ADM8843EB-EVALZ 데이터 시트보기 (PDF) - Analog Devices
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ADM8843EB-EVALZ Datasheet PDF : 16 Pages
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Data Sheet
ADM8843
THEORY OF OPERATION
The ADM8843 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 among 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 regulation
of the charge pump output voltage for supply voltages up to 5.5 V.
The ADM8843’s four LEDs are arranged into two groups, main
and sub. The main display can have up to three LEDs (FB1 to
FB3), and the sub display has one LED (FB4) (see Figure 18).
The CTRL1 and CTRL2 digital input control pins control the
shutdown operation and the brightness of the main and sub
displays (see Table 5).
Table 5. 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 ADM8843
C4
4.7µF
CHARGE PUMP
1×/1.5×/2× MODE
CTRL1
CTRL2
CONTROL
LOGIC
OSC
VREF
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 ADM8843 to produce
ILED currents of up to 20 mA/LED (ILED = ISET × 120 and ISET =
1.18 V/RSET). The ADM8843 uses four separate current sinks to
sense each LED current individually with a typical matching
performance of 0.3%. This current matching performance
ensures uniform brightness across a color display.
The ADM8843 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 ADM8843 also allows the brightness of the white
LEDs to be controlled using a dc voltage (see Figure 17). Soft
start circuitry limits the in-rush current flow at power-up. The
ADM8843 is fabricated using CMOS technology for minimal
power consumption and is packaged in a 16-lead lead frame
chip scale package.
ADM8843
ISET
VBRIGHT
0V–2.5V
R = 15kΩ
RSET = 13.4kΩ
Figure 17. PWM Brightness Control Using a DC Voltage Applied to VBRIGHT
VOUT
C3
2.2µF
MAIN
SUB
FB1
FB2
FB3
FB4
ISET
RSET
LED
CURRENT
CONTROL
CIRCUIT
CURRENT CONTROLLED SINKS
GND
Figure 18. Functional Block Diagram
Rev. E | Page 9 of 16
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