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aat2847 four-channel backlight driver with dual ldos 2847.2007.09.1.0 1 chargepump ? general description the aat2847 is a highly integrated power solution for single cell li-ion/polymer based liquid crystal display (lcd) display applications. it includes a four channel light emitting diode (led) backlight driver and two integrated 200ma low dropout volt- age regulators (ldos) as additional power sup- plies for display and camera related chipsets. the backlight driver in the aat2847 is a low noise tri-mode dc/dc charge pump converter. each of the four channels of the backlight driver is capable of delivering up to 20ma of bias currents for white leds. the white led (wled) backlight bias cur- rent matching is 1% which helps provide uniform display brightness. analogictechs as 2 cwire? (advanced simple serial control?) serial digital interface is used to enable, disable, and set the current for each back- light led channel. each led channel has sixteen available current level settings in three separate current scales, plus four available current level set- tings on a low level current scale. each led channel is equipped with built-in short circuit protection and auto disable functionality. a low shutdown current feature disconnects the load from the input and reduces quiescent current to less than 1a. the aat2847 is available in the thermally enhanced 20-pin 3x4x0.75mm tqfn package. features ? input supply voltage range: 2.7v to 5.5v ? tri-mode (1x/1.5x/2x) charge pump: delivers up to 120ma of output current ? integrated lcd display solution: four-channel wled backlight ? user-programmable wled current scales: 30ma, 20ma and 15ma. sixteen programmable current level settings ? user selectable low level current scale four programmable current level settings dual 200ma ldos (w/separate enables) ? single-wire as 2 cwire serial interface for configuration/control four addressable registers fast, 1mhz serial interface ? > 90% peak efficiency ? ldo output voltages: aat2847-ee: user-programmable aat2847-qg: 2.8v and 1.5v aat2847-qi: 2.8v and 1.8v ? over-temperature protection ? available in 3x4x0.75mm TQFN34-20 package ? -40c to +85c temperature range applications ? camera function power supplies ? camera phone displays ? lcd modules ? white led backlighting typical application c1 1f c2 1f c1- c1+ c2- c2+ in out d1 d2 d3 d4 200ma ldoa v bat 3.6v c in 2.2f aat 2847-ee en/set as 2 cwire backlight control c out 1f c ldoa 2. 2f 200ma ldob c ldob 2. 2f in fba fbb wleds osram lw m673 or equivalent gnd pgnd ena ldo a enable enb ldo b enable c1 1f c2 1f c1- c1+ c2- c2+ in out d1 d2 d3 d4 v outa 2. 8v, 200ma ldoa v bat 3.6v c in 2.2f aat 2847-qi gnd en/set as 2 cwire backlight control c out 1f c ldoa 2.2f ldob c ldob 2. 2f ena ldo a enable in wleds osram lw m673 or equivalent v outb 1.8v, 200ma enb ldo b enable pgnd
aat2847 four-channel backlight driver with dual ldos 2 2847.2007.09.1.0 pin descriptions pin # symbol function aat2847-ee aat2847-qg/qi 1 d2 d2 led2 current channel input. connect to the cathode of backlight led 2. if not used, connect d2 to the out pin. 2 d1 d1 led1 current channel input. connect to the cathode of backlight led 1. if not used, connect d1 to the out pin. 3 ldob ldob output of ldob. 4, 6 in in input voltage supply connection. 5 pgnd pgnd power ground. 7 fba nc aat2847-ee: feedback pin of ldoa. internally regulated at 1.2v. aat2847-qg/qi: no connection. do not make any connection to this pin. 8 ldoa ldoa output of ldoa. 9 c2- c2- negative terminal of flying capacitor 2. 10 c2+ c2+ positive terminal of flying capacitor 2. 11 out out charge pump output to drive load circuit. connect a 1f or larger ceramic capacitor between out and pgnd. 12 c1- c1- negative terminal of flying capacitor 1. 13 c1+ c1+ positive terminal of flying capacitor 1. 14 en/set en/set as 2 cwire control pin for backlight led current profile selection and control. 15 enb enb enable pin for ldob. active logic high. 16 d4 d4 led4 current channel input. connect to the cathode of backlight led 4. if not used, connect d4 to the out pin. 17 d3 d3 led3 current channel input. connect to the cathode of backlight led 3. if not used, connect d3 to the out pin. 18 gnd gnd ground. 19 ena ena enable pin for ldoa. active logic high. 20 fbb n/c aat2847-ee: feedback pin of ldob. internally regulated at 1.2v. aat2847-qg/qi: no connection. do not make any connection to this pin. ep exposed pad (bottom).
aat2847 four-channel backlight driver with dual ldos 2847.2007.09.1.0 3 1. stresses above those listed in absolute maximum ratings may cause permanent damage to the device. functional operation at co nditions other than the operating conditions specified in not implied. only one absolute maximum rating should be applied at any one tim e. 2. derate 20mw/c above 40c ambient temperature. 3. mounted on a fr4 circuit board. pin configuration aat2847-ee aat2847-qg/qi TQFN34-20 TQFN34-20 (top view) (top view) absolute maximum ratings 1 t a = 25c, unless otherwise noted. pin descriptions below apply to aat2847-ee (aat2847-qg/qi) thermal information 2, 3 1 3 2 5 4 6 16 14 15 12 13 11 9 10 7 8 18 17 20 19 d2 d1 ldob in pgnd in fba ldo a c2- c2+ c1+ en/set c1- out enb d4 d3 gnd ena fbb 1 3 2 5 4 6 16 14 15 12 13 11 9 10 7 8 18 17 20 19 d2 d1 ldob in pgnd in n/c ldo a c2- c2+ c1+ en/set c1- out enb d4 d3 gnd ena n/c symbol description value units ja thermal resistance 50 c/w p d maximum power dissipation 2 w symbol description value units v n [in, out, d1, d2, d3, d4] to gnd -0.3 to 6.0 v v n [c1+, c1-, c2+, c2-] to gnd -0.3 to v out + 0.3 v v n [ldoa, ldob, fba, fbb, en/set, ena, enb] to gnd -0.3 to v in + 0.3 v t j operating temperature range -40 to 150 c t lead maximum soldering temperature (at leads, 10 sec) 300 c
aat2847 four-channel backlight driver with dual ldos 4 2847.2007.09.1.0 electrical characteristics 1 v in = 3.6v; c in = c ldoa = c ldob = 2.2f; c out = 1f; c1 = c2 = 1f; t a = 25c, unless otherwise noted. typical values are at t a = 25c. 1. the aat2847 is guaranteed to meet performance specifications over the C40?c to +85c operating temperature range is assured by design, characterization and correlation with statistical process controls. 2. current matching is defined as the deviation of any sink current from the average of all active channels. symbol description conditions min typ max units power supply v in input voltage range 2.7 5.5 v i shdn total shutdown current at in v en/set = v ena = v enb = 0v 1.0 a charge pump i in input operating current 1x mode, 3.0 v in 5.5, active, no load, v ena = v enb = 0v 1.0 ma 1.5x mode, 3.0 v in 5.5, active, no load, v ena = v enb = 0v 4.0 2x mode, 3.0 v in 5.5, active, no load, v ena = v enb = 0v 5.0 i dx average current accuracy data 1, 20ma range 18 20 22 ma data 2, address 4 0.9 1.0 1.1 data 1, 30ma range 30 data 1, 15ma range 15 i (d-match) current matching 2 v in - v f = 1.5v, 20ma range 0.5 1 % v th 1x to 1.5x or 1.5x to 2x transition threshold at any dx pin 20ma range 150 mv f clk clock frequency 1 mhz t sd over-temperature shutdown threshold 140 ?c t hys over-temperature shutdown hysteresis 15 ?c en/set logic control t en/set lo en/set low time 0.3 75 s t en/set_hi_min minimum en/set high time 50 ns t en/set_hi_max maximum en/set high time 75 s t off en/set off timeout 500 s t lat en/set latch timeout 500 s v il(en/set) en/set logic low threshold voltage 0.4 v v ih(en/set) en/set logic high threshold voltage 1.4 v i en/set en/set input leakage v en/set = v in = 5v -1 1 a
aat2847 four-channel backlight driver with dual ldos 2847.2007.09.1.0 5 electrical characteristics 1 v in = 3.6v; c in = c ldoa = c ldob = 2.2f; c out = 1f; c1 = c2 = 1f; t a = 25c, unless otherwise noted. typical values are at t a = 25c. symbol description conditions min typ max units ldos: aat2847-ee i in in operating current v ena = v enb = v in , v en/set = 0v, no load 80 150 a v ena = v in ; v enb = 0v; v en/set = 0v; no load 60 112 v fba , v fbb feedback voltage i ldo[a/b] = 1ma to 200ma 1.17 1.2 1.23 v v do dropout voltage i ldo[a/b] = 150ma 150 300 mv v out / v out * v in line regulation v in = (v ldo[a/b] + 1v) to 5v 0.09 %/v psrr power supply rejection ratio i ldo[a/b] =10ma, 1khz 50 db ldos: aat2847-qg i in in operating current v ena = v enb = v in , v en/set = 0v, no load 80 150 a v ena = v in ; v enb = 0v; v en/set = 0v; no load 60 112 ldoa ldoa voltage tolerance i ldoa = 1ma to 150ma 2.716 2.8 2.884 v ldob ldob voltage tolerance i ldob = 1ma to 150ma 1.455 1.5 1.545 v v do ldoa dropout voltage 2 i ldoa = 150ma 150 300 mv v ldo[a/b] / v ldo[a/b] * v in ldoa, ldob line regulation v in = (ldoa + 1v) to 5v; v in = (ldob + 1.2v) to 5v 0.09 %/v psrr ldoa, ldob power supply rejection ratio i ldo[a/b] =10ma, 1khz 50 db ldos: aat2847-qi i in in operating current v ena = v enb = v in , v en/set = 0v, no load 80 150 a v ena = v in ; v enb = 0v; v en/set = 0v; no load 60 112 ldoa ldoa voltage tolerance i ldoa = 1ma to 150ma 2.716 2.8 2.884 v ldob ldob voltage tolerance i ldob = 1ma to 150ma 1.746 1.8 1.854 v v do ldoa dropout voltage 2 i ldoa = 150ma 150 300 mv v ldo[a/b] / v ldo[a/b ]* v in ldoa, ldob line regulation v in = (ldo[a/b] + 1v) to 5v 0.09 %/v psrr ldoa, ldob power supply rejection ratio i ldo[a/b] =10ma, 1khz 50 db ldo logic control C all options v il(ena) , v il(enb) ena, enb input logic low threshold voltage 0.4 v v ih(ena) , v ih(enb) ena, enb input logic high threshold voltage 1.4 v i en[a/b] ena, enb input leakage v en[a/b] = v in = 5v -1 1 a 1. the aat2847 is guaranteed to meet performance specifications over the C40?c to +85c operating temperature range is assured by design, characterization and correlation with statistical process controls. 2. v do is defined as v in - ldoa when ldoa is 98% of nominal.
typical characteristics logic high threshold voltage vs. input voltage input voltage (v) v ih(ena/enb) , v ih(en/set) (v) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 -40c 25c 85c logic low threshold voltage vs. input voltage input voltage (v) v il(ena/enb) , v il(en/set) (v) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 -40c 25c 85c en/set latch timeout vs. input voltage input voltage (v) t lat (s) 50 100 150 200 250 300 350 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 -40c 85c 25c en/set off timeout vs. input voltage input voltage (v) t off (s) 50 100 150 200 250 300 350 400 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 -40c 85c 25c backlight efficiency vs. input voltage input voltage (v) efficiency (%) 30 40 50 60 70 80 90 100 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 20ma/channel 1ma/channel 14.5ma/channel backlight current matching vs. temperature (i out = 20ma/channel; v in = 4.2v) temperature (c) led current (ma) 18.5 19 19.5 20 20.5 21 -40 -15 10 35 60 85 aat2847 four-channel backlight driver with dual ldos 6 2847.2007.09.1.0
typical characteristics backlight operating characteristic (v in = 2.9v; 2x mode; 14ma/channel load; ac coupled) time (500ns/div) v out (40mv/div) v dx (40ma/div) v in (20mv/div) backlight operating characteristic (v in = 3.5v; 1.5x mode; 14ma/channel load; ac coupled) time (500ns/div) v out (40mv/div) v dx (20ma/div) v in (20mv/div) backlight operating characteristic (v in = 2.9v; 2x mode; 20ma/channel load; ac coupled) time (500ns/div) v out (40mv/div) v dx (40ma/div) v in (20mv/div) shutdown current vs. input voltage ( v en/set = v ena/enb = 0v) input voltage (v) shutdown current (na) 0 5 10 15 20 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5. 5 -40c 25c 85c backlight operating characteristic (v in = 3.7v; 1.5x mode; 20ma/channel load; ac coupled) time (500ns/div) v out (40mv/div) v dx (20ma/div) v in (20mv/div) aat2847 four-channel backlight driver with dual ldos 2847.2007.09.1.0 7
typical characteristics ldoa/ldob load regulation load current (ma) output voltage error (%) -1.0 -0.5 0.0 0.5 1.0 0.1 1 10 100 1000 ldoa ldob ldoa/ldob line regulation input voltage (v) output voltage error (%) -1.000 -0.500 0.000 0.500 1.000 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 ldoa ldob turn on to 2x mode backlight (20ma/channel; address 0, data 1; v in = 3.2v) time (200s/div) v out (2v/div) v en/set (2v/div) v dx (500mv/div) i in (200ma/div) turn off from 1.5x mode backlight (20ma/channel; address 0, data 1; v in = 3.6v) time (100s/div) v out (2v/div) v en/set (2v/div) i led (20ma/div) turn on to 1x mode backlight (20ma/channel; address 0, data 1; v in = 4.2v) time (500ns/div) v out (2v/div) v en/set (2v/div) v dx (500mv/div) i in (100ma/div) turn on to 1.5x mode backlight (20ma/channel; address 0, data 1; v in = 3.5v) time (200s/div) v out (2v/div) v en/set (2v/div) v dx (500mv/div) i in (200ma/div) aat2847 four-channel backlight driver with dual ldos 8 2847.2007.09.1.0
typical characteristics ldob line transient response (10ma load) time (40s/div) v ldob (20mv/div) v in (400mv/div) v in = 3.6v v in = 4.2v ldob load transient response (v in = 3.6v; v ldob = 1.2v) time (20s/div) v ldob (100mv/div) i ldob (100ma/div) 200ma 10ma ldoa line transient response (10ma load) time (40s/div) v ldoa (20mv/div) v in (400mv/div) v in = 4.2v v in = 3.6v ldoa/ldob quiescent current vs. input voltage (v out = 1.2v) input voltage (v) quiescent current (a) 0 20 40 60 80 100 120 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 85c 25c -40c ldoa load transient response (v in = 3.6v; v ldoa = 1.2v) time (20s/div) v ldoa (100mv/div) i ldoa (100ma/div) 200ma 10ma aat2847 four-channel backlight driver with dual ldos 2847.2007.09.1.0 9
aat2847 four-channel backlight driver with dual ldos 10 2847.2007.09.1.0 typical characteristics ldoa/ldob turn on (v in = 3.6v; v ldo(a/b) = 1.5v; dc coupled) time (40s/div) v ldo(a/b) (500mv/div) v en(a/b) (2v/div) ldoa/ldob turn on (v in = 3.6v; v ldo(a/b) = 1.2v; dc coupled) time (40s/div) v ldo(a/b) (500mv/div) v en(a/b) (2v/div) ldoa/ldob turn on (v in = 3.6v; v ldo(a/b) = 2.8v; dc coupled) time (40s/div) v ldo(a/b) (1v/div) v en(a/b) (2v/div) ldoa/ldob turn on (v in = 3.6v; v ldo(a/b) = 1.8v; dc coupled) time (40s/div) v ldo(a/b) (1v/div) v en(a/b) (2v/div)
aat2847 four-channel backlight driver with dual ldos 2847.2007.09.1.0 11 functional block diagram tri-mode (1x, 1.5x and 2x) charge pump d/a d/a d/a d/a 6 x16 bit rom as 2 cwire interface 1mhz oscillator voltage reference in en/set gnd d4 d3 d2 d1 out c1+ c1- c2+ c2- ldo a in ldoa (2.8v/2.8v) fba (nc) ldo b ldob (1.5v/1.8v) fbb (nc) ena 1.2v reference aat 2847-ee (aat 2847-qg/-qi) (aat 2847-qg/-qi) pgnd enb functional description the aat2847 is an integrated solution for lcd dis- play applications with a built-in four channel white led driver (charge pump) and dual 200ma ldo voltage regulators. the aat2847 incorporates a tri-mode charge pump with load switch (1x) func- tionality and high efficiency (1.5x or 2x) perform- ance. to maximize power conversion efficiency, an internal sensing circuit monitors the voltage required at each white led cathode input pin (d1- d4) and sets the load switch and charge pump mode based on the input battery voltage and the white led cathode pin voltage. the voltage thresh- old for 1x to 1.5x and 1.5x to 2x mode transitions is v th . the charge pump in the aat2847 starts-up in 1x mode (default); under this condition, if the led cathode input pin voltage is below v th the aat2847 will transition into 1.5x mode. when 1.5x mode does not provide enough current through the white leds for backlight applications, which occurs during the normal discharge of the input battery power source, the led cathode input pin voltage will drop below v th and the aat2847 will transition into 2x mode. the charge pump requires only four external capacitors, 1f ceramic capacitors for the flying (c1 and c2), input (c in ), and output (c out ) capacitors. the four led cathode input pins (d1-d4) can drive individual leds with a maximum current of 30ma each. the unused led cathode input pins have to be connected to the out pin, otherwise the aat2847 will operate in 2x mode only. operating in 2x mode when it is not necessary will result in a significant reduction in efficiency. the as 2 cwire serial interface enables the charge pump and sets the current flowing into the led cathode input pins. the aat2847 also contains dual ldo voltage reg- ulators that have separate enable pins from each other and the charge pump. the separate enable pins for each dc/dc device in the aat2847 allow for every possible device operational combination. the two ldo regulators require 2.2f output capacitors for stable operation.
aat2847 four-channel backlight driver with dual ldos 12 2847.2007.09.1.0 figure 1: led current control profile. as 2 cwire serial interface each white led channel input on the aat2847 (d1-d4) is controlled by analogictech's as 2 cwire serial digital interface. the as 2 cwire interface uses the number of rising edges on the en/set pin to address and load the led configuration registers. as 2 cwire latches data or addresses after the en/set pin has been held logic high for longer than t lat (500s). addresses and data are differ- entiated by the number of en/set rising edges. since the data registers are 4 bits each, the differ- entiating number of pulses is 2 4 or 16, so that address 0 is signified by 17 rising edges, address 1 by 18 rising edges, address 2 by 19 rising edges, and so on. data is set to any number of rising edges between, and including, 1 to 16. a typical write protocol consist of the following: first a burst of en/set rising edges that identify/target a particular address followed by en/set being held logic high for the t lat timeout period to latch the address value in the address register, then another burst of rising edges that sig- nify data with the accompanying t lat timeout peri- od to latch the data value in the data register. once an address is set, then multiple writes to the corre- sponding data register are allowed without having to write to the address for every value change in the data register. when en/set is held low longer than t off (500s), the aat2847 enters shutdown mode operation and draws less than 1a from the input supply voltage. data and address registers are cleared (0 for the address register and 1 for the data registers) in shutdown mode operation. as 2 cwire serial interface addressing table 1: as 2 cwire serial interface addressing. current operation (address 0) use address 0 to program all four led channels with the current values in table 2. all four white led channels are programmed to the same cur- rent level by writing to address 0 followed by any data between, and including, 1 to 16. 0.0 5.0 10.0 15.0 20.0 25.0 30.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3 4 code led current (ma) address 0 address 4 figure 2: as 2 cwire serial interface timing. address data 1 en/set 2 19 20 a ddress data 3 data 0 1 2 . . . n 16 03 1 1 n t hi t lo t lat t lat address en/set edges addressed register 0 17 d1-d4 current control 3 20 current scale 4 21 low current control 5 22 independent led control
aat2847 four-channel backlight driver with dual ldos 2847.2007.09.1.0 13 current scale (address 3) the aat2847 has three selectable current scales for the four white led channels: 30ma, 20ma, and 15ma. only one of the three current scales can be active at any given time. by default, the 20ma current scale is active upon start-up. to change to the 30ma or 15ma current scale, or go back to the 20ma scale, write to address 3 with the appropriate data between, and including, 1 to 3. to enable low current mode, write data 4 to address 3. low current mode results in the four white led channels reverting to the cur- rent set by the low current settings in address 4. table 2: current settingsaddress 0. table 3: current scale settingsaddress 3. low current operation (address 4) all four led channels are programmed to the same low current level by writing to address 4 followed by any data between, and including, 1 to 4. this oper- ational mode is especially useful for low current applications where a continuous low current state is maintained for a substantial length of time. table 4: low current settingsaddress 4. independent led current control (address 5) independent led control allows for individual leds to be enabled and disabled to form custom arrangement of active leds. to enable independ- ent control write address 5 with data between, and including, 1 to 16. table 5: independent led control settings address 5. data current scale 1 20ma scale 2 30ma scale 3 15ma scale 4 low current mode data 30ma scale 20ma scale 15ma scale 1 30.0 20.0 15.0 2 28.5 19.0 14.3 3 26.6 17.7 13.3 4 25.2 16.8 12.6 5 23.3 15.5 11.6 6 21.8 14.5 10.9 7 19.8 13.2 9.9 8 18.5 12.3 9.2 9 16.5 11.0 8.3 10 15.0 10.0 7.5 11 13.1 8.7 6.5 12 11.6 7.7 5.8 13 9.8 6.5 4.9 14 8.3 5.5 4.1 15 6.3 4.2 3.2 16 4.8 3.2 2.4 data ma 1 0.5 2 1.0 3 1.5 4 2.0 data d4 d3 d2 d1 1 on on on on 2 on on on off 3 on on off on 4 on on off off 5 on off on on 6 on off on off 7 on off off on 8 on off off off 9 off on on on 10 off on on off 11 off on off on 12 off on off off 13 off off on on 14 off off on off 15 off off off on 16 off off off off
aat2847 four-channel backlight driver with dual ldos 14 2847.2007.09.1.0 auto disable feature the charge pump in the aat2847 is equipped with an auto-disable feature for each led channel. after the ic is enabled and successively starts-up, a test current of 100a (typical) is forced through each led channel. the channel will be disabled if the voltage on that particular dx pin does not drop to certain threshold. this feature is convenient for disabling an unused channel or during an led short circuiting event. low dropout regulators the aat2847 incorporates two ldo voltage regu- lators. the two regulators run from the same 2.7v to 5.5v input voltage as the charge pump and have separate on/off control inputs, ena and enb. for the aat2847-ee, the ldo output voltages are set through a resistive voltage divider from the out- put (ldoa or ldob) to the feedback input (fba or fbb). the ratio of the voltage divider resistor val- ues determines the ldo output voltage. for the aat2847-qg option, ldoa is internally set to 2.8v and ldob is internally set to 1.5v. for the aat2847-qi option, ldoa is also 2.8v and ldob is internally set to 1.8v. each ldo regulator can supply a continuous load current up to 200ma, and both ldos include current limiting and thermal overload protection to prevent damage to the load or to the ldo. thermal protection the charge pump has built-in thermal protection circuitry that will shut down the charge pump and the ldos if the die temperature rises above the thermal limit, as is the case during an out pin short circuit event. applications information led selection the charge pump in the aat2847 is specifically intended for driving white leds. however, the aat2847 can drive most types of leds with for- ward voltage specifications ranging from 2.0v to 4.7v. led applications may include mixed arrange- ments for display backlighting, color (rgb) leds, infrared (ir) diodes and any other load needing a constant current source generated from a varying input voltage. since the d1 to d4 constant current channels are matched with negligible voltage dependence, the constant current channels will be matched regardless of the specific led forward voltage (v f ) levels. multiple channels can be com- bined to obtain a higher led drive current without complication. aat2847-ee ldo output voltage programming the output voltages for ldoa and ldob are pro- grammed by an external resistor divider network. as shown in figure 3, the selection of r1 and r2 is a straightforward matter. r1 is chosen by consider- ing the tradeoff between the feedback network bias current and resistor value. higher resistor values allow stray capacitance to become a larger factor in circuit performance, whereas lower resistor values decrease efficiency. figure 3: selection of external resistors. to select appropriate resistor values, first choose a value for r1 that will produce a reasonable feed- back network bias current. then, according to the desired v ldo(a/b) , calculate r2 according to the equation below. an example calculation follows. v ldo(a/b) r2(a/b) v ref = 1.2v r1(a/b) ldo(a/b) fb(a/b)
aat2847 four-channel backlight driver with dual ldos 2847.2007.09.1.0 15 r1 is chosen to be 120k?, resulting in a small feedback network bias current of 10a (v fb(a/b) /r1 = 1.2v/120k?). the desired output voltage is 1.8v. from this information, r2 is calculated from the equation below: the result is r2 = 60k?. since 60k? is not a stan- dard 1% resistor value, 60.4k? is selected. from this example calculation, for v out = 1.8v, use r1 = 120k? and r2 = 60.4k?. a table of example output voltages and corresponding resistor values is pro- vided below. table 6: example output voltages and corresponding resistor values. device power efficiency the aat2847s charge pump conversion efficiency is defined as the power delivered to the white led load divided by the input power: v ledx = white led forward voltage (v f ) i ledx = white led bias current (i d ) the expression to define the estimated ideal effi- ciency () for the aat2847 in 1x mode is as follows: the aat2847s charge pump is a fractional charge pump which will boost the input supply voltage in the event where v in is less then the required output voltage across the white led load. the efficiency can be simply defined as a linear voltage regulator with an effective white led forward voltage that is equal to one and a half (1.5x mode) or two (2x mode) times the input voltage. with an ideal 1.5x charge pump, the input current is 1.5x of the output current. the expression to define the estimated ideal efficiency () for the aat2847 in 1.5x mode is as follows: the same calculations apply for the aat2847 in 2x mode where for an ideal 2x charge pump, the input current is 2x of the output current. the expression for the estimated ideal efficiency () for the aat2847 in 2x mode is as follows: capacitor selection careful selection of the six external capacitors c in , c1, c2, c ldoa , c ldob , and c out are important because they will affect turn on time, output ripple and transient performance. optimum performance will be obtained when low esr (<100m?) ceramic capacitors are used. in general, low esr is defined as a resistance that is less than 100m?. x7r and x5r type ceramic capacitors are highly recommended over all other types of capacitors for use with the aat2847. for the charge pump sec- tion, a 1f or greater capacitor is required for the fly (c1 and c2) and output (c out ) capacitors. the dual = = p leds p in v led1 i led1 + ... + v led4 i led4 v in i in = = p leds p in v led1 i led1 + ... + v led4 i led4 v in i in = = p leds p in v led1 i led1 + ... + v led4 i led4 v in i in = ; x = 1, 2, 3 or 4 and i in = 1.5(4 i ledx ) = 4 v ledx i ledx v in i in v ledx 1.5v in = ; x = 1, 2, 3 or 4 and i in = 2(4 i ledx ) = 4 v ledx i ledx v in i in v ledx 2v in r2 (a/b) = r1 (a/b) (v ldo(a/b) - 1.2v) 1.2v = ; x = 1, 2, 3 or 4 and i in = 4 i ledx = 4 v ledx i ledx v in i in v ledx v in = = p leds p in v led1 i led1 + ... + v led4 i led4 v in i in r2 standard 1% values (r1 = 120k) v ldo(a/b) (v) r2 () 2.8 160k 2.5 130k 2 79.6k 1.8 60.4k 1.5 30.1k
aat2847 four-channel backlight driver with dual ldos 16 2847.2007.09.1.0 ldos require a 2.2f or greater output capacitor. the required input capacitor (c in ) is 2.2f or greater. ceramic capacitors offer many advantages over their tantalum and aluminum electrolytic counter- parts. a ceramic capacitor typically has very low esr, is lowest cost, has a smaller printed circuit board (pcb) footprint, and is non-polarized. low esr ceramic capacitors maximize charge pump transient response. before choosing a particular capacitor, verify the capacitors performance with the characteristics illustrated in the components data sheet. performance verification will help avoid undesirable component related performance deficiencies. pcb layout to achieve adequate electrical and thermal per- formance, careful attention must be given to the pcb layout. in the worst-case operating condition, the chip must dissipate considerable power at full load. adequate heat-sinking must be achieved to ensure intended operation. figures 5 and 6 illustrate an example pcb layout. the bottom of the package features an exposed metal pad. the exposed pad acts, thermally, to transfer heat from the chip and, electrically, as a ground connection. the junction-to-ambient thermal resistance ( ja ) for the connection can be significantly reduced by fol- lowing a couple of important pcb design guidelines. the pcb area directly underneath the package should be plated so that the exposed paddle can be mated to the top layer pcb copper during the reflow process. multiple copper plated thru-holes should be used to electrically and thermally connect the top surface pad area to additional ground plane(s). the chip ground is internally connected to both the exposed pad and to the agnd and pgnd pins. it is good practice to connect the gnd pins to the exposed pad area with traces as shown in figure 4. the flying capacitors (c1 and c2), input capacitor (c4), and output capacitors (c3, c5, and c6) should be connected as close as possible to the ic. in addi- tion to the external passive components being placed as close as possible to the ic, all traces connecting the aat2847 should be as short and wide as possible to minimize path resistance and potential coupling. figure 4: aat2847 package layout. pin #1
aat2847 four-channel backlight driver with dual ldos 2847.2007.09.1.0 17 evaluation board layout figure 5: aat2847 evaluation board figure 6: aat2847 evaluation board component side layout. solder side layout. evaluation board schematic figure 7: aat2847 section schematic. c1 c2 c3 c4 d1 d2 d3 d4 c5 r8 r9 c6 r10 r11 dc+ 1 2 3 j1 d2 1 fbb 20 d1 2 ldob 3 in 4 pgnd 5 in 6 fba 7 l doa 8 c2- 9 c2+ 10 out 11 c1- 12 c1+ 13 en/set 14 enb 15 d4 16 d3 17 gnd 18 ena 19 u1 aat2847 dc+ 1 2 3 j2 dc+ 1 2 3 j3 r12 r13 r14 en/set ldob ldoa
aat2847 four-channel backlight driver with dual ldos 18 2847.2007.09.1.0 figure 8: mcu section schematic. vdd 1 gp5 2 gp4 3 gp3 4 gp2 5 gp1 6 gp0 7 vss 8 pic12f675 u2 c8 r5 red led7 1 2 3 j4 r7 r6 data light sw dc+ dc+ r1 r2 r3 en/set
aat2847 four-channel backlight driver with dual ldos 2847.2007.09.1.0 19 ordering information all analogictech products are offered in pb-free packaging. the term pb-free means semiconductor products that are in compliance with current rohs standards, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. for more information, please visit our website at http://www.analogictech.com/pbfree. package low dropout regulators marking 1 part number (tape and reel) 2 ldoa ldob TQFN34-20 programmable programmable ylxxy aat2847iml-ee-t1 TQFN34-20 2.8v 1.5v aat2847iml-qg-t1 TQFN34-20 2.8v 1.8v zdxyy aat2847iml-qi-t1 1. xyy = assembly and date code. 2. sample stock is generally held on part numbers listed in bold .
aat2847 four-channel backlight driver with dual ldos 20 2847.2007.09.1.0 advanced analogic technologies, inc. 3230 scott boulevard, santa clara, ca 95054 phone (408) 737-4600 fax (408) 737-4611 ? advanced analogic technologies, inc. analogictech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an analogictech pr oduct. no circuit patent licenses, copyrights, mask work rights, or other intellectual property rights are implied. analogictech reserves the right to make changes to their products or specifications or to discontinue any product or service with- out notice. except as provided in analogictechs terms and conditions of sale, analogictech assumes no liability whatsoever, an d analogictech disclaims any express or implied war- ranty relating to the sale and/or use of analogictech products including liability or warranties relating to fitness for a part icular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right. in order to minimize risks associated with the customers applications, adequa te design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. testing and other quality control techniques are utilized to the extent an alogictech deems necessary to support this warranty. specific testing of all parameters of each device is not necessarily performed. analogictech and the analogictech logo are trad emarks of advanced analogic technologies incorporated. all other brand and product names appearing in this document are registered trademarks or trademarks of their respective holder s. package information 1 TQFN34-20 all dimensions in millimeters. top view side view detail "a" bottom view 3.00

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