AAT2848

PRODUCT DATASHEET AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications Features • • • • • • • • • • • • • • Tri-Mode (1x/1.5x/2x) Charge Pump Four 30mA Backlight LED Channels Independent S2Cwire Control for Backlight 32-Level Programmable Backlight Current Control— Linear, Inverting Two 300mA Flash LED Channels Independent S2Cwire Control for Flash 16-Level Programmable Flash Current Control— Linear, Inverting User-Programmable Full Scale Current for Backlight and Flash User-Programmable Fade-In/Fade-Out Function for Backlight User-Programmable Safety Timer for Flash LED Protection Built-In Thermal Protection Automatic Soft-Start -40°C to +85°C Temperature Range Available in TQFN33-20 Package General Description The AAT2848 is a tri-mode charge pump optimized for systems operating with lithium-ion/polymer batteries. The charge pump provides power for both white LED backlight and flash, with an integrated four channel backlight LED current sink plus two channel flash LED current sinks. Each backlight channel can drive up to 30mA, while each flash current sink/channel can drive up to 300mA (600mA total). Two independent S2Cwire™ (AnalogicTech’s Simple Serial Control™) serial digital interface inputs enable, disable, and set LED current to one of 32 levels for the backlight and to one of 16 levels for the flash, with fullscale current independently set for backlight and flash using external resistors. The AAT2848 offers a built-in fade-in/out function for linear control of the backlight current during ON/OFF transitions. The fade time is programmable by an external capacitor. The AAT2848 also offers a built-in flash timeout function as a safety feature associated with the high power flash driver. The safety timer is also programmable via an external capacitor. The AAT2848 is equipped with built-in short-circuit and over-temperature protection. The soft-start circuitry prevents excessive inrush current at start-up and mode transitions. The AAT2848 is available in a Pb-free, space saving TQFN33-20 package and operates over the -40°C to +85°C ambient temperature range. Applications • Camera Enabled Mobile Devices • Digital Still Cameras • Multimedia Mobile Phones Typical Application C1 1μF C2 1μF C2+ OUT COUT 2.2μF Flash LEDs Lumileds PWF1 or equivalent WLEDs OSRAM LW M673 or equivalent C1- C1+ C2IN VBAT CIN 4.7μF AAT2848 BL1 BL2 BL3 BL4 FL1 FL2 S2Cwire Backlight Control S2Cwire Flash Control BENS FENS BSET FSET RBSET 15k RFSET 14.3k FLTMR CFLTMR 330nF FADE CFADE 47nF AGND PGND 2848.2008.05.1.0 www.analogictech.com 1 PRODUCT DATASHEET AAT2848 AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications Pin Descriptions Pin # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 EP Symbol OUT C2+ AGND FADE FLTMR BENS FENS FSET BSET BL1 BL2 BL3 BL4 FL1 FL2 PGND C2C1IN C1+ Description Charge pump output; supplies current to the backlight and flash LEDs. Connect the backlight and flash LED anodes to OUT. Bypass OUT to PGND with a 2.2μF or larger ceramic capacitor as close to the AAT2848 as possible. Positive terminal of charge pump capacitor 2. Connect a 1μF charge pump capacitor 1 from C2+ to C2-. Analog ground. Connect AGND to PGND at a single point as close to the AAT2848 as possible. Fade-In/Fade-Out timing control capacitor input. Connect a capacitor from FADE to AGND to set the automatic backlight current fade-in/fade-out response time. A 47nF capacitor sets the fade-in/fade-out backlight response time to 2.7 seconds. Connect to the IN pin to disable fade functionality. Flash timer control capacitor input. Connect a capacitor from FLTMR to AGND to set the flash timer. A 330nF capacitor sets the flash timer to 1.3 seconds. Connect to the AGND pin to disable timer functionality. Backlight enable and serial control input. BENS is the on/off control for the backlight and the S2Cwire input to serially control the backlight LED brightness relative to the maximum current set by the resistor at BSET. Flash enable and serial control input. FENS is the on/off control for the flash and the S2Cwire input to serially control the flash LED brightness relative to the maximum current set by the resistor at FSET. Flash current setting input. A 9.53kΩ resistor from FSET to AGND sets the maximum flash LED current to 300mA per flash output. Backlight current setting input. A 10kΩ resistor from BSET to AGND sets the maximum backlight LED current to 30mA per backlight output. Backlight LED 1 current sink/channel. BL1 controls the current through backlight LED 1. Connect the cathode of backlight LED 1 to BL1. If not used, connect BL1 to OUT. Backlight LED 2 current sink/channel. BL2 controls the current through backlight LED 2. Connect the cathode of backlight LED 2 to BL2. If not used, connect BL2 to OUT. Backlight LED 3 current sink/channel. BL3 controls the current through backlight LED 3. Connect the cathode of backlight LED 3 to BL3. If not used, connect BL3 to OUT. Backlight LED 4 current sink/channel. BL4 controls the current through backlight LED 4. Connect the cathode of backlight LED 4 to BL4. If not used, connect BL4 to OUT. Flash LED 1 current sink/channel. FL1 controls the current through flash LED 1. Connect the cathode of flash LED 1 to FL1. If not used, connect FL1 to OUT. Flash LED 2 current sink/channel. FL2 controls the current through flash LED 2. Connect the cathode of flash LED 2 to FL2. If not used, connect FL2 to OUT. Power ground. Connect PGND to AGND at a single point as close to the AAT2848 as possible. Negative terminal of charge pump capacitor 2. Negative terminal of charge pump capacitor 1. Power input. Connect IN to the input source voltage. Bypass IN to PGND with a 4.7μF or larger ceramic capacitor as close to the AAT2848 as possible. Positive terminal of charge pump capacitor 1. Connect a 1μF charge pump capacitor 1 from C1+ to C1-. Exposed paddle (bottom). Connect to ground as close as possible to the device. Pin Configuration PGND C2C1IN C1+ 20 19 18 17 16 OUT C2+ AGND FADE FLTMR 1 2 3 4 5 10 6 7 8 9 EP 15 14 13 12 11 FL2 FL1 BL4 BL3 BL2 2 www.analogictech.com BL1 BSET FSET FENS BENS 2848.2008.05.1.0 PRODUCT DATASHEET AAT2848 AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications Absolute Maximum Ratings1 Symbol VN VN VN TJ TLEAD Description IN, OUT, BL1, BL2, BL3, BL4, FL1, FL2, C1-, C2- Voltage to GND C1+, C2+ Voltage to GND BENS, FENS, BSET, FSET, FLTMR, FADE Voltage to GND Operating Junction 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, 4 Symbol PD θJA Description Maximum Power Dissipation Maximum Thermal Resistance Value 2.0 50 Units W °C/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 is not implied. Only one Absolute Maximum Rating should be applied at any one time. 2. Based on long-term current density limitation. 3. Derate 20mW/°C above 25°C ambient temperature. 4. Mounted on a 1.6mm-thick FR4 circuit board. 2848.2008.05.1.0 www.analogictech.com 3 PRODUCT DATASHEET AAT2848 AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications Electrical Characteristics1 VIN = 3.6V; CIN = 4.7μF; COUT = 2.2μF; C1 = C2 = 1μF; RBSET = 15kΩ; RFSET = 14.3kΩ; CFLTMR = 0.33μF; CFADE = 47nF; TA = -40°C to +85°C, unless otherwise noted. Typical values are TA = 25°C. Symbol VIN Description IN Operating Voltage Range Conditions BENS = IN, FENS = AGND, BL1 = BL2 = BL3 = BL4 = OUT (Excluding LED Current) BENS = IN, FENS = AGND, BL1 = BL2 = BL3 = BL4 = OPEN (1.5x CP, no load) BENS = AGND, FENS = IN, FL1 = FL2 = OUT (Excluding LED Current) BENS = AGND, FENS = IN, FL1 = FL2 = OPEN (2x CP, no load) BENS = FENS = AGND Min 2.7 Typ Max 5.5 600 5.5 600 5.5 1.0 Units V μA mA μA mA μA °C °C mA IIN(Q) IN Operating Current IIN(SHDN) IN Shutdown Current TSD Thermal Shutdown TSD(HYS) Thermal Shutdown Hysteresis Charge Pump Section IOUT(MAX) CP Maximum Output Current Charge Pump Mode Transition VIN(TH_H) Hysteresis Charge Pump Oscillator fOSC Frequency (1.5X mode) Charge Pump Soft Start Time tSS BL1-BL4 Backlight LED Outputs IBL_(MAX) ΔI(BL_) BL1-BL4 Current Accuracy 140 15 IN = 3V BENS = DATA1 TA = 25°C 720 100 1.0 100 BENS = DATA1; VIN – VF = 1.5V BENS = DATA32; VIN – VF = 1.5V BENS = DATA1; VIN – VF = 1.5V Any backlight current sink/channel BL1, BL2, BL3, BL4 FENS = DATA1; VIN – VF = 1.5V FENS = DATA16; VIN – VF = 1.5V FENS = DATA1; VIN – VF = 1.5V Any flash current sink/channel FL1, FL2 180 5.66 18 0.26 20 0.32 ±3 120 200 6.66 ±3 300 220 7.66 22 0.39 mV MHz μs mA % mV BL1-BL4 Current Matching BL1-BL4 Charge Pump Mode VBL_(TH) Transition Threshold FL1-FL2 Flash LED Outputs IFL_(MAX) ΔI(FL_) VFL_(TH) FL1-FL2 Current Accuracy FL1-FL2 Current Matching FL1-FL2 Charge Pump Mode Transition Threshold mA % mV 4 www.analogictech.com 2848.2008.05.1.0 PRODUCT DATASHEET AAT2848 AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications Electrical Characteristics1 VIN = 3.6V; CIN = 4.7μF; COUT = 2.2μF; C1 = C2 = 1μF; RBSET = 15kΩ; RFSET = 14.3kΩ; CFLTMR = 0.33μF; CFADE = 47nF; TA = -40°C to +85°C, unless otherwise noted. Typical values are TA = 25°C. Symbol Description Conditions Min Typ Max 0.4 1.4 VBENS = VFENS = VIN = 5V BENS Transition High-to-Low or Low-toHigh; DATA1 FENS Transition Low-to-High -1 2.7 1.3 0.3 50 75 500 500 75 1 Units V V μA s s μs ns μs μs μs BENS and FENS Logic Control VBENS(L), BENS, FENS Input Low Threshold Voltage VFENS(L) VBENS(H), BENS, FENS Input High Threshold Voltage VFENS(H) IBENS, IFENS BENS, FENS Input Leakage Current tFADE tFLASH tBENS(LOW), tFENS(LOW) tBENS(HI_MIN), tFENS(HI_MIN) tBENS(HI_MAX), tFENS(HI_MAX) tBENS(OFF), tFENS(OFF) tBENS(LAT), tFENS(LAT) Automatic Fade-In/Out Timer Automatic Flash Disable Time BENS, FENS Low Time Minimum BENS, FENS High Time Maximum BENS, FENS High Time BENS, FENS Off Timeout BENS, FENS Latch Timeout 2848.2008.05.1.0 www.analogictech.com 5 PRODUCT DATASHEET AAT2848 AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications Typical Characteristics Backlight Efficiency vs. Input Voltage 100 90 Backlight Current Matching vs. Temperature (VIN = 3.6V; 30mA/Channel) 30.5 80 70 60 50 40 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 Output Current (mA) Efficiency (%) 30mA/ch, VF = 3.4V 15mA/ch, VF = 3.2V 7.5mA/ch, VF = 3.0V 30.0 29.5 D1 D2 D3 D4 -15 10 35 60 85 29.0 -40 Input Voltage (V) Temperature (°C) Flash Efficiency vs. Input Voltage 100 90 Flash Current Matching vs. Temperature (VIN = 3.6V; 150mA/Channel) 152 Output Current (mA) 151mA/ch, VF = 3.0V 70mA/ch, VF = 2.9V 151 Efficiency (%) 80 70 60 50 40 2.7 150 149 148 D1 D2 -15 10 35 60 85 3.1 3.5 3.9 4.3 4.7 5.1 5.5 147 -40 Input Voltage (V) Temperature (°C) Turn-On to 1x Mode Backlight (VIN = 4.5V; 30mA/Channel) Turn-On to 1.5x Mode Backlight (VIN = 3.6V; 30mA/Channel) VEN/SET (2V/div) VOUT (2V/div) VBLX (2V/div) IIN (200mA/div) 0V VEN/SET (2V/div) VOUT (2V/div) VBLX (2V/div) IIN (200mA/div) 0V 0V 0V 0V 0V 0A 0A Time (100μs/div) Time (100μs/div) 6 www.analogictech.com 2848.2008.05.1.0 PRODUCT DATASHEET AAT2848 AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications Typical Characteristics Turn-On to 2x Mode Flash (VIN = 2.8V; 151mA/Channel) Turn-On to 1.5x Mode Flash (VIN = 3.6V; 151mA/Channel) VEN/SET (2V/div) VOUT (2V/div) VFLX (2V/div) IIN (400mA/div) 0V VEN/SET (2V/div) VOUT (2V/div) VFLX (2V/div) IIN (400mA/div) 0V 0V 0V 0V 0V 0A 0A Time (100μs/div) Time (100μs/div) Turn-Off from 1.5x Mode Backlight (VIN = 3.6V; 30mA/Channel; Fade-In/Out Disabled) VEN/SET (2V/div) VF (1V/div) VEN/SET (2V/div) VF (1V/div) Turn-Off from 1.5x Mode Flash (VIN = 3.6V; 151mA/Channel) 0V 0V 0V 0V IIN (200mA/div) 0A IIN (400mA/div) 0A Time (100μs/div) Time (100μs/div) Turn-Off from 2x Mode Flash (VIN = 2.8V; 151mA/Channel) VEN/SET (2V/div) VF (1V/div) Backlight Operating Characteristics (VIN = 3.6V; 1.5x Mode; 30mA/Channel; AC Coupled) VIN (20mV/div) VOUT (50mV/div) 0V 0V IIN (400mA/div) 0A VBLX (50mV/div) Time (100μs/div) Time (1μs/div) 2848.2008.05.1.0 www.analogictech.com 7 PRODUCT DATASHEET AAT2848 AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications Typical Characteristics Backlight Operating Characteristics (VIN = 3.6V; 1.5x Mode; 15mA/Channel; AC Coupled) VIN (20mV/div) VOUT (50mV/div) Flash Operating Characteristics (VIN = 3.6V; 1.5x Mode; 151mA/Channel; AC Coupled) VIN (20mV/div) VOUT (50mV/div) VBLX (50mV/div) VFLX (50mV/div) Time (1μs/div) Time (1μs/div) Flash Operating Characteristics (VIN = 3.3V; 1.5x Mode; 70mA/Channel; AC Coupled) VIN (20mV/div) VOUT (50mV/div) VFLX (50mV/div) Time (1μs/div) 8 www.analogictech.com 2848.2008.05.1.0 PRODUCT DATASHEET AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications Functional Block Diagram IN C1+ C1C2+ C2Tri-Mode Charge Pump 1x/1.5x/2x OUT BL1 BL2 BL3 BL4 Control Logic BENS FENS BSET FSET FADE FLTMR 32-Step S2Cwire Backlight Control 16-Step S2Cwire Flash Control FL1 FL2 PGND AGND Any unused sink inputs must be connected to the OUT pin; otherwise, the AAT2848 will operate in 1.5x charge pump mode regardless of the battery voltage or the VF of the diodes connected to the remaining current sinks. The S2Cwire serial interface at the BENS input enables the charge pump and sets the backlight current sink level. Since the current level is programmable, no PWM (pulse width modulation) or additional control circuitry is needed to control LED brightness. This feature greatly reduces the burden on a microcontroller or system IC to manage LED or display brightness, allowing the user to “set it, and forget it.” With its high speed serial interface (>1MHz data rate), the LED current drive can be changed successively to brighten or dim the LEDs in smooth transitions or in abrupt steps, giving the user complete programmability and real-time control of LED brightness. The flash driver uses a tri-mode load switch (1x) and high-efficiency (1.5x and 2x) charge-pump engine. To maximize power conversion efficiency, an internal sensing circuit monitors the voltage required on each constant flash current sink/channel input and sets the load switch and charge pump mode based on the input battery voltage and the flash current sink input voltage. As the battery discharges over time, the charge pump is enabled when any of the two flash current sink inputs nears dropout. The charge pump initially starts in 1.5x mode, and then transitions into 2x mode if the flash current sink inputs nears dropout. Functional Description The AAT2848 is an integrated solution for LCD display applications with a built-in four-channel, 30mA driver for white LED backlight and two 300mA drivers for flash LEDs. The backlight driver uses a dual-mode load switch (1x) and high-efficiency (1.5x) charge-pump engine. To maximize power conversion efficiency, an internal sensing circuit monitors the voltage required on each constant current sink output and sets the load switch and charge pump mode based on the input battery voltage and the current sink input voltage. As the battery discharges over time, the charge pump is enabled when any of the four current sink inputs nears dropout. The charge pump operates in 1.5x mode for backlight operation. The charge pump requires only four external components: two 1μF ceramic capacitors for the charge pump flying capacitors (C1 and C2), one 4.7μF ceramic input capacitor (CIN) and one 2.2μF ceramic charge pump output capacitor (COUT). The four constant current sink inputs (BL1 to BL4) can drive up to four individual LEDs with a maximum current of 30mA each. Because the inputs BL1 to BL4 are true independent constant current sinks, the voltage observed on any single given input will be determined by the difference between VOUT and the actual forward voltage (VF) of the LED being driven. 2848.2008.05.1.0 www.analogictech.com 9 PRODUCT DATASHEET AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications BENS Data 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 The two constant flash current sink outputs (FL1 and FL2) can drive up to two LEDs with a maximum current of 300mA each. Because the outputs FL1 and FL2 are true independent constant flash current sinks, the voltage observed on any single given output will be determined by the difference between VOUT and the actual forward voltage (VF) of the flash LED being driven. Any unused flash sink inputs must be connected to the OUT pin; otherwise, the AAT2848 will operate continuously in 2x charge pump mode regardless of the battery voltage or VF of the remaining diodes connected to the current sinks. The S2Cwire serial interface at the FENS input enables the charge pump and sets the flash current sink levels. The backlight and flash LED currents are controlled by a combination of an external programming resistor from BSET (for backlight) or FSET (for flash) to AGND and the backlight or flash serial S2Cwire interface BENS or FENS. The programming resistor sets the maximum LED current for each channel, and the serial S2Cwire interface controls the LED current relative to the maximum. For backlight LEDs, the maximum channel current can be set up to 30mA with a 10kΩ resistor connected at the BSET pin of the AAT2848. Depending upon the application and the white LEDs chosen, the preferred and most accurate method for a backlight LED current control is to choose RBSET for the maximum backlight current required by the application conditions and use the S2Cwire control to set the backlight current to lower levels. The 32 levels provided by the interface allows a smooth transition between intermediate values. Table 1 and Figure 1 illustrate the current magnitude control from the S2Cwire interface at the BENS pin, based on the maximum current set by the RBSET resistor. For flash white LEDs, the maximum channel current can be set up to 300mA with a 9.53kΩ resistor connected at the FSET pin of the AAT2848. Depending upon the application and the flash white LEDs chosen, the preferred method for flash LED current control is to choose RFSET for the maximum flash current required by the application conditions; use the S2Cwire control to set the backlight current to lower levels. Table 2 and Figure 2 illustrate the current magnitude control from the S2Cwire interface at the FENS pin, based on the maximum current set by the RFSET resistor. IBLX (%) 100 97 93 90 87 83 80 77 73 70 67 63 60 57 54 51 47 44 41 38 35 32 28 25 22 19 16 13 10 7 3 1.5 RBSET (Ω) 15k (20mA Max) Table 1: IBLX (%) vs. BENS Data. 24 BLX Pin Current (mA) 20 16 12 8 4 0 RBSET = 15kΩ 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 BENS Data (#) Figure 1: BL1-BL4 Current Control Profile. 10 www.analogictech.com 2848.2008.05.1.0 PRODUCT DATASHEET AAT2848 ChargePumpTM FENS Data 1 2 3 4 5 6 7 8 High-Current Charge Pump For Backlight Display and Flash Applications RFSET (Ω) FENS Data 9 10 11 12 13 14 15 16 IFLX (%) 100 94 87 81 73 67 60 53 IFLX (%) 46 40 34 27 20 13 6 3 RFSET (Ω) The AAT2848 features separate control interfaces for backlight and flash current control. The backlight current features 32 current steps, each step as a percentage of the maximum backlight current set by the BSET resistor. The flash has 16 current level settings; again, each step as a percentage of the maximum flash current set by the FSET resistor. 14.3k (206mA Max) 14.3k (206mA Max) Flash Timeout Functionality The AAT2848 contains an internal flash mode operation timeout safety circuit. The purpose of this timeout circuit is to prevent any thermal damage to the flash white LED when flash mode operation is unable to be disabled by the embedded microprocessor/microcontroller. Flash mode operation is initiated by strobing the FENS pin lowto-high, this also initiates the flash timer circuit which can be programmed via an external capacitor at the FLTMR pin. The flash timeout period can be calculated using the following equation: Table 2: IFLX (%) vs. FENS Data. 240 FLX Pin Current (mA) 200 160 120 80 40 0 1 3 5 7 9 11 RFSET = 14.3kΩ 13 15 Flash Timeout = 3.9 s/μF · CFLTMR where Flash Timeout is in seconds and CFLTMR is in μF. For example, for a 0.33μF capacitor: FENS Data (#) Figure 2: FL1-FL2 Current Control Profile. S2Cwire Serial Interface The S2Cwire single-wire serial interface used in the AAT2848 exhibits flexible timing in that data can be clocked-in at speeds ranging from 15kHz to higher than 1MHz. The S2Cwire serial interface recognizes rising edges at both the BENS pin and at the FENS pin. For setting the backlight current level, data applied at the AAT2848’s BENS input is decoded into 32 different levels. For setting the flash current level, data applied at the AAT2848’s FENS input is decoded into 16 different levels. After data is supplied, the BENS and the FENS inputs pins are held high to latch the data. Once the data at BENS or FENS has been held in the logic high state for longer than tLAT (500μs), the programmed backlight or flash current becomes active and the internal backlight and flash data registers are latched. To adjust the backlight and/or flash current levels once programmed, the requisite number of rising edges corresponding to the desired code must be applied at the BENS or the FENS pin. Flash Timeout = 3.9 s/μF · 0.33μF = 1.3s To disable the flash timer, connect the FLTMR pin to AGND. When both the BENS and the FENS inputs are strobed and held low for longer than tOFF (500μs), the AAT2848 enters shutdown mode and draws less than 1μA from VIN. Both internal backlight and flash data registers are cleared to zero when in shutdown. Backlight/Flash LED Current Setting The maximum backlight and flash LED currents are set, or programmed, with external resistors. The backlight LED current is programmed by the RBSET resistor with the following equation: RBSET = 300 IBLED RBSET = Backlight LED Current Programming Resistor IBLED = Backlight LED Current 2848.2008.05.1.0 www.analogictech.com 11 PRODUCT DATASHEET AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications t HI t LO t LAT t OFF BENS/FENS 1 2 n-1 n 32/16 Data Reg 0 n-1 0 Figure 3: S2Cwire Serial Interface Timing. The flash LED current is programmed by the RFSET resistor with the following equation: To disable the fade-in/fade-out timer, connect the FADE pin to IN. RFSET = 2880 IFLED Auto Disable Feature The charge pump in AAT2848 is equipped with an autodisable feature for each LED channel. After the IC is enabled and started up, a test current of 1.5mA (typical) is forced through each sink channel. The channel will be disabled if the voltage of that particular BLX or FLX pin does not drop to a certain threshold. This feature is convenient for disabling an unused channel or during a flash/backlight LED failed short event. RFSET = Flash LED Current Programming Resistor IFLED = Flash LED Current Backlight Fade-In/ Fade-Out Functionality The AAT2848 contains internal circuitry that automates fade-in/fade-out operation. Fade functionality simply allows for the turning off/on of the LEDs in a smooth controlled transition. The AAT2848 does not have to be manually programmed to avoid the abrupt changes in lighting when white LED drivers are shutdown/turned-off. The fade-in/fade-out operation occurs only during device on/off transitions. The response time is constant regardless of the LED current level. The backlight fade-in/fade-out response time (tFADE) can be calculated using the following equation: Thermal Protection The charge pump has a built-in thermal protection circuit that will shut down the charge pump if the die temperature rises above the thermal limit as is the case during a short circuit of the OUT pin. tFADE = 57 s/μF · CFADE where tFADE is in seconds and CFADE is in μF. For example, for a 0.047μF capacitor: tFADE = 57 s/μF · 0.047μF = 2.7s 12 www.analogictech.com 2848.2008.05.1.0 PRODUCT DATASHEET AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications This makes quantifying output power a difficult task when taken in the context of comparing to other white LED driver circuit topologies. A better way to quantify total device efficiency is to observe the total input power to the device for a given LED current drive level. The best white LED driver for a given application should be based on trade-offs of size, external component count, reliability, operating range and total energy usage...not just "% efficiency." Applications Information LED Selection The AAT2848 is specifically intended for driving white LEDs. However, the device design will allow the AAT2848 to drive most types of LEDs. 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 BL1 to BL4 and FL1 to FL2 constant current sinks are matched with negligible voltage dependence, the constant current channels will be matched regardless of the specific LED forward voltage (VF) levels. The low-dropout current sinks in the AAT2848 maximize performance and make it capable of driving LEDs with high forward voltages. Multiple channels can be combined to obtain a higher LED drive current without complication. Device Power Efficiency The AAT2848’s backlight charge pump conversion efficiency is defined as the power delivered to the white LED load divided by the input power: η= PLEDs VLED1 · ILED1 + ... + VLEDX · ILEDX = PIN VIN · IIN Device Switching Noise Performance The AAT2848 operates at a fixed frequency of approximately 1MHz to control noise and limit harmonics that can interfere with the RF operation of cellular telephone handsets or other communication devices. Back-injected noise appearing on the input pin of the charge pump is 20mV peak-to-peak, typically ten times less than inductor-based DC/DC boost converter white LED backlight solutions. The AAT2848 soft-start feature prevents noise transient effects associated with in-rush currents during the start up of the charge pump circuit. VLEDx = White LED Forward Voltage (VF) ILEDx = White LED Bias Current (ID) X = Number of Backlight White LEDs The expression to define the estimated ideal backlight efficiency (η) for the AAT2848 in 1x mode is as follows: η= PLEDs VLED1 · ILED1 + ... + VLEDX · ILEDX = PIN VIN · IIN X · VLEDX · ILEDX ; X = 1, 2, 3 or 4 and IIN = X · ILEDX VIN · IIN η= VLEDX η= V IN The AAT2848’s backlight 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 backlight white LED load. With an ideal 1.5x charge pump, the input current is 1.5x of the output current. The expression to define the estimated ideal backlight efficiency (η) for the AAT2848 in 1.5x mode is as follows: Power Efficiency and Device Evaluation Charge-pump efficiency discussion in the following sections accounts only for the efficiency of the charge pump section itself. Due to the unique circuit architecture and design of the AAT2848, it is very difficult to measure efficiency in terms of a percent value comparing input power over output power. Since the AAT2848 outputs are pure constant current sinks/channels and typically drive individual loads, it is difficult to measure the output voltage for a given output (BL1 to BL4 and FL1 to FL2) to derive an overall output power measurement. For any given application, white LED forward voltage levels can differ, yet the output drive current will be maintained as a constant. η= η= PLEDs VLED1 · ILED1 + ... + VLEDX · ILEDX = PIN VIN · IIN X · VLEDX · ILEDX ; X = 1, 2, 3 or 4 and IIN = 1.5(X · ILEDX) VIN · IIN IN VLEDX η = 1.5V 2848.2008.05.1.0 www.analogictech.com 13 PRODUCT DATASHEET AAT2848 ChargePumpTM High-Current Charge Pump For Backlight Display and Flash Applications η= X · VLEDX · ILEDX ; X = 1 or 2 and IIN = 2(X · ILEDX) VIN · IIN The AAT2848’s flash charge pump conversion efficiency is defined as the power delivered to the flash white LED load divided by the input power: V ·I + VLED2 · ILED2 P η = LEDs = LED1 LED1 PIN VIN · IIN VLEDx = White LED Forward Voltage (VF) ILEDx = White LED Bias Current (ID) X = Number of Flash White LEDs The expression to define the estimated ideal flash efficiency (η) for the AAT2848 in 1x mode is as follows: VLEDX η = 2V IN Capacitor Selection Careful selection of the four most critical external capacitors CIN, C1, C2, and COUT are important because they will affect turn on time, output ripple and transient performance (the flash timeout and fade functionality capacitors are not as critical to the normal operation of the AAT2848). Optimum performance will be obtained when low ESR (