An external resistor, RSET, is connected between the ISET pin and GND. This resistor sets up a reference current, ISET
, which is internally gained up by 120 within the ADM8845 to produce the ILED currents of up to 30 mA/LED (ILED = ISET × 120 and ISET = 1.18 V/RSET). The ADM8845 uses six individual current sinks to individually sense each LED current with a maximum matching performance of 1%. This current matching perform-ance ensures uniform brightness across a color display. The ADM8845 lets the user control the brightness of the white LEDs with a digital PWM signal applied to CTRL1 and/or CTRL2. The duty cycle of the applied PWM signal determines the brightness of the main and/or sub display backlight white LEDs. The ADM8845 also allows the brightness of the white LEDs to be controlled using a dc voltage (see Figure 22). Soft-start circuitry limits the in-rush current flow at power-up. The ADM8845 is fabricated using CMOS technology for minimal power consumption and is packaged in a 16-lead lead frame chip scale package.
THEORY OF OPERATION
The ADM8845 charge pump driver for LCD white LED back-lights implements a multiple gain charge pump (1×, 1.5×, 2×) to maintain the correct voltage on the anodes of the LEDs over a 2.6 V to 5.5 V (Li-Ion) input supply voltage. The charge pump automatically switches between 1×/1.5×/2× modes, based on the input voltage, to maintain sufficient drive for the LED anodes, with VCC input voltages as low as 2.6 V. It also includes regula-tion of the charge pump output voltage for supply voltages up to 5.5 V. The six LEDs of the ADM8845 are arranged into two groups, main and sub. The main display can have up to four LEDs, FB1 to FB4, and the sub display can have one or two LEDs, FB5 and FB6 (see Figure 23). Two digital input control pins, CTRL1 and CTRL2, control the shutdown operation and the brightness of the main and sub displays (see Table 4).
V04867-0-027
BRIGHT
C11µF
C24.7µR
Rev. C | Page 10 of 20
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