ADM8845 查看數據表（PDF） - Analog Devices

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ADM8845

Charge Pump Driver for LCD White LED Backlights

Analog Devices 

ADM8845

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 ADM8845’s six LEDs 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).

Table 4. Shutdown Truth Table

CTRL1 CTRL2 LED Shutdown Operation

0

0

Sub Display Off / Main Display Off

0

1

Sub Display Off / Main Display On

1

0

Sub Display On / Main Display Off

1

1

Sub Display On / Main Display On

C1

C2

1µF

1µF

VCC ADM8845

C4

4.7µF

CHARGE PUMP

1×/1.5×/2× MODE

CTRL1

CTRL2

CONTROL

LOGIC

OSC

VREF

ISET

RSET

LED

CURRENT

CONTROL

CIRCUIT

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.

ADM8845

ISET

VBRIGHT

0V–2.5V

R = 15kΩ

RSET = 13.4kΩ

Figure 22. PWM Brightness Control Using a DC Voltage Applied to VBRIGHT

VOUT

C3

2.2µF

MAIN

SUB

FB1

FB2

FB3

FB4

FB5

FB6

CURRENT CONTROLLED SINKS

GND

Figure 23. Functional Block Diagram

Rev. 0 | Page 10 of 20

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