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19-4266; Rev 0; 9/08
T EVALUATION KI AVAILABLE
Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer
General Description
The MAX8879 provides complete light management for main display backlight, subdisplay backlight (or RGB indicator), and white LED camera flash with regulated constant current up to 610mA total. By utilizing adaptive 1x/1.5x/2x charge-pump modes and very-lowdropout current regulators, it achieves high efficiency over the full 1-cell Li+ battery input voltage range. The 1MHz fixed-frequency switching allows for tiny external components while the regulation scheme is optimized to ensure low EMI and low input ripple. An integrated derating function protects the LEDs from overheating during high ambient temperatures. The MAX8879 features an internally trimmed reference to set the maximum LED current. An I2C serial port is used for on/off control and setting the LED currents in 32 linear steps for main, sub/RGB, and movie. When using the RGB indicator, the I2C port provides 32k colors and programmable ramp-up/down rates. The camera flash for flash-mode operation is enabled by an active-low signal on the FSH pin. A safety timer is activated on the falling edge of FSH, which has an I2Cadjustable (programmable) period of no timer, 0.5s, 1.0s, or 2.0s (default). If the safety timer period expires, both flash and movie modes are disabled.
Features
Guaranteed 610mA Continuous Drive Capability 4 LEDs at 30mA Each for Main Display 3 LEDs at 30mA Each for Sub or RGB 400mA Total for Flash 2-Wire I2C Serial Port 5-Bit (32-Step) Linear Dimming 32k Colors Ramp-Up/Down Rates 92% Peak/83% Avg Efficiency (PLED/PBATT) Adaptive 1x/1.5x/2x Mode Switchover 0.3% (typ) LED Current Accuracy and Matching Low Input Ripple and EMI Low 0.1A Shutdown Current Output Overvoltage Protection I2C Programmable Flash Safety Timer Thermal Derating Function Protects LEDs 24-Pin, 4mm x 4mm Thin QFN Package
MAX8879
Ordering Information
PART MAX8879ETG+ TEMP RANGE -40C to +85C PINPACKAGE 24 Thin QFN 4mm x 4mm
Applications
Cell Phones and Smartphones PDAs, Digital Cameras, Camcorders Displays with Up to 11 LEDs
Pin Configuration appears at end of data sheet. +Denotes a lead-free/RoHS-compliant package.
Typical Operating Circuit
1F INPUT 2.7V TO 5.5V 4.7F 1F OUTPUT UP TO 610mA 10F MAIN SUB OR RGB FLASH
C1P IN PIN GND
C1N
C2P
C2N OUT
MAX8879 VLOGIC 0.1F VDD
I2C PORT ON/OFF AND BRIGHTNESS FLASH ON/OFF
SCK SDA FSH PGND
M1 M2 M3 M4 S1 S2 S3 F1 F2 F3 F4
________________________________________________________________ Maxim Integrated Products
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer MAX8879
ABSOLUTE MAXIMUM RATINGS
VDD, IN, PIN, SCK, SDA, FSH, OUT to GND .........-0.3V to +6.0V M_, S_, F_ to GND ....................................-0.3V to (VOUT + 0.3V) C1N, C2N to GND .......................................-0.3V to (VIN + 0.3V) C1P, C2P to GND...-0.3V to greater of (VOUT + 0.3V) or (VIN + 0.3V) PGND to GND .......................................................-0.3V to +0.3V OUT Short Circuit to GND ..........................................Continuous Continuous Power Dissipation (TA = +70C) 24-Pin Thin QFN (derate 20.8mW/C above +70C)...1666mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN = VPIN = 3.6V, VDD = 1.8V, VGND = VPGND = 0V, temperature derating disabled, TA = -40C to +85C, typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER IN Operating Voltage VDD Operating Range Undervoltage-Lockout Threshold Undervoltage-Lockout Hysteresis Output Overvoltage-Protection Threshold IN + PIN No-Load Supply Current IN + PIN Shutdown Supply Current VDD Quiescent Current VOUT rising 1.5x or 2x mode 10% setting, 1x mode, flash off All LEDs off, FSH = SDA = SCK = VDD, I2C ready All LEDs off, SDA = SCK = VDD, I2C ready Startup into 1x mode Soft-Start Time LED Current Derating Function Start Temperature LED Current Derating Function Slope LED Current SUB Output Accuracy (Note 2) LED Current FLASH and MAIN Output Accuracy Maximum M_, S_, F_ Sink Current LED Dropout Voltage 1.5x and 2x Mode Regulation Voltage 1x to 1.5x and 1.5x to 2x Mode Transition Threshold VM_, VS_, VF_ falling Startup into 1.5x mode Startup into 2x mode Temperature derating enabled TA = +40C to +85C, temperature derating enabled Default current setting, TA = +25C Default current setting, TA = -40C to +85C Default current setting (Note 2) M_, S_ F_ 100% LED setting (Note 3) M_, S_ F_ -2 -5 -5 28.5 95 0.3 30.0 100 40 40 150 100 90 90 TA = +25C TA = +85C TA = +25C TA = +85C 4.75 VIN rising or falling CONDITIONS MIN 2.7 1.6 2.25 50 5.00 4.0 0.35 0.7 0.8 0.1 0.1 0.5 1.0 1.5 +40 -1.7 0.3 +2 +5 +5 C %/C % % mA mV mV mV ms 1 5 5.25 6.5 TYP MAX 5.5 5.5 2.60 UNIT V V V mV V mA A A
2
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VPIN = 3.6V, VDD = 1.8V, VGND = VPGND = 0V, temperature derating disabled, TA = -40C to +85C, typical values are at TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Input Voltage Mode Transition Hysteresis M_, S_, F_ Leakage in Shutdown OUT Pulldown Resistance in Shutdown Maximum OUT Current Open-Loop OUT Resistance Switching Frequency SDA = 111xxx00 S1, S2, S3 (RGB) Full-Scale Ramp Time SDA = 111xxx01 SDA = 111xxx10 SDA = 111xxx11 Logic-Input High Voltage Logic-Input Low Voltage Logic-Input Current SDA Output Low Voltage I2C Clock Frequency Bus-Free Time Between START and STOP Hold Time Repeated START Condition SCK Low Period SCK High Period Setup Time Repeated START Condition SDA Hold Time SDA Setup Time Setup Time for STOP Condition Thermal Shutdown Thermal-Shutdown Hysteresis tBUF tHD_STA tLOW tHIGH tSU_STA tHD_DAT tSU_DAT tSU_STO 1.3 0.6 1.3 0.6 0.6 0 100 0.6 0.1 0.2 0.2 0.1 -0.01 50 0.1 +160 20 VDD = 1.6V to 5.5V VDD = 1.6V to 5.5V VIL = 0V or VIH = 5.5V ISDA = 3mA TA = +25C TA = +85C 0.01 0.1 0.03 0.4 400 0.7 x VDD 0.3 x VDD 1 All LEDs off, FSH = VDD TA = +25C TA = +85C 610 0.5 1.5 2.0 1 29 218 219 220 V V A V kHz s s s s s s ns s C C s 2.5 3.5 4.1 MHz CONDITIONS MIN TYP 150 0.01 0.1 5 1 MAX UNIT mV A k mA
MAX8879
All LEDs off, FSH = VDD VIN 3.2V, VOUT = 3.9V 1x mode (VIN - VOUT) / IOUT 1.5x mode (1.5 x VIN - VOUT) / IOUT 2x mode (2 x VIN - VOUT) / IOUT
Note 1: All devices are 100% production tested at TA = +25C. Limits over the operating temperature range are guaranteed by design. Note 2: LED current specification includes both accuracy and matching tolerance. Note 3: Dropout voltage is defined as the M2 or F3 to GND voltage at which current into M2 or F3 drops 10% from the value at 0.2V. All other current regulators are tested functionally by the accuracy test and guaranteed for low dropout by design.
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer MAX8879
Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
EFFICIENCY vs. SUPPLY VOLTAGE DRIVING 4 MAIN LEDs
MAX8879 toc01
INPUT CURRENT vs. SUPPLY VOLTAGE DRIVING LUMILEDS LXCL-PWF1 FLASH
MAX8879 toc02
INPUT CURRENT vs. SUPPLY VOLTAGE DRIVING 4 MAIN LEDs AND RGB LEDs
SUPPLY VOLTAGE FALLING 30mA/LED 350 INPUT CURRENT (mA) 300 250 15mA/LED 200 150 100 1.9mA/LED 50 0 400
MAX8879 toc03 MAX8879 toc06
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 SUPPLY VOLTAGE FALLING 1.9mA/LED 30mA/LED 15mA/LED
900 800 700 INPUT CURRENT (mA) 600 500 400 300 200 100 0 200mA 100mA 400mA
450
5.5
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
INPUT CURRENT vs. SUPPLY VOLTAGE DRIVING 4 MAIN LEDs
MAX8879 toc04
INPUT CURRENT vs. SUPPLY VOLTAGE DRIVING RGB LEDs
SUPPLY VOLTAGE FALLING 400 350 INPUT CURRENT (mA) 300 250 200 150 100 15mA/LED 1.9mA/LED 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 30mA/LED
MAX8879 toc05
LED CURRENT MATCHING vs. SUPPLY VOLTAGE (MAIN AND RGB AT FULL CURRENT)
31.2 30.9 30.6 LED CURRENT (mA) 30.3 30.0 29.7 29.4 29.1 28.8 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 SUPPLY VOLTAGE (V) GREEN VF = 2V BLUE VF = 4.2V 4 WHITE VF = 3.4V RED VF = 1.9V
450 400 350 INPUT CURRENT (mA) 300 250 200 150 100 50 0 2.7 3.1 3.5 1.9mA/LED 15mA/LED 30mA/LED
SUPPLY VOLTAGE FALLING
450
50 0 3.9 4.3 4.7 5.1 5.5 SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
LED CURRENT MATCHING vs. SUPPLY VOLTAGE (MAIN AND RGB AT 1.9mA/LED)
MAX8879 toc07
LED CURRENT vs. TEMPERATURE
THERMAL DERATING OFF
MAX8879 toc08
1.96 1.94 1.92 LED CURRENT (mA) 1.90 1.88 1.86 1.84 1.82 1.80 2.7 3.1 3.5 3.9 4.3 4.7 5.1 GREEN VF = 1.9V BLUE VF = 3.4V 4 WHITE VF = 2.9V RED VF = 1.7V
30 25 LED CURRENT (mA) 20 15
4 MAIN OR 4 FLASH LEDs
THERMAL DERATING ON 10 5 0
5.5
-40
-15
10
35
60
85
SUPPLY VOLTAGE (V)
TEMPERATURE (C)
4
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer
Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
OPERATING WAVEFORMS 1X MODE
MAX8879 toc09
MAX8879
OPERATING WAVEFORMS 1.5X MODE
MAX8879 toc10
VOUT
50mV/div (AC-COUPLED) 50mV/div (AC-COUPLED) 100mA/div 0 1s/div
VOUT
50mV/div (AC-COUPLED) 50mV/div (AC-COUPLED) 100mA/div 0 1s/div
VIN
VIN
IIN
IIN
OPERATING WAVEFORMS 2X MODE
MAX8879 toc11
MAIN BACKLIGHT STARTUP WAVEFORM
MAX8879 toc12
VSDA VOUT 50mV/div (AC-COUPLED) VOUT VIN IIN 50mV/div (AC-COUPLED) 100mA/div IIN 0 1s/div 1ms/div
5V/div
1V/div
0 200mA/div 0
MAIN STARTUP WITH RGB ALREADY ON
MAX8879 toc13
FLASH STARTUP WAVEFORM
MAX8879 toc14
VSDA VOUT
5V/div 1V/div
VSDA
5V/div
1V/div VOUT
0 IIN 200mA/div 0 1ms/div IIN
0 200mA/div 0 1ms/div
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer MAX8879
Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
STROBE STARTUP WAVEFORM WITH MAIN ALREADY ON
MAX8879 toc15
MAIN BRIGHTNESS CHANGE (1.9mA/LED TO 30mA/LED)
MAX8879 toc16
V STB VOUT
5V/div 1V/div
VSDA
5V/div
VOUT
1V/div
0 IIN 200mA/div 0 1ms/div IOUT
0 100mA/div 0 1ms/div
LINE-TRANSIENT RESPONSE (4V TO 3.3V TO 4V)
MAX8879 toc17
LINE-TRANSIENT RESPONSE WITH MODE TRANSITION (3.3V TO 2.7V TO 3.3V)
MAX8879 toc18
VIN VOUT
1V/div 4V 1V/div
VIN VOUT
1V/div 4V 1V/div
200mA/div IOUT 0 100s/div IOUT MOVIE OR FLASH MODE ACTIVE 100s/div 200mA/div 0
RGB RAMP WAVEFORMS (BLUE-TO-YELLOW TRANSITION)
MAX8879 toc19
RGB RAMP WAVEFORMS (BLUE-TO-YELLOW TRANSITION)
MAX8879 toc20
FULL-BRIGHTNESS 1048ms RAMP TIME IRED
HALF-BRIGHTNESS DEFAULT RAMP TIME 20mA/div 0 IRED 10mA/div 0 IGREEN 10mA/div 0 10mA/div IBLUE 0 0 40ms/div
IGREEN
20mA/div 0 20mA/div
IBLUE
200ms/div
6
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer
Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 NAME PIN IN GND VDD M4 M3 M2 M1 F4 F3 F2 F1 S3 S2 S1 Flash Logic Input. Drive FSH low to turn on the flash LEDs (F1-F4) at the current specified in the internal flash register. The built-in flash timer is activated by synchronizing with the active-low falling edge on the pin. Drive FSH high to turn off the flash LEDs. This logic-high/low signal is only dedicated for enabling or deactivating flash LEDs. If the embedded flash timer expires in a certain time, it is not reactivated unless a falling edge is monitored again. I2C Clock Input. Data is read on the rising edge of SCK. I2C Data Input. Data is read on the rising edge of SCK. Transfer Capacitor 1 Negative Connection. Connect a 1F ceramic capacitor from C1N to C1P. C1N is shorted to IN during shutdown. Transfer Capacitor 1 Positive Connection. Connect a 1F ceramic capacitor from C1N to C1P. During shutdown, if VOUT > VIN, C1P is shorted to OUT. If VOUT < VIN, C1P is shorted to IN. Power Ground. Charge-pump switching ground. Connect to GND and EP as close to the IC as possible. Output. Connect a 10F ceramic capacitor from OUT to PGND. The anodes of all the LEDs connect to OUT. OUT is pulled to ground through an internal 5k resistor in shutdown. Transfer Capacitor 2 Positive Connection. Connect a 1F ceramic capacitor from C2N to C2P. During shutdown, if VOUT > VIN, C2P is shorted to OUT. If VOUT < VIN, C2P is shorted to IN. Transfer Capacitor 2 Negative Connection. Connect a 1F ceramic capacitor from C2N to C2P. C2N is shorted to IN during shutdown. Exposed Paddle. Connect to GND and PGND directly under the IC. Any combination of M_, S_, and F_ can be connected together to drive higher current LEDs. LED Cathode Connections. Current flowing into these pins is based on the internal I2C dimming registers. The charge pump regulates the lowest LED cathode voltage to 0.15V. Grounding any of these pins forces output overvoltage protection mode causing OUT to pulse on and off at approximately 5V. To avoid constantly operating in overvoltage protection mode, any unused LED cathode connection (M_, S_, or F_) must be connected to OUT. This disables the corresponding current regulator. These pins are high impedance in shutdown. M1 through M4 are for main display backlights. S1 through S3 are for subdisplay backlights or one RGB LED indicator. F1 through F4 are for LED flash. FUNCTION Power-Supply Voltage Input. Connect PIN to IN. Connect a 4.7F ceramic capacitor from PIN to PGND. The input voltage range is 2.7V to 5.5V. PIN is high impedance during shutdown. Analog Supply Voltage Input. Connect IN to PIN. The input voltage range is 2.7V to 5.5V. IN is high impedance during shutdown. Ground. Connect GND to system ground and the ground side of the input bypass capacitor as close to the IC as possible. Logic-Input Supply Voltage. Connect VDD to the logic supply driving SDA and SCK. Connect a 0.1F ceramic capacitor from VDD to GND.
MAX8879
16
FSH
17 18 19 20 21 22 23 24 --
SCK SDA C1N C1P PGND OUT C2P C2N EP
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer MAX8879
C1 1F C2 1F
INPUT 2.7V TO 5.5V
C1P PIN IN
C1N
C2P
C2N OUT OUT COUT 10F
1x/1.5x/2x REGULATING CHARGE PUMP
CIN 4.7F PGND ERROR AMP 1 REFERENCE 0.15V 0.6V
OVP
MAX8879
GND 1.25V
SELECT MIN SCK SDA FSH VDD 1.6V TO 5.5V CVDD 0.1F S3 D3 BRIGHTNESS CONTROL FLASH CONTROL S2 D2 S1 D1 I2C AND CONTROL
+ -
+ -
+ -
ERROR AMP 2 M1 RSET M2 M3 M4 D5 D6 D7 D4
+ -
+ -
+ -
+ -
F1 F2 FSH F3 FLASH CONTROL TIMER (NO TIMER, 0.5s, 1.0s, 2.0s) + - + - + - + - F4
D8 D9 D10 D11
Figure 1. Functional Diagram
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer
Detailed Description
The MAX8879 charge pump operates in three modes to maintain high efficiency over a wide supply voltage range. The IC automatically selects between these three modes as described in the 1x/1.5x/2x Mode Switchover section. Current-sinking LED cathode connections are provided to drive four main (M_) and three sub (S_) LEDs at a regulated current up to 30mA each. The sub LED connections can be used for either subdisplay backlighting or one RGB indicator. The IC also contains four flash LED connections (F_) that sink up to 100mA each. These LED connections can be connected together in any combination to provide increased current up to 610mA total. An I2C serial port is used for on/off control and setting the LED currents in 32 linear steps. When using the RGB indicator, the I2C port provides 32k colors and programmable ramp-up/down rates. The camera flash may be turned on/off through a separate digital logic input (FSH) with the maximum flash safety timer programmed to an I2C-selectable value. The movie mode may be turned on/off only though the I2C interface. The flash and movie modes have separately adjustable brightness levels through separate I2C registers. threshold, and the MAX8879 starts switching in 1.5x mode. When the input voltage is rising, the transition from 1.5x to 1x is made when VIN is greater than VOUT. When the MAX8879 is running in 1.5x mode and the input voltage is decreased, the lowest LED cathode voltage crosses the 100mV switchover threshold again. At this point, the MAX8879 changes to the 2x chargepump mode. With the input voltage rising and the MAX8879 in 2x mode, the IC changes to 1.5x mode once VIN is greater than 2/3 of the output voltage.
MAX8879
Soft-Start
The MAX8879 includes soft-start circuitry to limit inrush current at turn-on and mode transitions. When starting up, the output capacitor is charged directly from the input with a ramped current source (with no chargepump action) until the output voltage is near the input voltage. After 512s, if all the LED cathodes are not above 100mV, the MAX8879 switches to 1.5x mode with the LED output current ramped from 1/32 to the programmed current in 1/32 steps. After another 512s, if all the LED cathodes are not above 100mV, the MAX8879 switches to 2x mode, once again ramping the LED current from 1/32 to the programmed current in 1/32 steps. Any time the output voltage is less than 1.25V, the soft-start routine is reset to the 1x state. Thus, the startup time is 512s, 1024s, or 1536s, depending on what mode is required after the completion of startup.
1x/1.5x/2x Mode Switchover
When the input voltage is higher than the required output voltage needed to drive the LEDs, the MAX8879 pulls OUT up to the input voltage (in 1x mode), while still regulating the LED current with the current regulators. As the input voltage drops, the lowest LED cathode voltage falls below the 100mV switchover
Output-Current Settings
The output currents for the main, sub, and flash current regulators are set using the I2C serial interface (see the I2C Interface section). The current for the four main LEDs is always equal and set with a single command.
Table 1. Control Data Byte
SDA CONTROL BYTE FUNCTION C2 On/Off Control Main Brightness Sub1 Brightness Sub2 Brightness Sub3 Brightness Movie Brightness Flash Brightness Other Functions 0 0 0 0 1 1 1 1 COMMAND C1 0 0 1 1 0 0 1 1 C0 0 1 0 1 0 1 0 1 D4 Main D3 Sub3 DATA D2 Sub2 32-steps, 30mA/LED max 32-steps, 30mA max 32-steps, 30mA max 32-steps, 30mA max 16-steps, 51.0 mA/LED max, I2C enable 32-steps, 100mA/LED max, active-low enable on FSH Flash-on time No timer (00), 0.5s (01), 1.0s (10), 2.0s (11) Temp derate RGB ramp rate D1 Sub1 D0 Movie
Notes: C2 is MSB and D0 is LSB. Default in bold italics.
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer MAX8879
Table 2. Data and LED Currents
DATA D4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 D3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 D2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 D1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 D0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 MAIN 0.9 1.9 2.8 3.8 4.7 5.6 6.6 7.5 8.4 9.4 10.3 11.3 12.2 13.1 14.1 15.0 15.9 16.9 17.8 18.8 19.7 20.6 21.6 22.5 23.4 24.4 25.3 26.3 27.2 28.1 29.1 30.0 SUB1 0.9 1.9 2.8 3.8 4.7 5.6 6.6 7.5 8.4 9.4 10.3 11.3 12.2 13.1 14.1 15.0 15.9 16.9 17.8 18.8 19.7 20.6 21.6 22.5 23.4 24.4 25.3 26.3 27.2 28.1 29.1 30.0 LED CURRENT (mA) SUB2 0.9 1.9 2.8 3.8 4.7 5.6 6.6 7.5 8.4 9.4 10.3 11.3 12.2 13.1 14.1 15.0 15.9 16.9 17.8 18.8 19.7 20.6 21.6 22.5 23.4 24.4 25.3 26.3 27.2 28.1 29.1 30.0 SUB3 0.9 1.9 2.8 3.8 4.7 5.6 6.6 7.5 8.4 9.4 10.3 11.3 12.2 13.1 14.1 15.0 15.9 16.9 17.8 18.8 19.7 20.6 21.6 22.5 23.4 24.4 25.3 26.3 27.2 28.1 29.1 30.0 MOVIE 3.3 6.5 9.7 12.9 16.2 19.4 22.6 25.8 28.9 32.1 35.4 38.6 41.6 44.7 47.9 51.0 3.3 6.5 9.7 12.9 16.2 19.4 22.6 25.8 28.9 32.1 35.4 38.6 41.6 44.7 47.9 51.0 FLASH 3.3 6.5 9.7 12.9 16.2 19.4 22.6 25.8 28.9 32.1 35.4 38.6 41.6 44.7 47.9 51.0 54.1 57.2 60.3 63.4 66.3 69.6 72.7 75.8 78.8 81.9 84.9 87.9 91.0 94.0 97.0 100.0
Note: Defaults in bold italics.
10
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer
Table 3. Control Data Byte (Hexadecimal)
CONTROL BYTE (HEXADECIMAL) MAIN 20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F 30 31 32 33 34 35 36 37 38 39 3A 3B 3C 3D 3E 3F SUB1 40 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F 50 51 52 53 54 55 56 57 58 59 5A 5B 5C 5D 5E 5F SUB2 60 61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F 70 71 72 73 74 75 76 77 78 79 7A 7B 7C 7D 7E 7F SUB3 80 81 82 83 84 85 86 87 88 89 8A 8B 8C 8D 8E 8F 90 91 92 93 94 95 96 97 98 99 9A 9B 9C 9D 9E 9F MOVIE A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 AA AB AC AD AE AF B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 BA BB BC BD BE BF FLASH C0 C1 C2 C3 C4 C5 C6 C7 C8 C9 CA CB CC CD CE CF D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 DA DB DC DD DE DF MAIN 0.9 1.9 2.8 3.8 4.7 5.6 6.6 7.5 8.4 9.4 10.3 11.3 12.2 13.1 14.1 15.0 15.9 16.9 17.8 18.8 19.7 20.6 21.6 22.5 23.4 24.4 25.3 26.3 27.2 28.1 29.1 30.0 SUB1 0.9 1.9 2.8 3.8 4.7 5.6 6.6 7.5 8.4 9.4 10.3 11.3 12.2 13.1 14.1 15.0 15.9 16.9 17.8 18.8 19.7 20.6 21.6 22.5 23.4 24.4 25.3 26.3 27.2 28.1 29.1 30.0 LED CURRENT (mA) SUB2 0.9 1.9 2.8 3.8 4.7 5.6 6.6 7.5 8.4 9.4 10.3 11.3 12.2 13.1 14.1 15.0 15.9 16.9 17.8 18.8 19.7 20.6 21.6 22.5 23.4 24.4 25.3 26.3 27.2 28.1 29.1 30.0 SUB3 0.9 1.9 2.8 3.8 4.7 5.6 6.6 7.5 8.4 9.4 10.3 11.3 12.2 13.1 14.1 15.0 15.9 16.9 17.8 18.8 19.7 20.6 21.6 22.5 23.4 24.4 25.3 26.3 27.2 28.1 29.1 30.0 MOVIE 3.3 6.5 9.7 12.9 16.2 19.4 22.6 25.8 28.9 32.1 35.4 38.6 41.6 44.7 47.9 51.0 3.3 6.5 9.7 12.9 16.2 19.4 22.6 25.8 28.9 32.1 35.4 38.6 41.6 44.7 47.9 51.0 FLASH 3.3 6.5 9.7 12.9 16.2 19.4 22.6 25.8 28.9 32.1 35.4 38.6 41.6 44.7 47.9 51.0 54.1 57.2 60.3 63.4 66.3 69.6 72.7 75.8 78.8 81.9 84.9 87.9 91.0 94.0 97.0 100.0
MAX8879
Note: Defaults in bold italics.
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer MAX8879
Table 4. RGB Ramp Rate
CONTROL BYTE (HEXADECIMAL) E0 E1 E2 E3 RGB RAMP RAMP TIME FROM OFF TO RATE (A/s) FULL BRIGHTNESS (ms) 58.6 0.114 0.0572 0.0286 0.512 262 524 1048
The LED cathode connections (M_, S_, and F_) can be connected together in any combination to allow the use of higher current LEDs. For example, to drive a single flash LED at up to 400mA, connect F1, F2, F3, and F4 together to the cathode of the flash LED. To avoid constantly operating in overvoltage protection mode, any unused LED cathode connection (M_, S_, or F_) must be connected to OUT. This disables the corresponding current regulator.
*Default in bold italics.
RGB Color and Ramp-Rate Settings
The three sub LED currents are controlled independently by the I2C interface, allowing for use of a common anode RGB LED. Thirty-two programmable brightness levels (5 bits) per LED provide a total of 32k colors. To smooth the transition between different color/brightness settings, a controlled ramp is used when the sub LED current level is changed, when the sub LEDs are enabled, and when the LEDs are disabled. The ramp rate is set to one of four settings with the I2C interface (see Table 4).
The currents for the three sub LEDs are set independently, allowing them to drive an RGB LED. The current-level settings for both the main and sub LEDs range from 0.9mA to 30mA, defaulting to 15mA each (see Tables 1, 2, and 3). The flash LEDs are controlled by pulsing the FSH input low. There are two registers in the MAX8879 to set the flash/movie current level. The Movie register sets the F1-F4 LED current when the I2C interface is used to activate the LEDs and the Flash register sets the F1-F4 LED current when the FSH input is pulsed low. The current level settings for the flash LEDs ranges from 3.3mA to 100mA, with a default of 25.8mA (see Tables 1, 2, and 3). The Flash register has priority over the Movie register when both movie and flash are on. In the event that the flash safety timer period expires, the flash LEDs are turned off and the movie mode ON/OFF control bit is reset to 0. This is to prevent the MAX8879 from going back into movie mode in the event of a safety timer violation.
Temperature-Derating Function
The MAX8879 contains a temperature-derating function that automatically limits the LED current at high temperatures in accordance with the recommended derating curve of popular white LEDs. The derating function enables the safe usage of higher LED current at room temperature, thus reducing the number of LEDs required to backlight the display. In camera-light applications, the derating circuit protects the LEDs and PC board from overheating. The derating circuit limits the LED current
A
tLOW
B
tHIGH
C
D
E
F
G
H
I
J
K
L
M
SCK
SDA
tSU:STA tHD:STA A = START CONDITION B = MSB OF ADDRESS CLOCKED INTO SLAVE C = LSB OF ADDRESS CLOCKED INTO SLAVE D = R/W BIT CLOCKED INTO SLAVE E = SLAVE PULLS SMBDATA LINE LOW
tSU:DAT
tHD:DAT F = ACKNOWLEDGE BIT CLOCKED INTO MASTER G = MSB OF DATA CLOCKED INTO SLAVE (OP/SUS BIT) H = LSB OF DATA CLOCKED INTO SLAVE I = SLAVE PULLS SMBDATA LINE LOW
tSU:STO tBUF J = ACKNOWLEDGE CLOCKED INTO MASTER K = ACKNOWLEDGE CLOCK PULSE L = STOP CONDITION, DATA EXECUTED BY SLAVE M = NEW START CONDITION
Figure 2. I2C Timing Diagram
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer
by reducing the LED current above +40C by approximately 1.7%/C. The typical derating function characteristic is shown in the Typical Operating Characteristics. The temperature derating function is enabled/disabled using the I2C interface and is off by default.
Output Overvoltage Protection
In case an LED fails or the cathode is shorted to GND, the output overvoltage protection limits the output to 5V. When the MAX8879 detects the output voltage rising above 5V, it shuts off the charge pump. The charge pump restarts once the output voltage has dropped to 4.9V. To avoid constantly operating in overvoltage protection, any unused LED cathode connection (M_, S_, or F_) must be connected to OUT; this disables the corresponding current regulator.
MAX8879
I2C Interface
An 2-wire serial interface is provided on the MAX8879 to control LED brightness, flash, temperature deration, and RGB ramp rate. The serial interface consists of a serial data line (SDA) and a serial clock line (SCK). Standard I2C write-byte commands are used. Figure 2 shows a timing diagram for the I2C protocol. The MAX8879 is a slave-only device, relying upon a master to generate a clock signal. The master (typically a microprocessor) initiates data transfer on the bus and generates SCK to permit data transfer. A master device communicates to the MAX8879 by transmitting the proper 8-bit address (0x9A) followed by the 8-bit control byte. Each 8-bit control byte consists of a 3-bit command code and 5 bits of data (see Table 1). Each transmit sequence is framed by a START (A) condition and a STOP (L) condition (see Figure 2). Each word transmitted over the bus is 8 bits long and is always followed by an acknowledge clock pulse. I 2C
Thermal Shutdown
Thermal shutdown limits total power dissipation in the MAX8879. When the junction temperature exceeds +160C, the MAX8879 turns off, allowing the IC to cool. The MAX8879 turns on and begins soft-start after the junction temperature cools by 20C. This results in a pulsed output during continuous thermal-overload conditions.
Applications Information
Input Ripple
In 1x mode, the input ripple of the MAX8879 is negligible. When the charge pump is switching in 1.5x or 2x mode, the input ripple depends on the load current and the output impedance of the source supply. The worstcase ripple occurs when the charge pump is operating in 1.5x mode. The switching waveforms in the Typical Operating Characteristics show the typical input ripple. For noise-sensitive applications, input ripple can be reduced by increasing the input capacitance.
Shutdown Mode
When all the LEDs are off, the MAX8879 turns off the charge pump and enters low-power shutdown mode. When in shutdown, OUT is pulled to GND by an internal 5k resistor, discharging the output capacitor. IN and PIN are high impedance during shutdown, but the I2C interface (powered from VDD) remains active. To enter shutdown, send control byte 0x00 to the I2C interface, and drive STB high. To exit shutdown, enable any of the LEDs with the I2C interface.
Capacitor Selection
Ceramic capacitors are recommended due to their small size, low cost, and low ESR. Select ceramic capacitors that maintain their capacitance over temperature and DC bias. Capacitors with X5R or X7R temperature characteristics generally perform well. Recommended values are shown in the Typical Operating Circuit. Using a largervalue input capacitor helps to reduce input ripple (see the Input Ripple section).
FSH Logic Input
The FSH input is used to control the flash LEDs with the embedded safety timer. When FSH is driven low, the flash LEDs are driven to the current set in the Flash register. Driving FSH low activates the flash safety timer with the pre-set time (programmed period). The safety timer is not activated until an active-low signal is applied to FSH. This function also serves as a protection feature to avoid thermal damage to the flash LEDs because of software-related errors. This pin is the flashdedicated control pin.
PCB Layout and Routing
The MAX8879 is a high-frequency switched-capacitor regulator. For best circuit performance, use a solid ground plane and place the capacitors as close to the IC as possible. Connect the exposed pad to GND and PGND, and allow sufficient copper area for cooling the IC. Refer to the MAX8879 evaluation kit for an example PCB layout.
Movie-Mode Operation
The movie current level is set through the I2C interface according to the current scale in Movie register. The ON/OFF of movie LEDs is only through I2C interface.
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer MAX8879
Chip Information
SDA SCK FSH S1 S2 18 C1N 19 C1P 20 PGND 21 OUT 22 C2P 23 + C2N 24 1 PIN 2 IN 3 GND 4 VDD 5 M4 6 M3 7 M2 17 16 15 14 13 12 11 10 F1 F2 F3 F4 M1 S3
TOP VIEW
PROCESS: BiCMOS
MAX8879
9 8
THIN QFN 4mm x 4mm
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Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE 8 TQFN PACKAGE CODE T2444-4 DOCUMENT NO. 21-0139
MAX8879
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24L QFN THIN.EPS
Charge Pump for Backlight/Flash/RGB LEDs with Safety Timer MAX8879
Package Information (continued)
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2008 Maxim Integrated Products is a registered trademark of Maxim Integrated Products. Inc.

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