MAX8831
Detailed Description
The MAX8831 integrates a 60mA, 28V PWM DC-DC
step-up converter with five low-dropout LED current reg-
ulators for display and keypad backlighting in cell
phones, PDAs, and other portable devices. The IC pro-
vides up to 90% efficiency over the entire input voltage
range of 2.7V to 5.5V. The step-up converter operates at
a fixed 2MHz switching frequency, enabling the use of
very small external components to achieve a compact
circuit area. For improved efficiency, the step-up con-
verter automatically operates in pulse-skipping mode at
light loads. Figure 1 displays the functional diagram of
the MAX8831.
Each current regulator accommodates up to 9 series
LEDs (depending on LED string forward voltage), and
is independently programmed using an I
2
C interface.
Two of the current regulators (LED1, LED2) are intend-
ed to support display backlight functions and are pro-
grammable up to 25.25mA using a 128-step
logarithmic dimming scheme.
High-Efficiency, White LED Step-Up Converter
with I
2
C Interface in 2mm x 2mm WLP
8 _______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
A1 LED1
25mA LED Current Regulator. Connect LED1 to the cathode of the LED1 diode string. LED1 is high
impedance in shutdown.
A2 GND
Analog Ground. Connect GND directly to PGND at the output capacitor as close as possible to the
IC.
A3 LED3
5mA LED Current Regulator. Connect LED3 to the cathode of the LED3 diode string. LED3 is high
impedance in shutdown.
A4 COMP
Step-Up Compensation Node. Connect a 0.22µF ceramic capacitor from COMP to GND. The applied
COMP capacitance stabilizes the converter and sets the soft-start time. COMP discharges to GND
through a 20kΩ resistance when in shutdown. See the Compensation Network Selection section for
more details.
B1 LED2
25mA LED Current Regulator. Connect LED2 to the cathode of the LED2 diode string. LED2 is high
impedance in shutdown.
B2 LED5
5mA LED Current Regulator. Connect LED5 to the cathode of the LED5 diode string. LED5 is high
impedance in shutdown.
B3 LED4
5mA LED Current Regulator. Connect LED4 to the cathode of the LED4 diode string. LED4 is high
impedance in shutdown.
B4 SCL I
2
C Serial-Clock Input
C1, D1 LX
Step-Up Converter Switching Node. Connect an inductor between IN and LX. LX is high impedance
in shutdown.
C2, D2 PGND Power Ground. Connect PGND directly to GND at the output capacitor as close as possible to the IC.
C3 SDA I
2
C Serial-Data I/O. Data written on rising edge of SCL, data read on falling edge of SCL.
C4 V
DD
I
2
C Input Buffer Supply. Connect a 0.1µF capacitor from V
DD
to GND as close as possible to the IC.
Connect V
DD
to a 1.6V to 5.5V supply to enable the I
2
C interface. Drive V
DD
low to place the IC in
shutdown.
D3 IN
Power-Supply Input. Bypass IN to GND with a 1µF ceramic capacitor placed as close as possible to
the IC.
D4 OUT
LED Overvoltage Protection Input. Connect OUT to the positive terminal of the output capacitor. OUT
monitors voltage at the LEDs. If an overvoltage condition is detected, all LED_ current regulators and
the step-up converter are shut down. OUT is high impedance during shutdown.
