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jul. 12. 2012 / preliminary 1/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver features typical application circuit applications ? feedback reference voltage : 0.2v ? current mode buck led driver with 925khz fixed frequency ? input voltage range : 4.5v to 28v ? continuous output current : 2a ? up to 93% efficiency ? integrated power mosfet switch : 80m ? ?10 ? shutdown mode ? thermal shutdown & current limit protection ? under voltage lockout figure 1. typical application circuit en l1 c2 r1 d2 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 r2 ? high power led/ir-led lighting ? automotive and marine lighting ? architecture lighting ? general lighting solutions the ADT6780 is a thermally enha nced current mode step down led driver. that is designed to deliver constant current to high power leds. the device is suitable for various high power led application due to the wide operating range(vin 4.5v~28v) and high output capability(continuous 2a). with a very low feedback voltage(v fb =0.2v) power dissipation can be minimized. fault condition pr otection includes cycle-by-cycl e current limiting and thermal shutdown. the package is available in a standard sop8-pp(with exposed pad) package. thermally enhanced low v fb step-down led driver ADT6780 general description
jul. 12. 2012 / preliminary 2/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver part list 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 0.2v 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. component type value (model) manufacturer u1 ic ADT6780 adtech d1 schottky barrier diode b230a diodes l1 chip inductor 6.8uh / 3a tdk c1 mlcc 10 ? / 35v - c2 mlcc 10 ? / 10v - c3 mlcc 8.2 ? - c4 mlcc 100 ? - c5 mlcc 10 ? - r1 chip resistor 0.2 ? / 1% - r2 chip resistor 2 ? - 1 2 3 45 6 7 8 bst vin sw gnd fb comp en ss exposed pad * connect to gnd package outline ADT6780
jul. 12. 2012 / preliminary 3/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver 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 0.2 - 16 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
jul. 12. 2012 / preliminary 4/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver 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 - ? supply current (quiescent) i q v en = 3v, v fb = 1.4v - 0.7 - ? feedback voltage v fb 4.5v v in 28v, v comp < 2v 0.184 0.200 0.216 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 ? peak current limit duty=50% - 3 - a current sense to comp transconductance g cs --11-a/v oscillator frequency f sw - - 925 - ? fold-back frequency v fb = 0v - 125 - ? maximum duty cycle d max v fb = 0.15v, io=1a 76 86 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 - 2.0 - ? soft-start period c4 = 100 ? -3- ? thermal shutdown (note4) - - 145 - note4. guaranteed by design.
jul. 12. 2012 / preliminary 5/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver typical operating characteristics vin=12v, load : 1a / one 4w white led and ta=25 , unless otherwise noted v sw 10v/div v out (ac) 20mv/div i inductor 0.5a/div 1 ? /div steady state operation led current (a) efficiency (%) 60% 65% 70% 75% 80% 85% 90% 95% 100% 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 efficiency vin=24v vin=12v vin=6v 0.184 0.188 0.192 0.196 0.200 0.204 0.208 0.212 0.216 -40 -20 0 20 40 60 80 100 120 750 800 850 900 950 1000 1050 -40 -20 0 20 40 60 80 100 120 v out 2v/div v sw 10v/div i inductor 2a/div 4 ? /div temperature ( ) temperature ( ) feedback voltage (v) switching frequency (khz) inductor peak current (a) 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 20406080100 duty (%) peak current vs duty switching frequency vs die temperature vfb vs die temperature output short
jul. 12. 2012 / preliminary 6/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver v en 5v/div v out 2v/div v sw 10v/div i led 1a/div 1 ? /div 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 20406080100 led current (a) pwm dimming duty (%) pwm dimming through enable (c4 open, pwm frequency=500hz, duty=50%) led current vs pwm dimming duty (c4 open, pwm frequency=500hz) enable start-up (c4=100nf) v sw 10v/div v en 5v/div v out 2v/div i led 0.5a/div 1 ? /div v sw 10v/div v en 5v/div v out 2v/div i led 0.5a/div 40 ? /div enable turn-off (c4=100nf) enable start-up (c4 open) v sw 10v/div v en 5v/div v out 2v/div i led 0.5a/div 40 ? /div enable turn-off (c4 open) v sw 10v/div v en 5v/div v out 2v/div i led 0.5a/div 40 ? /div typical operating characteristics vin=12v, load : 1a / one 4w white led and ta=25 , unless otherwise noted
jul. 12. 2012 / preliminary 7/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver overview detailed description the ADT6780 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 2a of load current. features include enable control, under voltage lockout, programmable soft start, current limit ,thermal shutdown and pwm dimming of leds. the ADT6780 uses current mode cont rol to regulate the led current. the led current is measured at fb pin voltage and amplified through th e internal error amplifier. the error amplifier output voltage is used to control the high side nmos powe r switch and consequently led current is regulated. enable and soft start en pin of the ADT6780 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. a en pin voltage over 2.7v , the soft start capacitor will begin to charge and 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 3msec at 100nf of c4. switching frequency the ADT6780 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 80mv, the switching frequency is changed to 125khz for reducing abrupt inrush current. power boosting the ADT6780 uses an internal nmos power switch to st ep-down the input voltage to the regulated output current. since the nmos power switch requires a gate 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 2.2a and 4a. 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 0.2v for the ADT6780. 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.
jul. 12. 2012 / preliminary 8/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver application information figure 1 is the typical ADT6780 application circuit. and figure 2 is the functional block diagram of the ADT6780. for the application information, refer to the figure 1 & 2 unless otherwise noted. led current resistor selection the led current is set with a current sense resistor r1 between fb and gnd. it is recommended to use 1% tolerance or better resistor. led current is calculated by the below equation. for 1a led current, choose r1 = 0.2 ? 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. r1 0.2v i led = ? ? ? ? ? ? ? ? ? = 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. led pwm dimming the led brightness can be controlled by applying a pulse-width modulation(pwm) signal to the en pin. pwm frequency is limited by turn-on and turn-off time of the led current. so, using a pwm dimming application, soft-start time control capacitor, c4 is not used for higher pwm dimming frequency. pwm frequency is recommended in range of 100hz to 1khz to get a good dimming linearity.
jul. 12. 2012 / preliminary 9/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver 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. for most applications, a 10 ? ceramic capacitor is sufficient. 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 concerned because of its low ripple current. for most applications, a 10 ? ceramic capacitor is sufficient. ? ? ? ? ? ? ? ? ? = 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 =
jul. 12. 2012 / preliminary 10/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver application information (continued) loop compensation the ADT6780 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 r1 is the current sense 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 output capacitor and the led?s ac resistor(r led = v out / i led ) . 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 (r2-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 r2 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. cs ea 1 vdc g a r a = 1 f a c3 2 g f = = r2 c3 2 1 f z1 = esr c 2 1 f o z2 = r2 c6 2 1 f p3 =
jul. 12. 2012 / preliminary 11/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver the output voltage is calculated by the below equation. where, n is the number of leds connected in series, vf is the forward voltage of the led and vfb is the voltage drop across the current sense resistor. application information (continued) r1 g g r c2 f 2 r2 cs ea led c = r2 f 2 4 c3 c r2 esr c2 c6 = vin (v) # of series irleds vout max (v) r2 ( ? )c3 ( ? )c6 ( ? )l1 (uh)c2 ( ? ) 12 1 ~ 5 10 2 8.2 none 6.8 ~ 10 10 24 1 ~ 8 16 2 8.2 none 15 ~ 22 10 vin (v) # of series wleds vout max (v) r2 ( ? )c3 ( ? )c6 ( ? )l1 (uh)c2 ( ? ) 12 1 ~ 2 8 2 8.2 none 6.8 ~ 10 10 24 1 ~ 4 16 2 8.2 none 15 ~ 22 10 fb f out v v v + = n table 1. components values for irled application (refer to the typical applicati on circuit, for other components.) table 2. components values for wled application (refer to the typical applicatio n circuit, for other components.) 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. 2. select r2 (compensation resistor) to operate the desired crossover frequency in a given condition. r2 value is calculated by the following equation: 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: table1 and table2 list the typical values of compensation components and external components for general applications.
jul. 12. 2012 / preliminary 12/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver application information (continued) thermal management the ADT6780 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 +145 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 10 c typically, the device automatically restarts, using the soft-start sequence. the ADT6780 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 curre nt sense resistor 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.
jul. 12. 2012 / preliminary 13/13 * this specifications are subjec t to be changed without notice http://www.ad-tech.co.kr ADT6780 thermally enhanced low v fb step-down led driver package ; sop8-pp(e-pad), 4.9 mm 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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