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four - string, white led driver for lcd backlight applications data sheet ADD5207 rev. a information furnished by analog devices is believed to be accurate and reliable. however, no responsibility is assumed by analog devices for its use, nor for any in fringements of patents or other rights of third parties that may result from its use. specifications subject to change without notice. no license is granted by implication or otherwise under any patent or patent rights of analog devices. trademarks and reg istered trademarks are the property of their respective owners. one technology way, p.o. box 9106, norwood, ma 02062 - 9106, u.s.a. tel: 781.329.4700 www.analog.com fax: 781.461.3113 ? 2009 C 2012 analog devices, inc. all rights reserved. features white led driver based on inductive boost converter integrated 40 v mosfet with 1 . 5 a peak current limit input voltage range: 6 v to 21 v maximum output adjustable up to 36 v 600 khz to 1 mhz adjustable operating frequency typical 39 v fixed o verv oltage protection (ovp) built - in soft start for boost converter drives up to 4 led current strings led current adjustable up to 25 ma for each channel headroom control to maximize efficiency fixed led dimming frequency: 8 khz led open fault protection bri ghtness control with pwm input d imming controls 4 - channel operation : 90 degree phase shift between channels 3 - channel operation : 120 degree phase shift between channels general thermal shutdown undervoltage lockout 14- lead, 4 mm 3 mm lfcsp applications notebook pcs, umpcs, and monitor displays general description the add520 7 is a white led driver for backlight a pplications based on high efficiency, current mode, step - up converter tech - nology. it is designed with a 0.15 , 1 .5 a i nternal switch and a pin - adjustable operating frequency between 600 khz and 1 mhz. the ADD5207 contains four regulated current sources for uniform led brightness. each current source can drive up to 25 ma and the led - driving current is pin adjustable by an external resistor. the ADD5207 drives up to four parallel strings of multiple series - connected leds with a 1.5% current matching between s trings . the ADD5207 provides phase shift pwm brightness control methods. led dimming control is achieved through the pwm input. the device includes an 8 khz led - dimming oscillator for driving each current sour ce. the ADD5207 operates over an input voltage range of 6 v to 21 v, but the device can function with a voltage as low as 5.6 v. the ADD5207 also has multiple safe ty protection features to prevent damage during fault conditions. if any led is open, the device automatically disables the faulty current source . the internal soft start circuit prevents a high inrush current during startup. thermal shutdown protection pr events thermal damage. the ADD5207 is available in a low profile, thermally enhanced, 4 mm 3 mm 0.75 mm, 14- lead, rohs - compliant lead frame chip scale package (lfcsp) and is specified over the industrial t emperature range of ?25c to +85c. typical application circuit 08350-101 l1 10h vin r c 6.8k? c c 2.2nf r set 180k? c2 open c in 10f v in iset comp gnd sw d1 ovp fb1 fb2 fb3 fb4 14 13 4 5 6 7 12 11 3 1 + ? pwm 9 r f 100k? fslct 2 c bypass 1f vdd 8 c out 4f ADD5207 c in2 0.1f shdn off on 10 figure 1.
ADD5207 data sheet rev. a | page 2 of 16 table of contents features .............................................................................................. 1 applications ....................................................................................... 1 general description ......................................................................... 1 typical application circuit ............................................................. 1 revision history ............................................................................... 2 functional block diagram .............................................................. 3 specifications ..................................................................................... 4 step - up switching regulator specifications ............................. 4 led current regulation specifications .................................... 5 general specifications ................................................................. 6 a bsolute m aximum r atings ............................................................ 7 thermal resistance ...................................................................... 7 esd caution .................................................................................. 7 pin configuration and function descriptions ............................. 8 typical performance characteristics ..............................................9 theory of operation ...................................................................... 11 current mode, step - up switching regulator operation ..... 11 internal 3.3 v regulator ............................................................ 11 boost converter switching frequency .................................... 11 dimming frequency (f pwm ) ...................................................... 11 current source ............................................................................ 11 pwm dimming mode .............................................................. 11 safety features ............................................................................ 11 external component sel ection guide ..................................... 12 layout guidelines ....................................................................... 13 typical application circuits ......................................................... 15 outline dimensions ....................................................................... 16 ordering guide .......................................................................... 16 revision history 2 /1 2 rev. sp0 to rev. a replaced block diagram with typical application circuit ........ 1 changes to fe atures section and general description section . 1 changes to current mode, step - up switching regulator operation section, boost converter switching frequency section, pwm dimming mode section, phase shift pwm dimming section, and safety features section .......................... 11 changes to overvoltage protection (ovp) section .................. 11 changes to open - loop protection (olp) section, undervoltage lockout (uvlo) section , and thermal protection section .......................................................................... 12 changes to layout guidelines section ........................................ 13 7/09 revision sp0: initial version
data sheet ADD5207 rev. a | page 3 of 16 functio nal block diagram ADD5207 vin vdd shdn ov p vout_fb sw linear regul a t or volt age reference shutdown error am p uv p com p pwm com p uv p ref g m ll ref vout_fb ll com p dcom p headroom contro l current source 1 current source 2 current source 3 current source 4 pwm 500k? iset ref vdd gnd 500k? gnd com p pwm dut y extrac t or soft s t art current sense osc led open/short f au l t detec t or dref ov p ref gnd therma l shutdown r s q fslct gnd fb1 fb2 current source driver fpwm oscill a t or 1 10 8 13 14 2 12 4 5 fb3 6 fb4 7 9 3 11 08350-002 + + figure 2 . functional block diagram
ADD5207 data sheet rev. a | page 4 of 16 specifications step - up switching regulator specifications v in = 12 v, shdn = high , t a = ? 25 c to +85 c, unless otherwise noted. typical values are at t a = +25 c. table 1 . parameter symbol test conditions /comments min typ max unit supply input voltage range v in 6 21 v boost output output voltage v out 36 v switch on resistance r ds(on) v in = 12 v, i sw = 1 00 ma 150 300 m leakage current i lkg 1 a peak current limit i cl duty cycle ( d ) = d max 1.5 a oscillator switching frequency f sw r f = 97 k 800 1000 1200 khz maximum duty cycle d max r f = 97 k 84 90 % soft start soft start time t ss 1.1 ms ov ervoltage protection overvoltage rising threshold on ovp pin v ovpr 36.5 39 40 v overvoltage hysteresis on ovp pin v ovp_hys 0.1 0.7 1.4 v
data sheet ADD5207 rev. a | page 5 of 16 led current regulati on specifications v in = 12 v, shdn = high , t a = ? 25 c to +85 c, unl ess otherwise noted. typical values are at t a = +25 c. table 2 . parameter symbol test conditions /comments min typ max unit current source iset pin voltage v set 6 v v in 21 v 1. 14 1. 18 1.2 2 v adjustable led current 1 i led 0 25 ma constant current sink of 20 ma 2 i led20 r set = 180 k 19.4 20 20.6 ma minimum headroom voltage 2 v hr20 r set = 180 k 0.66 0.9 v current matching between strings 2 r set = 180 k ?1.5 +1.5 % led current accuracy 2 r set = 180 k ?3 +3 % current source leakage current 1 a fpwm generator dimming frequency f pwm 6.8 8.0 9.2 k hz led fault detection open fault delay 1 t d_openfault 6.5 s 1 this electrical specification is guaranteed by design . 2 test ed at t a = +25 c .
ADD5207 data sheet rev. a | page 6 of 16 general specificatio ns v in = 12 v, shdn = high , t a = ? 25 c to +85 c, unless otherwise noted. typical values are at t a = +25 c. table 3 . parameter symbol test conditions /comments min typ max unit supply input voltage range v in 6 21 v quiescent current i q 6 v v in 21 v, shdn = high 3.5 7 ma shutdown supply current i sd 6 v v in 21 v, shdn = low 2 a v dd regulator v dd regulated output v vdd _reg 6 v v in 21 v 3. 1 3.3 3. 5 v pwm input pwm voltage high v pwm_high 2. 0 5.5 v pwm voltage low v pwm_low 0.8 v pwm input range 1 00 10,000 hz thermal shutdown thermal shutdown threshold 1 t sd 160 c thermal shutdown hysteresis 1 t sdhys 30 c uvlo v in falling threshold v uvlo f v in fall ing 4 4. 2 v v in rising threshold v uvlo r v in rising 5.0 5.6 v shdn control input voltage hig h v ih 2 . 5 5.5 v input voltage low v il 0.5 v shdn pin input current i shdn shdn = 3.3 v 6 a 1 this electrical specification is guaranteed by design .
data sheet ADD5207 rev. a | page 7 of 16 a bsolute m ax imum r atings t a = 25c, unless otherwise noted. table 4 . parameter rating vin ?0.3 v to +23 v sw ?0.3 v to + 4 0 v shdn , pwm ?0.3 v to +6 v iset, fslct, comp ?0.3 v to +3.5 v vdd ?0.3 v to +3. 7 v fb1, fb2, fb3, fb4 ?0.3 v to + 4 0 v ovp ?0.3 v to + 40 v maximum junction temperature (t j max) 150 c operating temperature range (t a ) ? 25 c to +85 c storage temperature range (t s ) ?6 5 c to +150 c reflow peak temperature (20 sec to 40 sec) 260 c stresses above t hose listed under absolute maximum ratings may cause permanent damage to the device. this is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. thermal resistance ja is specified for the worst - case conditions, that is, a device soldered in a circuit board for surface - mount packages. table 5 . thermal resistance package type ja jc unit 14- lead lfcsp 33.24 2.42 c/w esd caution
ADD5207 data sheet rev. a | page 8 of 16 pin configuration and fu nction descriptions top view (not to scale) ovp sw shdn pwm fslct vin iset fb1 fb2 fb3 08350-003 14 13 gnd 12 comp 11 10 9 vdd 8 6 5 4 2 3 1 fb4 notes 1. connect the exposed paddle to ground. 7 ADD5207 figure 3. pin configuration table 6. pin function descriptions pin no. mnemonic description 1 vin supply input. must be locally bypassed with a capacitor to ground. 2 fslct frequency select. a resistor from this pin to ground sets the boost switching frequency from 600 khz to 1 mhz. 3 iset full-scale led current set. a resistor from this pin to ground sets the led current up to 25 ma. 4 fb1 regulated current sink. connect the bottom cathode of the led string to this pin. 5 fb2 regulated current sink. connect the bottom cathode of the led string to this pin. 6 fb3 regulated current sink. connect the bottom cathode of the led string to this pin. 7 fb4 regulated current sink. connect the bottom cathode of the led string to this pin. if unused, connect fb4 to gnd. 8 vdd internal linear regulator output. this regulator provides power to the ADD5207 . 9 pwm pwm signal input. 10 shdn shutdown control for pwm input operation mode. active low. 11 comp compensation for the boost converter. two capacitors and a resistor are connected in series between ground and this pin for stable operation. 12 gnd ground. 13 ovp overvoltage protection. the boost converter output is connected to this pin directly. 14 sw drain connection of the internal power fet. ep exposed paddle. connect the exposed paddle to ground.
data sheet ADD5207 rev. a | page 9 of 16 typical performance characteristics 80 78 5 10 15 input voltage (v) boost converter efficiency (v) 20 84 82 88 86 90 i led = 20ma f sw = 800khz brightness = 100% leds = 10 series 4 parallel 08350-004 figure 4. boost converter efficiency vs. input voltage 2 8 2 6 2 4 2 2 2 0 led current (ma) 1 8 1 6 14 12 10 6 8 4 13 5 15 0 16 5 18 0 19 5 21 0 22 5 24 0 25 5 27 0 r se t (k?) 08350-005 figure 5 . led current vs. r set led current (ma) pwm duty cycle (%) 08350-006 0 5 10 15 20 0 5.08 10.16 15.23 20.31 25.39 30.47 35.55 40.63 45.70 50.78 55.86 60.94 66.02 71.09 76.17 81.25 86.33 91.41 96.48 figure 6 . led current vs. pwm input duty cycle 25 20 15 10 5 0 5 10 15 20 input voltage (v) led current (ma) 08350-007 figure 7. led curre nt vs. input voltage (i led = 20 ma) 1.5 1.2 0.9 0.6 0.3 0 ?0.3 ?0.6 ?0.9 ?1.5 ?1.2 6 10 14 18 8 12 16 20 22 led current matching (%) input voltage (v) brightness = 100% leds = 10 series 4 parallel i led = 20ma 08350-008 figure 8 . led current matching vs. input voltage 5ms/div 0v 0v 0v 0a v out 20v/div v sw 20v/div shdn 5v/div i l 600ma/div v in = 12v brightness = 100% leds = 10 series 4 p aralle l 08350-009 figure 9. start - up waveforms (brightness = 100%)
ADD5207 data sheet rev. a | page 10 of 16 1s/div 0v 0v 0a v out 100mv/div ac v sw 20v/div i l 500ma/div v in = 6 v , f sw = 800khz brightness = 100% leds = 10 series 4 p aralle l 08350-010 figure 10 . switching waveforms (v in = 6 v) 0v 0v 0a v out 100mv/div ac v sw 20v/div i l 500ma/div v in = 21 v , f sw = 800khz brightness = 100% leds = 10 series 4 p aralle l 1s/div 08350-0 1 1 figure 11 . switching waveforms (v in = 21 v) 0v 0v 0a pwm 2v/div fb1 5v/div i fb1 10ma/div v in = 12v brightness = 1.5% leds = 10 series 4 p aralle l 100s/div 08350-012 figure 12 . led current waveforms (brightness = 1.5%) 0v 0v 0v 0v fb1 7v/div fb2 7v/div fb3 7v/div fb4 7v/div v in = 12v brightness = 25% leds = 10 series 4 p aralle l 50s/div 08350-013 figure 13 . led fb x waveforms (brightness = 25%)
data sheet ADD5207 rev. a | page 11 of 16 theory of opera tion current mode, step - up switching regulator operation the ADD5207 uses a current mode pwm boost regulator to generate the minim um voltage needed to drive the led string at the programmed led current. the cu rrent mode regulation system allows a fast transient response while maintaining a stable output voltage. by selecting the proper resistor - capacitor net work from comp to gnd, the regulator response is optimized for a wide range of input voltages, output vo ltages, and load conditions. the ADD5207 can provide a 36 v maxi - mum output voltage and drive up to 1 0 leds (3.4 v/ 25 ma type of leds) for each channel. internal 3.3 v regul ator the ADD5207 contains a 3.3 v linear regulator that is used for biasing internal circuitry . the internal regulator requires a 1 f bypass capacitor. place this bypass capacitor between pin v dd (pin 8 ) and gnd, as close as possible to pin vdd . boost converter swit ching frequency the ADD5207 boost converter switching frequency is user adjustable, between 60 0 khz to 1 mhz, by using an external resistor , r f . a frequency of 600 khz is recommended to op tim - ize the regulator for high efficiency, and a frequency of 1 mhz is recommended to minimize the size of external components. see figure 14 for considerations when selecting a switching frequency and an adjustme nt resistor (r f ). 1000 900 800 700 600 switching frequency (khz) 500 400 300 80 100 120 140 160 180 200 220 r f (k ?) 08350-014 figure 14 . switching frequency vs. r f dimming frequency ( f pwm ) the ADD5207 contains an internal oscillator to generate the pwm dimming signal for led brightness co ntrol. the led dimming frequency ( f pwm ) is fixed at 8 khz internally. current source the ADD5207 contains four current sources to provide accu - rate current sinking for each led string. string - to - string tolera nce is kept within 1.5% at 20 ma. each led string current is adjusted up to 25 ma using an external resistor. the ADD5207 contains an led open fault protection circuit for each channel. if the headroom volta ge of the current source remains below 150 mv while the boost converter output reaches the ovp level, the ADD5207 recognizes that the current source has an open - load fault for the current source, and the curren t source is disabled. i f an application requires three led strings, each led string should be connected using fb1 to fb 3 . the unused fb4 pin should be tied to gnd. the ADD5207 contains hysteresis to prevent th e led current change that is caused by a 0.195% jitter of the pwm i nput. programming the led current as shown in the figure 2 , the ADD5207 has an led current set pin ( iset). a resistor (r set ) from this pin to ground adjusts the led current up to 25 ma. led current level can be set with following equation: ) ( 3600 a r i set led = pwm dimming mode the ADD5207 supports 8 - bit re solution to control brightness. however, e ach current source has a minimum on time require - ment for led current regulation such that the dimming is in the range of 1.5 % to 100% . accordingly , even when the pwm input duty cycle is more than 0% and less than 1.5%, the led duty cycle is held at 1.5%. phase shift pwm dimming there is a phase delay between each current source channel that is programmed by the number of current sources in operation. i f the applica tion requires four separate led strings , each stri ng has a 90 degree phase delay between channels. if three led strings are connected at the fb 1 to fb3 pins (fb4 = gnd), each string has a 120 degree phase delay. safety features the ADD5207 contains several s afety features to provide stable and reliable operation. soft start the ADD5207 contains an internal soft start function to reduce inrush current at startup. the soft start time is typically 1. 1 ms. overvoltag e protection (ovp) the ADD5207 contains ovp circuits to prevent boost converter damage if the output voltage becomes excessive for any reason. to keep a safe output level, the integrated ovp circuit monitors
ADD5207 data sheet rev. a | page 12 of 16 th e output voltage. when the ovp pin voltage reaches the ovp rising threshold, the boost converter stops switching , which causes the output voltage to drop. when the ovp pin voltage drops below the ovp falling threshold, the boot converter begins switching a gain , causing the output to rise. there is about 0.8 v hysteresis between the rising and falling thresholds. the ovp level is fixed at 39 v ( typical ) . open - load protection (olp) the ADD5207 contains a headroom control circuit to minimize power loss at each current source. therefore, the min imum feedback voltage is achieved by regulating the output voltage of the boost converter. if any led string is open circuit during normal operation, the current source head room voltage (v hr ) is pulled to gnd. in this condition, olp is act ivated if v hr is less than 150 mv until the boost converter output voltage rises up to the ovp level. undervoltage lockout (uvlo) an undervoltage lockout circuit is included with built - in hy steresis. the ADD5207 turns on when v in rises above 5 . 0 v (typ ical ) and shuts down when v in falls below 4 .2 v (typ ical ) . thermal protection thermal overload protection prevents excessive power dissipa - tion fro m overheating and damaging the ADD5207 . when the junction temperature (t j ) exceeds 160c, a thermal sensor immediately activates the fault protection, which shuts down the device and allow s it to cool. the devi ce self - starts when the junction temperature (t j ) of the die falls below 130c. external component selection guide inductor selection the inductor is an integral part of the step - up converter. it stores energy during the switch s on time and transfers tha t energy to the output through the output diode during the switch s off time. an inductor in the range of 4.7 h to 22 h is recommended. in general, lower inductance values result in higher saturation current and lower series resistance for a given physic al size. however, lower inductance results in higher peak current, which can lead to reduced efficiency and greater input and/or output ripple and noise. peak - to - peak inductor ripple current at close to 30% of the maximum dc input current typically yields an optimal compromise. the input (v in ) and output (v out ) voltages determine the switch duty cycle (d), which , in turn , is used to determine the inductor ripple current. out in out v v v d ? = use the duty cycle and switching frequency (f sw ) to determine the on time. sw on f d t = the inductor ripple current (i l ) in a steady state is : l t v i in l on = ? solve for the inductance value ( l) : l in i t v l ? = on make sure that the peak inductor current (that is, the maximum input curren t plus half of the inductor ripple current) is less than the rated saturation current of the inductor. in addition, ensure that the maximum rated rms current of the inductor is greater than the maximum dc input current to the regulator. for duty cycles gr eater than 50% that occur with input voltages greater than half the output voltage, slope compensation is required to maintain stability of the current mode regulator. the inherent open - loop stability causes subharmonic instab ility when the duty ratio is g reater than 50%. to avoid subharmonic instability, the slope of the inductor current should be less than half of the compensation slope. inductor manufacturers include : coilcraft, inc. , sumida corporation , and toko. input and output capacitor selection the ADD5207 requires input and output bypass capacitors to supply transient currents while maintaining a constant input and output voltage. use a low effective series resistance (esr) 10 f or greater capacitor for the input capacitor to prevent noise at the ADD5207 input. place the input between vin and gnd, as close as possible to the ADD5207 . cer amic capacitors are preferred because of their low esr characteristics. alternatively, use a high value, medium esr capacitor in parallel with a 0.1 f low esr capacitor as close as possible to the ADD5207 . th e output capacitor maintains the output voltage and supplies current to the load while the ADD5207 switch is on. the value and characteristics of the output capacitor greatly affect the output voltage ripple an d stability of the regulator. use a low esr output capacitor; ceramic dielectric capacitors are preferred. for very low esr capacitors, such as ceramic capacitors, the ripple current due to the capacitance is calculated as follows. because the capacitor d ischarges during the on time (t on ), the charge removed from the capacitor (q c ) is the load current multiplied by the on time. therefore, the output voltage ripple (v out ) is out on l out c out c t i c q v = = ? where: c out is the output capacitance. i l is the avera ge inductor current.
data sheet ADD5207 rev. a | page 13 of 16 using the duty cycle and switching frequency (f sw ), users can determine the on time with the following equation: sw on f d t = the input (v in ) and output (v out ) voltages determine the switch duty cycle (d) with the followi ng equation: out in out v v v d ? = choose the output capacitor based on the following equation: ( ) out out sw in out l out v v f v v i c ? ? capacitor manufacturers include : murata manufacturing co., ltd. , avx , sanyo , and taiyo yuden co., ltd. diode selection the output d iode conducts the inductor current to the output capacitor and loads while the switch is off. for high efficiency, minimize the forward voltage drop of the diode. schottky diodes are recommended. however, for high voltage, high temperature applications, wh ere the schottky diode reverse leakage current becomes significant and degrade s efficiency, use an ultrafast junction diode. the output diode for a boost regulator must be chosen depending on the output voltage and the output current. the diode must be rat ed for a reverse voltage equal to or greater than the output voltage used. the average current rating must be greater than the maximum load current expected, and the peak current rating must be greater than the peak inductor current. using schottky diodes with lower forward voltage drop decreases power dissipation and increases efficiency. the diode must be rated to handle the average output load current. many diode manufacturers derate the current capability of the diode as a function of the duty cycle. ve rify that the output diode is rated to handle the average output load current with the minimum duty cycle. the minimum duty cycle of the ADD5207 is : out in_max out min v v v d ? = where v in_max is the maximum input voltage. for example, d min is 0. 5 when v out is 30 v and v in_max is 15 v. schottky diode manufacturers include on semiconductor, diodes incorporated, central semiconductor corp., and sanyo. loop compensation t he external inductor, output capacitor, and t he compensation resistor and capacitor determine the loop stability. the induc - tor and output capacitor are chosen based on performance, size , and cost. t he compensation resistor ( r c ) and compensation capacitor ( c c ) at comp are selected to optimize contro l loop stability. for typical led application of the ADD5207 , a 6.8 k? compensation resistor in series with a 2.2 nf compensation capacitor at comp is adequate. r c g m c c c2 vout_fb headroom contro l 08350-015 figure 15 . compensation components a step - up converter produces an undesirable right - half plane zero in the regulation feedback loop. capacitor c2 is chosen to cancel the zero introduced by output capacitance esr. solving for c2, c out r c esr c2 = for low esr output capacitance, such as with a ceramic capacitor, c2 is optional. layout guidelines when designing a high frequency, switching, regulated power supply, layout is very important. using a good layout can solve many problems associated with these types of supplies. the main problems are loss of regulation at high output current and/or large input - to - output voltage different ials, excessive noise on the output and switch waveforms, and instability. using the following guidelines help s minimize these problems. make all power (high current) traces as short, direct, and thick as possible. it is good practice on a standard printe d circuit board (pcb) to make the traces an absolute minimum of 15 mil (0.381 mm) per ampere. the inductor, output capacitors, and output diode should be as close to each other as possible. this helps reduce emi radiated by the power traces that carry high switching currents. close proximity of the components also reduces lead inductance and resistance, which in turn reduce noise spikes, ringing, and resistive losses that produce voltage errors.
ADD5207 data sheet rev. a | page 14 of 16 the grounds of the ic, input capacitors, output capacit ors, and output diode (if applicable), should be connected close together, and directly to a ground plane. it is also a good idea to have a ground plane on both sides of the pcb. this reduces noise by reducing ground loop errors and by absorbing more of th e emi radiated by the inductor. for multilayer boards of more than two layers, a ground plane can be used to separate the power plane (power traces and com - ponents) and the signal plane (feedback, compensation, and components) for improved performance. on multilayer boards, the use of vias is required to connect traces and different planes. if a trace needs to conduct a significant amount of current from one plane to the other, it is good practice to use one standard via per 200 ma of current. arrange the components so that the switching current loops curl in the same direction. due to how switching regulators operate, there are two power states: one state when the switch is on, and one when the switch is off. during each state, there is a current loop mad e by the power components currently conducting. place the power components so that the current loop is conducting in the same direction during each of the two states. this prevents magnetic field reversal caused by the traces between the two half cycles an d reduces radiated emi. layout procedure to achieve high efficiency, good regulation, and stability, a good pcb layout is required. it is recommended that the reference board layout be followed as closely as possible because it is already optimized for hi gh efficiency and low noise. use the following general guidelines when designing pcbs: ? keep c in close to the v in and gnd leads of the ADD5207 . ? keep the high current path from c in (through l1) to the sw and gnd leads as short as possible. ? keep the high current path from c in (through l1), d1, and c out as short as possible. ? keep high current traces as short and as wide as possible. ? keep nodes connected to sw away from sensitive traces, such as comp, to preve nt coupling of the traces. if such traces must be run near each other, place a ground trace between the two as a shield. ? place the compensation components as close as possible to the comp pin. ? place the led current setting resistors as close as possible to each pin to prevent noise pickup. ? avoid routing noise - sensitive traces near high current traces and components , especially the led current setting node (i set ) . ? use a thermal pad size that is the same dimension as the exposed pad on the bottom of the p ackage. heat sinking when using a surface - mount power ic or external power switches, the pcb can often be used as the heat sink. this is done by using the copper area of the pcb to transfer heat from the device. users should maximize this area to optimize thermal performance.
data sheet ADD5207 rev. a | page 15 of 16 typical application circuits l1 10h shdn vin off on r f 100k? r set 180k? r c 6.8k? c c 2.2nf c2 open c in 10f v in c bypass 1f vdd pwm iset comp gnd sw d1 ovp fb1 fb2 fb3 fb4 fslct 10 9 2 14 13 4 5 6 7 12 11 3 1 + ? 8 c out 4f ADD5207 c in2 0.1f 08350-016 figure 16 . typical four - string application circuit l1 10h shdn vin off on c in2 0.1f r f 100k? r set 180k? r c 6.8k? c c 2.2nf c 2 open c in 10f c bypass 1f vdd pwm iset comp gnd sw d1 ovp fb1 fb2 fb3 fb4 fslct ADD5207 10 9 2 14 13 4 5 6 7 12 11 3 1 8 c out 4f 08350-017 + ? figure 17 . typical three - string application circuit
ADD5207 data sheet rev. a | page 16 of 16 outline dimensions 052509-a bottom view top view 1 7 8 14 4.00 bsc 3.00 bsc seating plane 0.80 0.75 0.70 0.30 0.25 0.20 0.05 max 0.02 nom 0.15 ref 0.50 bsc coplanarity 0.08 pin 1 indicator for proper connection of the exposed pad, refer to the pin configuration and function descriptions section of this data sheet. exposed pad 0.50 0.40 0.30 1.80 1.70 1.55 3.40 3.30 3.15 0.20 min p i n 1 i n d i c a t o r ( r 0 . 2 0 ) compliant to jedec standards mo-220-wged figure 18. 14-lead lead frame chip scale package [lfcsp_wd] 4 mm 3 mm body, very very thin dual (cp-14-1) dimensions shown in millimeters ordering guide model 1 temperature range package description package option ADD5207acpz-rl ?25c to +85c 14-lead lfcsp_wd cp-14-1 1 z = rohs compliant part. ?2009C2012 analog devices, inc. all rights reserved. trademarks and registered trademarks are the prop erty of their respective owners. d08350-0-2/12(a)

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