TQFN34-20

AAT2847 Four-Channel Backlight Driver with Dual LDOs 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 voltage regulators (LDOs) as additional power supplies 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 current matching is 1% which helps provide uniform display brightness. AnalogicTech’s AS2Cwire™ (Advanced Simple Serial Control™) serial digital interface is used to enable, disable, and set the current for each backlight LED channel. Each LED channel has sixteen available current level settings in three separate current scales, plus four available current level settings 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 1µA. The AAT2847 is available in the thermally enhanced 20-pin 3x4x0.75mm TQFN package. Features • • • ChargePump™ • • • • • • 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 AS2Cwire 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 -40°C to +85°C Temperature Range Applications • • • • Camera Function Power Supplies Camera Phone Displays LCD Modules White LED Backlighting C1 1μF C2 1μF C2+ OUT C OUT 1μF WLEDs OSRAM LW M673 or equivalent Typical Application C1 1μF C2 1μF C2+ C1- C1+ C2- C1- C1+ C2- OUT C OUT 1μF D1 D2 D3 D4 IN LDOA IN WLEDs OSRAM LW M673 or equivalent IN AAT 2847-EE VBAT 3.6V C IN 2.2μF VBAT 3.6V CIN 2.2μF AAT 2847-QI D1 D2 D3 D4 200mA IN LDOA CLDOA 2.2μF C LDOA 2.2μF AS2Cwire Backlight Control LDO A Enable LDO B Enable FBA EN/SET ENA ENB FBB GND PGND LDOB 200mA C LDOB 2.2μF VOUTA 2.8V, 200mA AS2Cwire Backlight control LDO A Enable LDO B Enable EN/SET ENA ENB LDOB VOUTB 1.8V, 200mA CLDOB 2.2μF GND PGND 2847.2007.09.1.0 1 AAT2847 Four-Channel Backlight Driver with Dual LDOs Pin Descriptions Symbol Pin # 1 2 3 4, 6 5 7 8 9 10 11 12 13 14 15 16 17 18 19 20 EP AAT2847-EE AAT2847-QG/QI Function LED2 current channel input. Connect to the cathode of backlight LED 2. If not used, connect D2 to the OUT pin. LED1 current channel input. Connect to the cathode of backlight LED 1. If not used, connect D1 to the OUT pin. Output of LDOB. Input voltage supply connection. Power Ground. AAT2847-EE: Feedback pin of LDOA. Internally regulated at 1.2V. AAT2847-QG/QI: No connection. Do not make any connection to this pin. Output of LDOA. Negative terminal of flying capacitor 2. Positive terminal of flying capacitor 2. Charge pump output to drive load circuit. Connect a 1µF or larger ceramic capacitor between OUT and PGND. Negative terminal of flying capacitor 1. Positive terminal of flying capacitor 1. AS2Cwire control pin for backlight LED current profile selection and control. Enable pin for LDOB. Active logic high. LED4 current channel input. Connect to the cathode of backlight LED 4. If not used, connect D4 to the OUT pin. LED3 current channel input. Connect to the cathode of backlight LED 3. If not used, connect D3 to the OUT pin. Ground. Enable pin for LDOA. Active logic high. AAT2847-EE: Feedback pin of LDOB. Internally regulated at 1.2V. AAT2847-QG/QI: No connection. Do not make any connection to this pin. Exposed pad (bottom). D2 D1 LDOB IN PGND FBA LDOA C2C2+ OUT C1C1+ EN/SET ENB D4 D3 GND ENA FBB D2 D1 LDOB IN PGND NC LDOA C2C2+ OUT C1C1+ EN/SET ENB D4 D3 GND ENA N/C 2 2847.2007.09.1.0 AAT2847 Four-Channel Backlight Driver with Dual LDOs Pin Configuration AAT2847-EE TQFN34-20 (Top View) D3 GND ENA FBB 20 19 18 17 AAT2847-QG/QI TQFN34-20 (Top View) D3 GND ENA N/C 20 19 18 17 D2 D1 LDOB IN PGND IN 1 2 3 4 5 6 10 9 7 8 16 15 14 13 12 11 D4 ENB EN/SET C1+ C1OUT D2 D1 LDOB IN PGND IN 1 2 3 4 5 6 10 9 7 8 16 15 14 13 12 11 D4 ENB EN/SET C1+ C1OUT C2+ C2LDOA N/C Absolute Maximum Ratings1 TA = 25°C, unless otherwise noted. Pin descriptions below apply to AAT2847-EE (AAT2847-QG/QI) Symbol VN VN VN TJ TLEAD C2+ C2LDOA FBA Description [IN, OUT, D1, D2, D3, D4] to GND [C1+, C1-, C2+, C2-] to GND [LDOA, LDOB, FBA, FBB, EN/SET, ENA, ENB] to GND Operating Temperature Range Maximum Soldering Temperature (at leads, 10 sec) Value -0.3 to 6.0 -0.3 to VOUT + 0.3 -0.3 to VIN + 0.3 -40 to 150 300 Units V V V °C °C Thermal Information2, 3 Symbol θJA PD Description Thermal Resistance Maximum Power Dissipation Value 50 2 Units °C/W W 1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions specified in not implied. Only one Absolute Maximum Rating should be applied at any one time. 2. Derate 20mW/°C above 40°C ambient temperature. 3. Mounted on a FR4 circuit board. 2847.2007.09.1.0 3 AAT2847 Four-Channel Backlight Driver with Dual LDOs Electrical Characteristics1 VIN = 3.6V; CIN = CLDOA = CLDOB = 2.2µF; COUT = 1µF; C1 = C2 = 1µF; TA = 25°C, unless otherwise noted. Typical values are at TA = 25°C. Symbol VIN ISHDN Charge Pump 1x Mode, 3.0 ≤ VIN ≤ 5.5, Active, No Load, VENA = VENB = 0V IIN Input Operating Current 1.5x Mode, 3.0 ≤ VIN ≤ 5.5, Active, No Load, VENA = VENB = 0V 2x Mode, 3.0 ≤ VIN ≤ 5.5, Active, No Load, VENA = VENB = 0V DATA 1, 20mA Range IDX Average Current Accuracy DATA 2, ADDRESS 4 DATA 1, 30mA Range DATA 1, 15mA Range VIN - VF = 1.5V, 20mA Range 18 0.9 20 1.0 30 15 0.5 150 1 140 15 1 % mV MHz ˚C ˚C 1.0 4.0 5.0 22 1.1 mA mA Description Input Voltage Range Total Shutdown Current at IN Conditions Min Typ Max Units 2.7 5.5 1.0 V µA Power Supply VEN/SET = VENA = VENB = 0V I(D-Match) VTH fCLK TSD THYS Current Matching 2 1x to 1.5x or 1.5x to 2x Transition 20mA Range Threshold at Any DX Pin Clock Frequency Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis 0.3 EN/SET Logic Control TEN/SET LO EN/SET Low Time TEN/SET_HI_MIN Minimum EN/SET High Time TEN/SET_HI_MAX Maximum EN/SET High Time TOFF TLAT VIL(EN/SET) VIH(EN/SET) IEN/SET EN/SET Off Timeout EN/SET Latch Timeout EN/SET Logic Low Threshold Voltage EN/SET Logic High Threshold Voltage EN/SET Input Leakage VEN/SET = VIN = 5V 1.4 -1 1 75 50 75 500 500 0.4 µs ns µs µs µs V V µA 1. The AAT2847 is guaranteed to meet performance specifications over the –40˚C to +85°C 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. 4 2847.2007.09.1.0 AAT2847 Four-Channel Backlight Driver with Dual LDOs Electrical Characteristics1 VIN = 3.6V; CIN = CLDOA = CLDOB = 2.2µF; COUT = 1µF; C1 = C2 = 1µF; TA = 25°C, unless otherwise noted. Typical values are at TA = 25°C. Symbol Description Conditions VENA = VENB = VIN, VEN/SET = 0V, No Load VENA = VIN; VENB = 0V; VEN/SET = 0V; No Load ILDO[A/B] = 1mA to 200mA ILDO[A/B] = 150mA VIN = (VLDO[A/B] + 1V) to 5V 1.17 Min Typ 80 60 1.2 150 0.09 50 Max Units 150 112 1.23 300 LDOs: AAT2847-EE IIN VFBA, VFBB VDO ΔVOUT/ VOUT*ΔVIN PSRR IN Operating Current Feedback Voltage Dropout Voltage Line Regulation µA V mV %/V dB Power Supply Rejection ILDO[A/B] =10mA, 1kHz Ratio VENA = VENB = VIN, VEN/SET = 0V, No Load 2.716 1.455 LDOs: AAT2847-QG IIN LDOA LDOB VDO IN Operating Current 80 60 2.8 1.5 150 0.09 50 150 112 2.884 1.545 300 VENA = VIN; VENB = 0V; VEN/SET = 0V; No Load LDOA Voltage Tolerance ILDOA = 1mA to 150mA LDOB Voltage Tolerance ILDOB = 1mA to 150mA LDOA Dropout Voltage 2 µA V V mV %/V dB ILDOA = 150mA VIN = (LDOA + 1V) to 5V; VIN = (LDOB + 1.2V) to 5V ILDO[A/B] =10mA, 1kHz ΔVLDO[A/B]/ LDOA, LDOB Line VLDO[A/B]*ΔVIN Regulation PSRR LDOA, LDOB Power Supply Rejection Ratio LDOs: AAT2847-QI IIN LDOA LDOB VDO IN Operating Current VENA = VENB = VIN, VEN/SET = 0V, No Load VENA = VIN; VENB = 0V; VEN/SET = 0V; No Load 2.716 1.746 80 60 2.8 1.8 150 0.09 50 150 112 2.884 1.854 300 µA V V mV %/V dB LDOA Voltage Tolerance ILDOA = 1mA to 150mA LDOB Voltage Tolerance ILDOB = 1mA to 150mA LDOA Dropout Voltage2 ILDOA = 150mA VIN = (LDO[A/B] + 1V) to 5V ILDO[A/B] =10mA, 1kHz ΔVLDO[A/B]/ LDOA, LDOB Line VLDO[A/B]*ΔVIN Regulation PSRR LDOA, LDOB Power Supply Rejection Ratio ENA, ENB Input Logic Low Threshold Voltage ENA, ENB Input Logic High Threshold Voltage ENA, ENB Input Leakage LDO Logic Control – All Options VIL(ENA), VIL(ENB) VIH(ENA), VIH(ENB) IEN[A/B] 0.4 1.4 VEN[A/B] = VIN = 5V -1 1 V V µA 1. The AAT2847 is guaranteed to meet performance specifications over the –40˚C to +85°C operating temperature range is assured by design, characterization and correlation with statistical process controls. 2. VDO is defined as VIN - LDOA when LDOA is 98% of nominal. 2847.2007.09.1.0 5 AAT2847 Four-Channel Backlight Driver with Dual LDOs Typical Characteristics Backlight Efficiency vs. Input Voltage 100 90 Backlight Current Matching vs. Temperature (IOUT = 20mA/Channel; VIN = 4.2V) 21 LED Current (mA) 5.5 20.5 20 19.5 19 18.5 -40 Efficiency (%) 80 70 60 50 40 30 2.7 3.1 3.5 3.9 4.3 20mA/Channel 1mA/Channel 14.5mA/Channel 4.7 5.1 -15 10 35 60 85 Input Voltage (V) Temperature (°C) EN/SET Latch Timeout vs. Input Voltage 350 300 400 EN/SET Off Timeout vs. Input Voltage -40°C TOFF (µs) 350 300 250 200 150 100 50 2.7 -40°C TLAT (µs) 250 200 150 100 50 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 85°C 25°C 85°C 3.1 3.5 3.9 25°C 4.3 4.7 5.1 5.5 Input Voltage (V) Input Voltage (V) Logic High Threshold Voltage vs. Input Voltage 1.2 1.2 Logic Low Threshold Voltage vs. Input Voltage VIL(ENA/ENB), VIL(EN/SET) (V) 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 VIH(ENA/ENB), VIH(EN/SET) (V) 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 2.7 3.1 -40°C -40°C 25°C 85°C 25°C 85°C 3.5 3.9 4.3 4.7 5.1 5.5 Input Voltage (V) Input Voltage (V) 6 2847.2007.09.1.0 AAT2847 Four-Channel Backlight Driver with Dual LDOs Typical Characteristics Shutdown Current vs. Input Voltage (VEN/SET = VENA/ENB = 0V) 20 Backlight Operating Characteristic (VIN = 3.7V; 1.5X Mode; 20mA/Channel Load; AC Coupled) VIN (20mV/div) VOUT (40mV/div) Shutdown Current (nA) 15 -40°C 10 5 25°C 85°C VDX (20mA/div) 0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 Input Voltage (V) Time (500ns/div) Backlight Operating Characteristic (VIN = 3.5V; 1.5X Mode; 14mA/Channel Load; AC Coupled) VIN (20mV/div) VOUT (40mV/div) VIN (20mV/div) VOUT (40mV/div) Backlight Operating Characteristic (VIN = 2.9V; 2X Mode; 20mA/Channel Load; AC Coupled) VDX (20mA/div) VDX (40mA/div) Time (500ns/div) Time (500ns/div) Backlight Operating Characteristic (VIN = 2.9V; 2X Mode; 14mA/Channel Load; AC Coupled) VIN (20mV/div) VOUT (40mV/div) VDX (40mA/div) Time (500ns/div) 2847.2007.09.1.0 7 AAT2847 Four-Channel Backlight Driver with Dual LDOs Typical Characteristics Turn On to 1X Mode Backlight (20mA/Channel; Address 0, Data 1; VIN = 4.2V) VEN/SET (2V/div) VOUT (2V/div) VDX (500mV/div) IIN (100mA/div) Time (500ns/div) Turn On to 1.5X Mode Backlight (20mA/Channel; Address 0, Data 1; VIN = 3.5V) VEN/SET (2V/div) VOUT (2V/div) VDX (500mV/div) IIN (200mA/div) Time (200µs/div) Turn On to 2X Mode Backlight (20mA/Channel; Address 0, Data 1; VIN = 3.2V) VEN/SET (2V/div) VOUT (2V/div) VDX (500mV/div) IIN (200mA/div) Time (200µs/div) Turn Off from 1.5X Mode Backlight (20mA/Channel; Address 0, Data 1; VIN = 3.6V) VEN/SET (2V/div) VOUT (2V/div) ILED (20mA/div) Time (100µs/div) LDOA/LDOB Load Regulation Output Voltage Error (%) Output Voltage Error (%) 1.0 1.000 LDOA/LDOB Line Regulation 0.5 LDOA 0.500 LDOA 0.0 0.000 -0.5 LDOB -0.500 LDOB -1.0 0.1 1 10 100 1000 -1.000 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 Load Current (mA) Input Voltage (V) 8 2847.2007.09.1.0 AAT2847 Four-Channel Backlight Driver with Dual LDOs Typical Characteristics LDOA/LDOB Quiescent Current vs. Input Voltage (VOUT = 1.2V) 120 LDOA Load Transient Response (VIN = 3.6V; VLDOA = 1.2V) 200mA Quiescent Current (µA) 100 80 60 40 20 0 2.7 85°C 25°C ILDOA (100mA/div) 10mA -40°C VLDOA (100mV/div) 3.1 3.5 3.9 4.3 4.7 5.1 5.5 Input Voltage (V) Time (20µs/div) LDOB Load Transient Response (VIN = 3.6V; VLDOB = 1.2V) 200mA LDOA Line Transient Response (10mA Load) VIN = 4.2V ILDOB (100mA/div) 10mA VIN (400mV/div) VIN = 3.6V VLDOB (100mV/div) VLDOA (20mV/div) Time (20µs/div) Time (40µs/div) LDOB Line Transient Response (10mA Load) VIN = 4.2V VIN (400mV/div) VIN = 3.6V VLDOB (20mV/div) Time (40µs/div) 2847.2007.09.1.0 9 AAT2847 Four-Channel Backlight Driver with Dual LDOs Typical Characteristics LDOA/LDOB Turn On (VIN = 3.6V; VLDO(A/B) = 2.8V; DC Coupled) LDOA/LDOB Turn On (VIN = 3.6V; VLDO(A/B) = 1.8V; DC Coupled) VEN(A/B) (2V/div) VEN(A/B) (2V/div) VLDO(A/B) (1V/div) VLDO(A/B) (1V/div) Time (40µs/div) Time (40µs/div) LDOA/LDOB Turn On (VIN = 3.6V; VLDO(A/B) = 1.5V; DC Coupled) LDOA/LDOB Turn On (VIN = 3.6V; VLDO(A/B) = 1.2V; DC Coupled) VEN(A/B) (2V/div) VEN(A/B) (2V/div) VLDO(A/B) (500mV/div) VLDO(A/B) (500mV/div) Time (40µs/div) Time (40µs/div) 10 2847.2007.09.1.0 AAT2847 Four-Channel Backlight Driver with Dual LDOs Functional Block Diagram C1+ AAT 2847-EE (AAT 2847-QG/-QI) IN C1- C2+ C2- Tri-Mode (1x, 1.5x and 2x) Charge Pump OUT 1MHz Oscillator Voltage Reference 1.2V Reference D/A D/A D1 D2 D3 D4 EN/SET AS Cwire Interface 2 6 x16 bit ROM D/A D/A IN LDO A LDOA (2.8V/2.8V) FBA (NC) LDOB (1.5V/1.8V) FBB (NC) ENA LDO B ENB (AAT 2847-QG/-QI) GND PGND Functional Description The AAT2847 is an integrated solution for LCD display 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) functionality and high efficiency (1.5X or 2X) performance. To maximize power conversion efficiency, an internal sensing circuit monitors the voltage required at each white LED cathode input pin (D1D4) and sets the load switch and charge pump mode based on the input battery voltage and the white LED cathode pin voltage. The voltage threshold for 1X to 1.5X and 1.5X to 2X mode transitions is VTH. The charge pump in the AAT2847 starts-up in 1X mode (default); under this condition, if the LED cathode input pin voltage is below VTH 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 2847.2007.09.1.0 input battery power source, the LED cathode input pin voltage will drop below VTH and the AAT2847 will transition into 2X mode. The charge pump requires only four external capacitors, 1µF ceramic capacitors for the flying (C1 and C2), input (CIN), and output (COUT) 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 AS2Cwire serial interface enables the charge pump and sets the current flowing into the LED cathode input pins. The AAT2847 also contains dual LDO voltage regulators 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 2.2µF output capacitors for stable operation. 11 AAT2847 Four-Channel Backlight Driver with Dual LDOs 30.0 25.0 20.0 15.0 10.0 5.0 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3 4 Address 0 Address 4 Code Figure 1: LED Current Control Profile. identify/target a particular address followed by EN/SET being held logic high for the TLAT timeout period to latch the address value in the address register, then another burst of rising edges that signify data with the accompanying TLAT timeout period to latch the data value in the data register. Once an address is set, then multiple writes to the corresponding 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 TOFF (500µs), the AAT2847 enters shutdown mode operation and draws less than 1µA 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. AS2Cwire Serial Interface Each white LED channel input on the AAT2847 (D1-D4) is controlled by AnalogicTech's AS2Cwire serial digital interface. The AS2Cwire interface uses the number of rising edges on the EN/SET pin to address and load the LED configuration registers. AS2Cwire latches data or addresses after the EN/SET pin has been held logic high for longer than TLAT (500µs). Addresses and data are differentiated by the number of EN/SET rising edges. Since the data registers are 4 bits each, the differentiating number of pulses is 24 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 Address T HI T LO TLAT LED Current (mA) AS2Cwire Serial Interface Addressing Address 0 3 4 5 EN/SET Edges 17 20 21 22 Addressed Register D1-D4 Current Control Current Scale Low Current Control Independent LED Control Table 1: AS2Cwire 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 current level by writing to Address 0 followed by any Data between, and including, 1 to 16. Data TLAT EN/SET 1 2 19 20 1 2... n ≤ 16 Address 0 3 DATA3 1 n DATA0 1 Figure 2: AS2Cwire Serial Interface Timing. 12 2847.2007.09.1.0 AAT2847 Four-Channel Backlight Driver with Dual LDOs 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 current set by the low current settings in Address 4. Data 30mA Scale 20mA Scale 15mA Scale 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 30.0 28.5 26.6 25.2 23.3 21.8 19.8 18.5 16.5 15.0 13.1 11.6 9.8 8.3 6.3 4.8 20.0 19.0 17.7 16.8 15.5 14.5 13.2 12.3 11.0 10.0 8.7 7.7 6.5 5.5 4.2 3.2 15.0 14.3 13.3 12.6 11.6 10.9 9.9 9.2 8.3 7.5 6.5 5.8 4.9 4.1 3.2 2.4 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 operational mode is especially useful for low current applications where a continuous low current state is maintained for a substantial length of time. Data 1 2 3 4 mA 0.5 1.0 1.5 2.0 Table 4: Low Current Settings—Address 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 independent control write Address 5 with Data between, and including, 1 to 16. Data 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 D4 on on on on on on on on off off off off off off off off D3 on on on on off off off off on on on on off off off off D2 on on off off on on off off on on off off on on off off D1 on off on off on off on off on off on off on off on off Table 2: Current Settings—Address 0. Data 1 2 3 4 Current Scale 20mA Scale 30mA Scale 15mA Scale Low Current Mode Table 3: Current Scale Settings—Address 3. Table 5: Independent LED Control Settings— Address 5. 2847.2007.09.1.0 13 AAT2847 Four-Channel Backlight Driver with Dual LDOs 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 100µA (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. 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 forward voltage specifications ranging from 2.0V to 4.7V. LED applications may include mixed arrangements 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 (VF) levels. Multiple channels can be combined to obtain a higher LED drive current without complication. Low Dropout Regulators The AAT2847 incorporates two LDO voltage regulators. 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 output (LDOA or LDOB) to the feedback input (FBA or FBB). The ratio of the voltage divider resistor values 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. AAT2847-EE LDO Output Voltage Programming The output voltages for LDOA and LDOB are programmed 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 considering 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. LDO(A/B) R2(A/B) FB(A/B) VREF = 1.2V R1(A/B) VLDO(A/B) 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. Figure 3: Selection of External Resistors. To select appropriate resistor values, first choose a value for R1 that will produce a reasonable feedback network bias current. Then, according to the desired VLDO(A/B), calculate R2 according to the equation below. An example calculation follows. 14 2847.2007.09.1.0 AAT2847 Four-Channel Backlight Driver with Dual LDOs R1 is chosen to be 120kΩ, resulting in a small feedback network bias current of 10µA (VFB(A/B)/R1 = 1.2V/120kΩ). The desired output voltage is 1.8V. From this information, R2 is calculated from the equation below: R2(A/B) = R1(A/B)(VLDO(A/B) - 1.2V) 1.2V The AAT2847’s charge pump is a fractional charge pump which will boost the input supply voltage in the event where VIN 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 (η) for the AAT2847 in 1.5X mode is as follows: η= η= The result is R2 = 60kΩ. Since 60kΩ is not a standard 1% resistor value, 60.4kΩ is selected. From this example calculation, for VOUT = 1.8V, use R1 = 120kΩ and R2 = 60.4kΩ. A table of example output voltages and corresponding resistor values is provided below. R2 Standard 1% Values (R1 = 120kΩ) VLDO(A/B) (V) R2 (Ω) 2.8 2.5 2 1.8 1.5 160k 130k 79.6k 60.4k 30.1k PLEDs VLED1 · ILED1 + ... + VLED4 · ILED4 = PIN VIN · IIN 4 · VLEDX · ILEDX ; x = 1, 2, 3 or 4 and IIN = 1.5(4 · ILEDX) VIN · IIN IN VLEDX η = 1.5V Table 6: Example Output Voltages and Corresponding Resistor Values. 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 (η) for the AAT2847 in 2X mode is as follows: η= η= Device Power Efficiency The AAT2847’s charge pump conversion efficiency is defined as the power delivered to the white LED load divided by the input power: V ·I + ... + VLED4 · ILED4 P η = LEDs = LED1 LED1 PIN VIN · IIN VLEDx = White LED Forward Voltage (VF) ILEDx = White LED Bias Current (ID) The expression to define the estimated ideal efficiency (η) for the AAT2847 in 1X mode is as follows: η= η= PLEDs VLED1 · ILED1 + ... + VLED4 · ILED4 = PIN VIN · IIN 4 · VLEDX · ILEDX ; x = 1, 2, 3 or 4 and IIN = 2(4 · ILEDX) VIN · IIN VLEDX η = 2V IN Capacitor Selection Careful selection of the six external capacitors CIN, C1, C2, CLDOA, CLDOB, and COUT are important because they will affect turn on time, output ripple and transient performance. Optimum performance will be obtained when low ESR (