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charge pump driver for lcd white led backlights adm8845 rev. b 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 infringements 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 ri ghts of analog devices. trademarks and registered 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? 200 4 C2010 analog devices, inc. all rights reserved. features drives 6 leds from 2.6 v to 5.5 v (li-ion) input supply 1/1.5/2 fractional charge pump to maximize power efficiency 1% max led current matching up to 88% power efficiency over li-ion range powers main and sub display leds with individual shutdown package footprint only 9 mm 2 (3 mm 3 mm) package height only 0.9 mm low power shutdown mode shutdown function soft-start limiting in-rush current applications cellular phones with main and sub displays white led backlighting camera flash/strobes and movie lights micro tft color displays dsc pdas general description the adm8845 uses charge pump technology to provide the power required to drive up to six leds. the leds are used for backlighting a color lcd display, having regulated constant current for uniform brightness intensity. the main display can have up to four leds, and the sub display can have one or two leds. the digital ctrl1 and ctrl2 input control pins control the shutdown operation and the brightness of the main and sub displays. to maximize power efficiency, the charge pump can operate in a 1, 1.5, or 2 mode. the charge pump automatically switches between 1/1.5/2 modes, based on the input voltage, to maintain sufficient drive for the led anodes at the highest power efficiency. improved brightness matching of the leds is achieved by a feedback pin to sense individual led current with a maximum matching accuracy of 1%. functional block diagram current control 1 current control 2 current control 3 current control 4 current control 5 current control 6 control logic v ref charge pump 1 /1.5 /2 mode osc led current control circuit fb1 current controlled sinks fb2 fb3 fb4 fb5 fb6 v out c4 4.7 f v cc c1 1 f ctrl1 ctrl2 c2 1 f r set i set gnd c3 2.2 f adm8845 04867-0-001 main sub figure 1.
adm8845 rev. b | page 2 of 20 table of contents features .............................................................................................. 1 applications ....................................................................................... 1 general description ......................................................................... 1 functional block diagram .............................................................. 1 specifications ..................................................................................... 3 absolute maximum ratings ............................................................ 4 thermal characteristics .............................................................. 4 esd caution .................................................................................. 4 pin configuration and function descriptions ............................. 5 typical performance characteristics ............................................. 6 theory of operation ...................................................................... 10 output current capability ........................................................ 11 automatic gain control ............................................................ 11 current matching ....................................................................... 11 brightness control with a digital pwm signal ..................... 11 led brightness control using a pwm signal applied to v pwm .......................................................................... 13 led brightness control using a dc voltage applied to v bright ....................................................................... 13 applications ..................................................................................... 14 layout considerations and noise ............................................ 14 white led shorting .................................................................. 14 driving fewer than six leds ................................................... 14 driving flash leds .................................................................... 15 driving camera light, main, and sub leds.......................... 15 driving four backlight white leds and flash leds ........... 16 power efficiency ......................................................................... 17 outline dimensions ....................................................................... 18 ordering guide .......................................................................... 18 revision history 4/10rev. a to rev. b changes to figure 2 ........................................................................... 5 changes to figure 4 ........................................................................... 6 changes to figure 16 to figure 19 ................................................... 8 changes to brightness control with a digital pwm signal section ................................................................................... 11 added exposed pad notation to outline dimensions .............. 18 changes to ordering guide ........................................................... 18 7/05rev. 0 to rev. a changes to table 3 ............................................................................. 5 changes to table 7 ........................................................................... 12 updated outline dimensions ........................................................ 18 changes to ordering guide ........................................................... 18 10/04revision 0: initial version
adm8845 rev. b | page 3 of 20 specifications v cc = 2.6 v to 5.5 v; t a = ?40c to +85 c, unless otherwise noted; c1, c2 = 1.0 f; c3 = 2.2 f; c4 = 4.7 f. table 1. parameter min typ max unit test conditions input voltage, v cc 2.6 5.5 v supply current, i cc 2.6 5 ma all six leds disabled, v cc = 3.3 v, r set = 7.08 k ctrl1 = 1, crtl2 = 1 shutdown current 5 a charge pump frequency 1.5 mhz charge pump mode thresholds 1.5 to 2 3.33 v accuracy 4 % 2 to 1.5 3.36 v accuracy 4 % hysteresis 40 mv 1 to 1.5 4.77 v accuracy 4 % 1.5 to 1 4.81 v accuracy 4 % hysteresis 40 mv i set pin led: led matching ?1 +1 % i led = 20 ma, v fb =0.4 v led: i set accuracy ?1 +1 % i led = 20 ma, r set = 7.08 k, v fb = 0.4 v, v cc = 3.6 v, t a = 25c i set pin voltage 1.18 v i led to i set ratio 120 min compliance on fbx pin 0.2 0.3 v i set = 20 ma charge pump output resistance 1.2 1.8 1 mode 3.5 5.1 1.5 mode 8.0 14 2 mode led current 30 ma guaranteed by design. not 100% production tested. see figure 21 . pwm 0.1 200 khz digital inputs input high 0.5 v cc v input low 0.3 v cc v input leakage current 1 a charge pump power efficiency 88 % ctrl1 = 1, crtl2 = 1, v cc = 3.4 v, v fb = 0.2 v, i fb = 20 ma v out ripple 30 m v v cc = 3.6 v, i led = 20 ma, all six leds enabled
adm8845 rev. b | page 4 of 20 absolute maximum ratings t a = 25c, unless otherwise noted. table 2. parameter rating supply voltage, v cc C0.3 v to +6.0 v i set C0.3 v to +2.0 v ctrl1, ctrl2 C0.3 v to +6.0 v v out shorted 1 indefinite feedback pins fb1 to fb6 C0.3 v to +6.0 v operating temperature range six leds enabled with 30 ma/led 2 C40c to +65c six leds enabled with 20 ma/led 2 C40c to +85c v out 3 180 ma storage temperature range C65c to +125c power dissipation 2 mw esd class 1 1 short through led. 2 led current should be derated above t a > 65c, refer to figure 21. 3 based on long-term c urrent density limitations. stresses above those 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 characteristics 16-lead lfcsp package: ja = 50c/w esd caution esd (electrostatic discharge) sensitive device. electros tatic charges as high as 4000 v readily accumulate on the human body and test equipment and can discharge wi thout detection. although this product features proprietary esd protection circuitry, permanent dama ge may occur on devices subjected to high energy electrostatic discharges. therefore, proper esd precautions are recommended to avoid performance degradation or loss of functionality.
adm8845 rev. b | page 5 of 20 pin configuration and function descriptions pin 1 indicator 1v out 2 c2+ 3i set 4 fb1 11 c2? 12 ctrl2 10 gnd 9fb6 notes 1. connect exposed paddle to gnd. 5 f b 2 6 f b 3 7 f b 4 8 f b 5 1 5 v c c 1 6 c 1 + 1 4 c 1 ? 1 3 c t r l 1 top view (not to scale) adm8845 04867-0-003 figure 2. pin configuration table 3. pin function descriptions pin no. mnemonic function 1 v out charge pump output. a 2.2 f capacitor to gr ound is required on this pin. connect v out to the anodes of all the leds. 2 c2+ flying capacitor 2 positive connection. 3 i set bias current set input. the current, i set , flowing through the resistor, r set , is gained up by 120 to give the i led current. connect r set to gnd to set the bias current as v set /r set . note that v set = 1.18 v. 4 to 9 fb1 to fb6 led1 to led6 cathode connection and charge pump feedback. the current, i set , flowing in these leds is 120 times the current flowing through resistor, r set . when using fewer than six leds, this pin can be left unconnected or connected to gnd. 10 gnd device ground pin. 11 c2? flying capacitor 2 negative connection. 12 ctrl2 digital input. 3 v cmos logic. used with ctrl1 to control the shutdown operation of the main and sub leds. 13 ctrl1 digital input. 3 v cmos logic. used with ctrl2 to control the shutdown operation of the main and sub leds. 14 c1? flying capacitor 1 negative connection. 15 v cc positive supply voltage input. connect this pin to a 2.6 v to 5.5 v supply with a 4.7 f decoupling capacitor. 16 c1+ flying capacitor 1 positive connection. - ep exposed paddle. connect the exposed paddle to gnd.
adm8845 rev. b | page 6 of 20 typical performance characteristics 5 10 15 20 25 30 35 10.75 04867-0-004 r set (k ) led current (ma) 4.75 6.75 12.75 14.75 8.75 figure 3. i led (ma) current vs. r set 20.00 20.10 20.15 20.20 20.25 20.30 20.35 4.1 04867-0-005 supply voltage (v) led current (ma) 2.6 3.1 4.6 5.6 3.6 5.1 ?40 c 25 c 85 c 20.05 figure 4. i led (ma) vs. temperature (c), six leds enabled ?0.3 ?0.2 ?0.1 0 0.1 0.2 0.3 25 04867-0-006 temperature ( c) % error ?40 ?20 45 85 0 65 figure 5. i led matching (%) over temperature (c), v cc = 3.6 v, i led = 20 ma, six leds enabled ?0.4 ?0.2 ?0.1 0 0.1 0.2 0.4 3.8 04867-0-007 supply voltage (v) matching error (%) 2.6 4.2 5.0 3.4 4.6 ?0.3 0.3 3.0 5.4 max negative matching error max positive matching error figure 6. i led (ma) matching error (%) vs. supply voltage (v), t a = 25c and i led = 20 ma 04867-0-008 temperature ( c) led current (ma) ?40 0 80 40 20.08 20.12 20.14 20.16 20.18 20.20 20.24 20.10 20.22 figure 7. i led (ma) variation over temperature (c), v cc = 3.6 v 5 10 15 20 25 30 35 3.8 04867-0-009 supply voltage (v) led current (ma) 2.6 3.0 4.6 5.4 3.4 5.0 4.2 figure 8. i led (ma) vs. supply voltage (v)
adm8845 rev. b | page 7 of 20 0 4 8 12 16 20 04867-0-010 duty cycle (%) led current (ma) 0 20 60 100 80 40 figure 9. i led (ma) vs. pwm dimming (varying duty cycle), six leds enabled, frequency = 1 khz 0 50 100 150 250 300 04867-0-011 supply voltage (v) supply current i cc (ma) 2.6 3.0 3.8 4.6 4.2 3.4 20ma/led 15ma/led 200 5.0 5.4 figure 10. input current vs. supply voltage, six leds enabled 04867-0-012 ch2 20.0mv ch1 20.0mv m 400ns ch1 220mv 2 1 b w b w v out v cc figure 11. 1.5 mode operating waveforms 60 65 70 80 90 95 04867-0-013 duty cycle (%) efficiency (%) 04 0 6 0 8 0 70 50 85 90 100 10 20 30 75 figure 12. led efficiency vs. varying duty cycle of 1 khz pwm signal, six leds enabled, 20 ma/led 04867-0-014 ch2 160ma ch1 2.00v m 5.00 s ch2 180mv 2 1 b w b w current 3 v out ctrl1/2 ch3 1.00v figure 13. soft start showing the initial in-rush current and v out variation, six leds @ 20 ma/led, v cc = 3.6 v 04867-0-015 ch2 20.0mv ch1 20.0mv m 400ns ch1 220mv 2 1 b w b w v out v cc figure 14 .2 mode operating waveform
adm8845 rev. b | page 8 of 20 04867-0-016 ch2 20.0mv ch1 20.0mv m 400ns ch1 220mv 2 1 b w b w v out v cc figure 15. 1 mode operating waveforms 40 45 50 60 80 90 04867-0-017 v cc (v) power efficiency (%) 70 55 65 75 85 v f = 4.0v v f = 3.8v v f = 4.3v v f = 3.2v v f = 3.6v 2.8 3.0 3.4 3.8 3.6 3.2 4.0 4.2 2.9 3.1 3.3 3.5 3.7 3.9 4.1 figure 16. power efficiency vs. supply voltage over li-ion range, six leds @ 15 ma/led 40 45 50 60 80 90 04867-0-018 v cc (v) power efficien c y (%) 70 55 65 75 85 v f = 4.0v v f = 3.8v v f = 4.3v v f = 3.2v v f = 3.6v 2.8 3.0 3.4 3.8 3.6 3.2 4.0 4.2 2.9 3.1 3.3 3.5 3.7 3.9 4.1 figure 17. power efficiency vs. supply voltage over li-ion range, four leds @ 15 ma/led 40 45 50 60 80 90 04867-0-019 v cc (v) power efficien c y (%) 70 55 65 75 85 v f = 4.0v v f = 3.6v v f = 4.3v v f = 3.2v v f = 3.8v 2.8 3.0 3.4 3.8 3.6 3.2 4.0 4.2 2.9 3.1 3.3 3.5 3.7 3.9 4.1 figure 18. power efficiency vs. supply voltage over li-ion range, six leds @ 20 ma/led 40 45 50 60 80 90 04867-0-020 v cc (v) power efficien c y (%) 2.8 3.0 3.4 3.8 3.6 3.2 70 4.0 4.2 55 65 75 85 2.9 3.1 3.3 3.5 3.7 3.9 4.1 v f = 4.0v v f = 4.3v v f = 3.8v v f = 3.2v v f = 3.6v figure 19. power efficiency vs. supply voltage over li-ion range four leds @ 20 ma/led 04867-0-021 ch1 2.00v ch2 2.00v m10.0ms ch2 4.36v 2 1 v out ctrl1/2 : 44.0ms @: ?44.4ms c2 fall 200 s low signal amplitude figure 20. tpc delay
adm8845 rev. b | page 9 of 20 04867-0-022 3 0m a 2 0m a 65c 85c figure 21. maximum i led (ma) vs. ambient temperature, six leds connected
adm8845 rev. b | page 10 of 20 theory of operation the adm8845 charge pump driver for lcd white led back- lights implements a multiple gain charge pump (1, 1.5, 2) to maintain the correct voltage on the anodes of the leds over a 2.6 v to 5.5 v (li-ion) input supply voltage. the charge pump automatically switches between 1/1.5/2 modes, based on the input voltage, to maintain sufficient drive for the led anodes, with v cc input voltages as low as 2.6 v. it also includes regula- tion of the charge pump output voltage for supply voltages up to 5.5 v. the six leds of the adm8845 are arranged into two groups, main and sub. the main display can have up to four leds, fb1 to fb4, and the sub display can have one or two leds, fb5 and fb6 (see figure 23 ). two digital input control pins, ctrl1 and ctrl2, control the shutdown operation and the brightness of the main and sub displays (see table 4 ). table 4. shutdown truth table ctrl1 ctrl2 led shutdown operation 0 0 sub display off/main display off 0 1 sub display off/main display on 1 0 sub display on/main display off 1 1 sub display on/main display on an external resistor, r set , is connected between the i set pin and gnd. this resistor sets up a reference current, i set , which is internally gained up by 120 within the adm8845 to produce the i led currents of up to 30 ma/led (i led = i set 120 and i set = 1.18 v/r set ). the adm8845 uses six individual current sinks to individually sense each led current with a maximum matching performance of 1%. this current matching perform- ance ensures uniform brightness across a color display. the adm8845 lets the user control the brightness of the white leds with a digital pwm signal applied to ctrl1 and/or ctrl2. the duty cycle of the applied pwm signal determines the brightness of the main and/or sub display backlight white leds. the adm8845 also allows the brightness of the white leds to be controlled using a dc voltage (see figure 22 ). soft- start circuitry limits the in-rush current flow at power-up. the adm8845 is fabricated using cmos technology for minimal power consumption and is packaged in a 16-lead lead frame chip scale package. i set r set = 13.4k r = 15k v bright 0v?2.5v adm8845 04867-0-027 figure 22. pwm brightness control using a dc voltage applied to v bright current control 1 current control 2 current control 3 current control 4 current control 5 current control 6 control logic v ref charge pump 1 /1.5 /2 mode osc led current control circuit fb1 current controlled sinks fb2 fb3 fb4 fb5 fb6 v out c4 4.7 f v cc c1 1 f ctrl1 ctrl2 c2 1 f r set i set gnd c3 2.2 f adm8845 04867-0-001 main sub figure 23. functional block diagram
adm8845 rev. b | page 11 of 20 output current capability the adm8845 can drive up to 30 ma of current to each of the six leds given an input voltage of 2.6 v to 5.5 v. the led currents have a maximum current matching of 1% between any two led currents. an external resistor, r set , sets the output current, approximated by the following equation: r set = 120 (1.18 v/ i led ) to regulate the led currents properly, sufficient headroom voltage (compliance) must be present. the compliance refers to the minimum amount of voltage that must be present across the internal current sinks to ensure that the desired current and matching performance can be realized. to ensure that the desired current is obtained, use the following equation to find the minimum input voltage required: v out ? v f compliance where v f is the led forward voltage. for 20 ma/led, the compliance is 0.20 v typ and 0.30 v max (see table 5 ). table 5. i led , r set , and compliance table i led r set typ compliance 15 ma 9.44 k 0.17 v 20 ma 7.08 k 0.20 v 30 ma 4.72 k 0.34 v when the adm8845 charge pump is loaded with 180 ma (six leds at 30 ma/led), the ambient operating temperature is reduced (see figure 21 ). automatic gain control the automatic gain control block controls the operation of the charge pump by selecting the appropriate gain for the charge pump. this maintains sufficient drive for the led anodes at the highest power efficiency over a 2.6 v to 5.5 v input supply range. the charge pump switching thresholds are described in table 6 . table 6. charge pump switching thresholds gain threshold 1.5 to 2 3.33 v 2 to 1.5 3.36 v 1 to 1.5 4.77 v 1.5 to 1 4.81 v current matching the 1% maximum current matching performance is defined by the following equations: i av g = ( i max + i min )/2 max matching error = [( i max ? i av g )/ i av g ] 100 or min matching error = [( i min ? i av g )/ i av g ] 100 where i max is the largest i led current, and i min is the smallest i led current. brightness control with a digital pwm signal pwm brightness control provides the widest brightness control method by pulsing the white leds on and off using the digital input control pins, ctrl1 and/or ctrl2. pwm brightness control also removes any chromaticity shifts associated with changing the white led current, because the leds operate either at zero current or full current (set by r set ). the digital pwm signal applied with a frequency of 100 hz to 200 khz turns the current control sinks on and off using ctrl1 and/or ctrl2. the average current through the leds changes with the pwm signal duty cycle. if the pwm frequency is much less than 100 hz, flicker could be seen in the leds. for the adm8845, zero duty cycle turns off the leds, and a 50% duty cycle results in an average led current i led being half the pro- grammed led current. for example, if r set is set to program 20 ma/led, a 50% duty cycle results in an average i led of 10 ma/led, i led being half the programmed led current. fb1 fb2 fb3 fb4 fb5 fb6 v out c1 1 f ctrl1 ctrl2 c2 1 f r set i set c3 2.2 f adm8845 pwm input or high/low pwm input or high/low 04867-0-024 figure 24. digital pwm brightne ss control application diagram by applying a digital pwm signal to the digital input control pins, ctrl1 and/or ctrl2 can adjust the brightness of the sub and/or main displays. the six white leds of the adm8845 are organized into two groups: main display, fb1 to fb4, and sub display, fb5 and fb6. for more information, refer to the theory of operation section.
adm8845 rev. b | page 12 of 20 the main and sub display brightness of the adm8845 can be controlled together or separately by applying a digital pwm signal to both ctrl1 and ctrl2 pins. the duty cycle of the applied digital pwm signal determines the brightness of the main and sub displays together. varying the duty cycle of the applied pwm signal also varies the brightness of the main and sub displays from 0% to 100%. by holding ctrl1 low and applying a digital pwm signal to ctrl2, the sub display is turned off and the main display is turned on. the brightness of the main display is then determined by the duty cycle of the applied digital pwm signal. by applying a digital pwm signal to ctrl1 and holding ctrl2 low, the sub display is turned on and the main display is turned off. then the brightness of the sub display is determined by the duty cycle of the applied digital pwm signal. by applying a digital pwm signal to ctrl1 and holding ctrl2 high, the sub display is turned on and the main display is turned on. then the brightness of the sub display is determined by the duty cycle of the applied digital pwm signal. the brightness of the main display is set to the maximum, which is set by r set . by holding ctrl1 high and applying a digital pwm signal to ctrl2, the sub display is turned on and the main display is turned on. then the brightness of the main display is determined by the duty cycle of the applied digital pwm signal. the bright- ness of the sub display is set to the maximum, which is set by r set . when ctrl1 and ctrl2 go low, the led current control sinks shutdown. shutdown of the charge pump is delayed by 15 ms. this timeout period (t cp ) allows the adm8845 to determine if a digital pwm signal is present on ctrl1 and ctrl2 or if the user has selected a full chip shutdown (see figure 25 ). if digital pwm brightness control of the leds is not required, a constant logic level 1 (v cc ) or 0 (gnd) must be applied. the six white leds in the adm8845 are arranged in two groups, sub and main. it is possible to configure the six leds as in table 7 . for more information, also refer to figure 25 . table 7. digital inputs truth table ctrl1 ctrl2 led operation 0 0 sub display off/main display off (full shutdown) 1 , 2 0 1 sub display off/main display on 1 , 3 1 0 sub display on/main display off 4 , 2 1 1 sub display on/main display on (full on) 4 , 3 0 pwm sub display off/digital pwm brightness control on main display 1 , 5 pwm 0 digital pwm brightness control on sub display/main display off 5 , 2 1 pwm sub display on/ digital pwm brightness control on main display 4 , 5 pwm 1 digital pwm brightness control on sub display/main display on 5 pwm pwm digital pwm brightness control on sub and main display 5 1 sub display off means the sub display leds only is off. ctrl1 = 0 means a constant logic level (gnd) is applied to ctrl1. 2 main display off means the main display only is off. ctrl2 = 0 means a constant logic level (gnd) is applied to ctrl2. 3 main display on means the display is on with the maximum brightness set by the r set resistor. ctrl2 = 1 means a constant logic level (v cc ) is applied to ctrl2. 4 sub display on means the display is on with the maximum brightness set by the r set resistor. ctrl1 = 1 means a constant logic level (v cc ) is applied to ctrl1. 5 pwm means a digital pwm signal is applied to the ctrl1 and/or the ctrl2 pin with a frequency from 100 hz to 200 khz.
adm8845 rev. b | page 13 of 20 led config. full on main and sub off ctrl1 ctrl2 v out i led (sub) i led (main) sub display brightness main display brightness 37ms > t cp > 15ms shdn shdn 50% 50% 100% 100% 80% t cp 04867-0-025 sub and main 50% duty cycle main 80% duty cycle, sub off figure 25. application timing led brightness control using a pwm signal applied to v pwm adding two external resistors and a capacitor, as shown in figure 26 , can also be used to control pwm brightness. this pwm brightness control method can be used instead of ctrl1 and/or ctrl2 digital pwm brightness control. with this con- figuration, the ctrl1 and ctrl2 digital logic pins can be used to control shutdown of the white leds, while v pwm can be used to control the brightness of all the white leds by applying a high frequency pwm signal (amplitude 0 v to 2.5 v) to drive an r-c-r filter on the i set pin of the adm8845. a 0% pwm duty cycle corresponds to 20 ma/led, while a 100% pwm duty cycle corresponds to a 0 ma/led. at pwm frequencies above 5 khz, c5 may be reduced (see figure 26 ). to have 20 ma flowing in each led, the amplitude of the pwm signal must be 0 v and 2.5 v only. 100 ) 1( 120 2 2 _ cycleduty rr rr voltage i i set set set led ? + = i set r set = 13.4k r = 7.5k v pwm 0v?2.5v adm8845 04867-0-026 100% = i led = 0ma 0% = i led = 20ma r = 7.5k c5 = 1 f figure 26. pwm brightness contro l using filtered pwm signal led brightness control using a dc voltage applied to v bright by adding one resistor, as in figure 22 , this configuration can control the brightness of the white leds using a dc voltage applied to the v bright node. figure 27 shows an application example of led brightness control using a dc voltage with a amplitude of 0 v to 2.5 v, applied to v bright . the equation for i led is i set = [(1/ r set + 1/ r )(v set )] C [(1/ r )(v bright )] i led = 120 i set where r = 15 k and v set = voltage at i set pin (1.18 v). v bright 04867-0-028 i led 2.5v 0ma 1.6v 7.2ma 0.8v 13.6ma 0v 20ma figure 27. pwm brightness contro l application diagram using a dc voltage applied to v bright
adm8845 rev. b | page 14 of 20 applications layout considerations and noise because of the s switching behavior of the adm8845, pcb trace layout is an important consideration. to ensure optimum performance, a ground plane should be used, and all capacitors (c1, c2, c3, c4) must be located with minimal track lengths to the pins of the adm8845. white led shorting if an led is shorted, the adm8845 continues to drive the remaining leds with i led per led (i led = i set 120 ma). this is because the adm8845 uses six internal currents sinks to produce the led current. if an led is shorted, the adm8845 continues to sink (i set 120 ma) as programmed by r set through the shorted led. driving fewer than six leds the adm8845 can be operated with fewer than six leds in parallel by simply leaving the unused fbx pins floating or connected to gnd. for example, figure 28 shows five leds being powered by the adm8845, and figure 29 shows three main leds and one sub led. v out fb1 fb2 fb3 fb4 fb5 fb6 v cc 2.6v?5.5v ctrl1 ctrl2 gnd adm8845 04867-0-029 lcd figure 28. driving five white leds v out fb1 fb2 fb3 fb4 fb5 fb6 v cc 2.6v?5.5v ctrl1 ctrl2 gnd adm8845 r set i set 04867-0-030 main display sub display figure 29. driving three main leds and one sub led v out fb1 fb2 fb3 fb4 fb5 fb6 v cc 2.6v?5.5v ctrl1 ctrl2 gnd adm8845 r set i set 04867-0-002 main display sub display figure 30. typical application diagram
adm8845 rev. b | page 15 of 20 driving flash leds the adm8845 can be operated with any two fbx pins used in parallel to double the combined led current supplied by the adm8845. for example, if three flash leds need to be driven with 60 ma/led, the adm8845 can be configured as in figure 31 (see also figure 21 ). v out fb1 fb2 fb3 fb4 fb5 fb6 v cc 2.6v?5.5v ctrl1 ctrl2 gnd adm8845 r set 4 .27k 04867-0-031 60ma 60ma 60ma figure 31. driving three flash leds driving camera light, main, and sub leds the adm8845 can be configured to power a camera light that is composed of four white leds in parallel, along with the main and sub display bundled into one package. fb1 to fb4 now power the camera light, and fb5 and fb6 power the main display. the sub display led is powered from the adm8845 by using an external current mirror to control the current flowing through the sub white led (see figure 32 ) . all white leds have 15 ma/led. total load on the adm8845 charge pump is therefore 105 ma, and the maximum load on the adm8845 charge pump is 180 ma (see figure 21 ). current control 1 current control 2 current control 3 current control 4 current control 5 current control 6 control logic v ref charge pump 1 /1.5 /2 mode osc led current control circuit fb1 current controlled sinks fb2 fb3 fb4 fb5 fb6 v out c4 4.7 f vcc c1 1 f ctrl1 ctrl2 c2 1 f r set 9.44k i set gnd c3 2.2 f adm8845 04867-0-032 15ma/led 15ma/led 15ma/led current control7 r camera main sub figure 32. driving camera light, two main leds, and one sub led
adm8845 rev. b | page 16 of 20 driving four backlight white leds and flash leds the adm8845 can be configured to power four backlight white leds and a camera flash, bundled into one package. fb1 to fb4 power the backlight light, fb5 and fb6 power two of the flash leds, while the third is powered by an external current mirror to control the current flowing through the third flash led (see figure 33 ). all the backlight white leds have 15 ma/led, and the flash current is 20 ma/led. the total load on the adm8845 charge pump is 120 ma; the maximum load on the adm8845 charge pump is 180 ma (see figure 21 ). ctrl1 controls the flash on/off, and ctrl2 controls the back- light on/off and brightness control. because the r set resistor sets the current that each of the six current control blocks can sink, a pwm signal is used to change the current in the backlight from 20 ma to 5 ma/led. the ctrl2 duty cycle is 15/20 to give 15 ma/backlight led. 04867-0-034 current control 1 current control 2 current control 3 current control 4 current control 5 current control 6 control logic v ref charge pump 1/1.5/2 mode osc led current control circuit fb1 current controlled sinks fb2 fb3 fb4 fb5 fb6 v out c4 4.7 f v cc ctrl1 ctrl2 c2 1 f c1 1 f r set 7.32k i set gnd c3 2.2 f adm8845 15ma/led 20ma/led current control 7 r back- light flash figure 33. driving four backlight leds and flash led
adm8845 rev. b | page 17 of 20 power efficiency the adm8845 power efficiency ( ) equations are = p out / p in p in = (( v cc i load gain ) + (i q v cc )) p out = 6 ( v f i led ) where: i q is the quiescent current of the adm8845, 2.6 ma. v f is the led forward voltage. gain is equal to the charge pump mode (1 , 1.5 , 2 ). fb1 fb2 fb3 fb4 fb5 fb6 v out ctrl1 ctrl2 r set i set adm8845 04867-0-033 v cc 3.4v v cc gnd i load i in figure 34. charge pump power efficiency diagram examples 1 and 2 show calculations of the adm8845 power efficiency. see figure 34 as well. example 1 the adm8845 driving six white led with 20 ma/led at v cc = 3.4 v (1.5 mode), led v f = 4.5 v. p in = (( v cc i load gain ) + (v cc i q )) p in = ((3.4 120 ma 1.5) + (3.4 2.6 ma)) p in = ((0.612) + (0.00884)) p in = 0.62084 p out = 6 (v f i led ) p out = 6 (4.5v 20 ma) p out = 0.54 = p out / p in = 0.54/0.62084 = 87% example 2 the adm8845 driving six white led with 20 ma/led at v cc = 3.4 (1.5 mode), led v f = 3.6 v. p in = (( v cc i load g ain ) + (v cc i q )) p in = ((3.4 120 ma 1.5) + (3.4 2.6 ma)) p in = ((0.612) + (0.00884)) p in = 0.62084 p out = 6 ( v f i led ) p out = 6 (3.6 v 20 ma) p out = 0.432 = p out / p in = 0.432/0.62084 = 70%
adm8845 rev. b | page 18 of 20 outline dimensions 1 0.50 bsc 0.60 max pin 1 indicator 1.50 ref 0.50 0.40 0.30 0.25 min 0.45 2.75 bsc sq top view 12 max 0.80 max 0.65 typ seating plane pin 1 indicator 1.00 0.85 0.80 0.30 0.23 0.18 0.05 max 0.02 nom 0.20 ref 3.00 bsc sq * 1.45 1.30 sq 1.15 exposed pad 16 5 13 8 9 12 4 (bottom view) * compliant to jedec standards mo-220-veed-2 except for exposed pad dimension. 072208-a for proper connection of the exposed pad, refer to the pin configuration and function descriptions section of this data sheet. figure 35. 16-lead lead frame chip scale package [lfcsp_vq] 3 mm x 3 mm body, very thin quad (cp-16-2) dimensions shown in millimeters ordering guide model 1 temperature range package description package option branding ADM8845ACP-REEL ? 40c to + 85c 16-lead lfcsp_vq cp-16-2 m0p adm8845acpz-reel ? 40c to + 85c 16-lead lfcsp_vq cp-16-2 m5g adm8845acpz-reel7 ? 40c to + 85c 16-lead lfcsp_vq cp-16-2 m5g adm8845eb-evalz evaluation board 1 z = rohs compliant part.
adm8845 rev. b | page 19 of 20 notes
adm8845 rev. b | page 20 of 20 notes ?200 4 C2010 analog devices, inc. all rights reserved. trademarks and registered tra demarks are the prop erty of their respective owners. d04867-0-4/10(b)

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