div 1sec/div startup through enable i out =0a, v out =3.3v v out 2v/div v en 2v/div v sw 10v/div i ind 1a/div 4 ? /div startup through enable i out =3a, v out =3.3v v out 2v/div v en 2v/div v sw 10v/div i ind 2a/div 4 ? /div shutdown through enable i out =3a, v out =3.3v v out 2v/div v en 2v/div v sw 10v/div i ind 2a/div 40 ? /div short v out 2v/div v en 2v/div v sw 10v/div i ind 2a/div 4 ? /div
oct. 31. 2011 / rev0.0 9/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 overview detailed description the ADT7351 is a current mode step-down converter with integrated high side nmos power switch. it operates from a 4.5v to 28v input voltage range and supplies up to 3a of load current. features include enable control, under voltage lockout, programmable soft start, current limit and thermal shutdown protection. the ADT7351 uses current mode control to regu late the output voltage. the output voltage is measured at fb through a resistive voltage divider an d amplified through the intern al error amplifier. the error amplifier output voltage is co mpared to the internally sensed load current and consequently generated pwm signal. the pwm signal has the in formation of the input and output voltage relationship and therefore output voltage is regulated by its pwm signal control function. enable and soft start en pin of the ADT7351 operates both chip enable and uvlo function. en pin voltage under 800mv shuts down all the chip function except for pu lling up en pin. when the en pi n voltage exceeds 1.1v, the internal regulator will be enabled and the soft start capacitor will begin to charge. a en pin voltage over 2.7v enables all the operations including switching function. when the en pin is floating, en voltage is high for its pull-up function. the soft start function is adjustable. wh en the en pin becomes high, a tens of ? current begins charging the capacitor which is connected from the ss pin to gnd. smooth control of the output voltage is maintained during start up. the soft start time is adjusted by changing capacitance of c4 and the typical soft start time is 7.5msec at 100nf of c4. switching frequency the ADT7351 switching frequency is fixed and set by an internal oscillator. the practical switching frequency could range from 777khz to 1050khz due to devi ce variation. if the fb voltage is under 0.3v, the switching frequency is changed to 125khz for reducing abrupt inrush current. power boosting the ADT7351 uses an internal nmos power switch to st ep-down the input voltage to the regulated output voltage. since the nmos power switch requires a ga te voltage greater than the input voltage, a boost capacitor connected between sw and bst drives the gate . the capacitor is interna lly charged when sw is low. current limit protection the output over-current protection (o cp) is implemented using a cycle-by-cycle peak detect control circuit. the switch current is monitored by measuring the hi gh side nmos switch current. the measured switch current is compared against a preset voltage which repr esents the current limit, between 3.3a and 7a. when the output current is more than current limit, the hi gh side switch will be turned off and pwm duty is reduced. the output current is mo nitored in the same manner at each cycle and finally the power switch almost turned off not to be damaged under fault conditions. error amplifier the high gain error amplifier extrac ts the difference between the referen ce voltage and the feedback voltage. this extracted difference, called error signal, amplifie d and fed into the comp, which is for compensation. the feedback voltage is regulated to the reference voltage, typical 0.92v for the ADT7351. current sensing the current sensing output is proportional to the current flowing into the inductor, this output goes to the comparator to make a proper pwm control signal. this output waveform resembles normally ramp shape.
oct. 31. 2011 / rev0.0 10/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 application information figure 1 is the typical ADT7351 application circuit. and figure 2 is the functional block diagram of the ADT7351. for the application information, refer to the figure 1 & 2 unless otherwise noted. output voltage resistors selection the output voltage is set with a resistor divider from th e output node to the fb pin. it is recommended to use 1% tolerance or better divider resistors. output voltage is calculated by the below equation. usually, a design is started by setting a fixed r 2 value and calculating the required r 1 with upper equation. some standard value of r 1 , r 2 and most frequently used output volta ge values are listed in below table. table 1. standard output voltage setting to improve efficiency at very light loads consider using larger value resistors. note too high of resistance will be more susceptible to noise and voltage errors from the fb input current will be more noticeable. inductor the inductor required to supply cons tant current to the output load when it is driven by a switching voltage. for given input and output voltage, inductance and switc hing frequency together decide the inductor ripple current, that is: the peak inductor current is: higher inductance gives low inductor ripple current but requires larger size inductor to avoid saturation. low ripple current reduces inductor core losses. also it reduces rms current thro ugh inductor and switches, which results in less conduction loss. usually, peak to peak ri pple current on inductor is designed to be 20% to 30% of the output current limit. make sure it is capable to handle the peak current without saturation. surface mount inductors in different shape and styles are available from tdk, toko 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. ? ? ? ? ? ? ? ? + = 2 1 out r r 1 0.92 v vout (v) r1 ( ? ) r2 ( ? ) 1.0 1.0 11 1.2 3.4 11 1.8 10.5 11 2.5 18.7 11 3.3 28.0 11 4.2 39.0 11 5.0 48.7 11 12.0 133.0 11 ? ? ? ? ? ? ? ? ? = in out sw out l v v 1 l f v i 2 i i i l out l.peak + = output freewheeling diode when the high side switch is of f, freewheeling diode supplies the cu rrent to the inductor. the forward voltage and reverse recovery times of the freewheeling diode are the key loss factors, so schottky diode is mostly used for the freewheeling diode. choose a diode whose maximum reverse voltage rating is greater than the maximum input voltage, and whose current rating is greater than the maximum load current.
oct. 31. 2011 / rev0.0 11/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 application information (continued) input capacitor the input capacitor is used to filter out discontinuous, pulsed inpu t current and to maintain input voltage stable. therefore input capacitor should be able to supply the ac current to the step-down converter. its input ripple voltage can be estimated by: where, cin is input capacitor value. the voltage rating of input capacitor must be greater than the maximum input voltage plus ripple voltage. since the input capacitor abso rbs the input switching current, it requir es an proper ripple current rating. the rms current in the input capaci tor can be approximated by: the worst-case condition occurs at v in =2 v out (50% duty condition), and its worst rms current is approximately half of the i out . for reliable operation and best performance, the input capacitors must have current rating higher than i cin_rms at worst operating conditions. cerami c capacitors are preferred for input capacitors because of their low esr and high current rating. when selecting ceramic capacitors, x5r or x7r type dielectric ceramic capacitors should be used for their better temperature and voltage characteristics. output capacitor the output capacitor is required to maintain the dc output voltage. in a step-down converter circuit, output ripple voltage is determined by the inductor value, switching frequency, output capacitor value and esr. that is: where, c o is output capacitor value, esr is the equivalent series re sistance of the output capacitor. low esr capacitors are preferred to keep the output voltage ripple low. when low esr ceramic capacitor is used as output capacitor, its esr va lue can be waived. so, the impedan ce at the switching frequency is dominated by the capacitance. theref ore the output voltage ripple is: on the other hand, in the case of tantalum or electr olytic capacitors, the esr dom inates the impedance at the switching frequency. in this case, the output voltage ripple is: in a step-down converter, output capacitor current is continuous. usually, the ripple current rating of the output capacitor is not concerne d because of its low ripple current. ? ? ? ? ? ? ? ? ? = in out in out in sw out in v v 1 v v c f i v ? ? ? ? ? ? ? ? ? = in out in out out cin_rms v v 1 v v i i ? ? ? ? ? ? ? ? + = o sw l out c f 8 1 esr i v ? ? ? ? ? ? ? ? = o sw l out c f 8 1 i v ( ) esr i v l out =
oct. 31. 2011 / rev0.0 12/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 application information (continued) loop compensation the ADT7351 uses a fixed frequency, peak current mode control scheme to provide easy compensation and fast transient response. peak current mode control eliminate the double pole effect of the output lc filter. therefore, the step-down converter can be simplified to be a one-pole system in frequency domain. the goal of compensation design is to shape the converter transfer function to get the desired gain and phase. system stability is provided with the addition of a simple series capacitor -resistor from comp to gnd. this pole-zero combination serves to adjust the desired response of the closed-loop system. the dc gain of the voltage feedback loop is given by: where a ea is the error amplifier voltage gain. g cs is the current sense transconductance and r l is the load resistor value. the system has two dominant poles. one is made by the combination of both the output resistor of the error amplifier and the compensation capacitor (c3). and th e other is due to the out put capacitor and the load resistor. these poles are expressed as: where, gea is the error amplifier transconductance. for a stable one-pole converter system, one of two dom inant poles needs to be elim inated by one zero. one zero made by the series capacitor-resistor (r3-c3) cancels f p2 out. this zero is: if the output capacitor has a large capacitance and/or a high esr value, unwanted zero is generated to the location of: in this case, third pole is needed to compensate f z2 . this pole, f p3 , is made by the r3 and the selectively added optional capacitor (c6) between comp to gnd. f p3 is expressed to: the system crossover frequency (fc), where the feedback loop has the unity gain, is important. the system crossover frequency is called the converter bandwidth. generally higher fc means faster transient response and load regulation. however, higher fc could cause sy stem unstable. a standard rule of thumb sets the crossover frequency to be equal or less than 1/10 of switching frequency (for the ADT7351, this is approximately 80khz for the 925khz fixed switching frequency). table 2 lists the typical values of compensation compone nts for some standard output voltages. the values of the compensation components have been optimized for fast transient response and good stability at given conditions. l cs ea out fb vdc r g a v v a = l o p2 ea ea p1 r c 2 1 f a c3 2 g f = = r3 c3 2 1 f z1 = esr c 2 1 f o z2 = r3 c6 2 1 f p3 =
oct. 31. 2011 / rev0.0 13/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 table 2. compensation values for standa rd output voltage/capacitor combinations a general procedure to choose the compensation components for conditions is following: 1. select the desired crossover frequency. set the cr ossover frequency to be equal or less than 1/10 of switching frequency. for the ADT7351, this is approximately 80khz for the 925khz fixed switching frequency. 2. select r3 (compensation resistor) to operate the desired crossover frequency in a given condition. r3 value is calculated by the following equation: for r l much greater than esr of the output capacitor (c2), the equati on can be simplified as follows: most cases, especially for the ceramic capacitors, esr of the output capacitor is much lower than r l , so this equation is good approximation. 3. select c3 (compensation capacito r) to achieve the desired loop ph ase margin. c3 determines the desired first system zero, f z1 . typically, set f z1 below 1/4 of the fc to provides su fficient phase margin. c3 value is calculated by: 4. if the esr output zero (f z2 ) is located at less than one-half the switching frequency, use the (optional) secondary compensation capacitor (c6) to cancel it. as f p3 =f z2 , then: application information (continued) vout (v) r3 ( ? ) c3 ( ? ) c6 ( ? ) l1 (uh) c2 ( ? ) 1.0 1.5 18 none 1 47 1.2 2.4 12 none 1 47 1.8 3.9 10 none 2.2 47 2.5 5.1 10 none 2.2 ~ 4.7 47 3.3 7.5 10 none 2.2 ~ 4.7 47 4.2 8.2 10 none 4.7 47 5.0 15 8.2 51 4.7 ~ 6.8 2 x 22 12.0 22 5.6 51 6.8 ~ 10 2 x 22 l cs ea l c fb out r g g ) r (esr c2 f 2 v v r3 + = cs ea c fb out g g c2 f 2 v v r3 = r3 f 2 4 c3 c r3 esr c2 c6 =
oct. 31. 2011 / rev0.0 14/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 application information (continued) thermal management the ADT7351 contains an internal thermal sensor that limits the total power dissipation in the device and protects it in the even t of an extended thermal fault condition. when the die temperature exceeds +148 c typically, the thermal sensor shuts down the device, tu rning off the dc-dc converter to allow the die to cool. after the die temperature falls by 6 c typically, the device automatically restarts, using the soft-start sequence. the ADT7351 is available in a thermally enhanced so p package and can dissipate up to 1.25w at ta=50 c (t j =125 c). the exposed pad should be connected to gnd externally, preferably soldered to a large ground plane to maximize thermal performance. maximum available power dissipation should be de-rated by 17mw/ above ta=25 not to damage the device. pcb layout consideration pcb layout is very important to achieve clean and st able operation. it is highly recommended to follow below guidelines for good pcb layout. 1. input capacitor (c1) should be placed as near as possible to the ic and connected with direct traces. 2. keep the high current path s as short and wide as possible. 3. keep the switching current path short and minimi ze the loop area, formed by sw, the output capacitors and the input capacitors. 4. route high-speed switching nodes (such as sw and bst) away from sensitive analog areas (such as fb and comp). 5. ensure all feedback connections are short and direct. place the fe edback resistors and compensation components as close as possible to the ic. 6. exposed pad of device must be connected to gnd with solder. for single layer, do not solder exposed pad of the ic.
oct. 31. 2011 / rev0.0 15/15 * this specifications are subject to be changed without notice http://www.ad-tech.co.kr ADT7351 package ; sop8-pp , 4.9mm x 3.94mm body (units : mm) symbol dimensions in millimeters dimensions in inches min max min max a 1.350 1.750 0.053 0.069 a1 0.050 0.150 0.004 0.010 a2 1.350 1.550 0.053 0.061 b 0.330 0.510 0.013 0.020 c 0.170 0.250 0.006 0.010 d 4.700 5.100 0.185 0.200 d1 3.202 3.402 0.126 0.134 e 3.800 4.000 0.150 0.157 e1 5.800 6.200 0.228 0.244 e2 2.313 2.513 0.091 0.099 e 1.270 (bsc) 0.050 (bsc) l 0.400 1.270 0.016 0.050 0 o 8 o 0 o 8 o


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