AD5253 datasheet(25/28 Pages) AD

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AD5253/AD5254

Rev. 0 | Page 25 of 28

APPLICATIONS

RGB LED LCD BACKLIGHT CONTROLLER

High power (>1 W) RGB LEDs have been improved so

dramatically in efficiency and cost that they are likely to replace

CCFLs (cold cathode florescent lamps) as backlighting sources

in high end LCD panels in the near future. Unlike conventional

LEDs, high power LEDs have a forward voltage of 2 V to 4 V,

and consume more than 350 mA at maximum brightness. The

LED brightness is a linear function of the conduction current

but not the forward voltage. To increase brightness of a given

color, multiple LEDs can be connected in series, rather than in

parallel, to achieve uniform brightness. For example, three red

LEDs configured in series require an average of 6 V to 12 V

voltage headroom, but the circuit operation requires current

control. As a result, Figure 45 shows the implementation of one

high power RGB LED controller using a digital potentiometer

AD5254, a boost regulator, an op amp, and power MOSFETs.

The ADP1610 (U2 in Figure 45) is an adjustable boost regulator

with its output adjusted by the AD5254’s RDAC3. Such an

output should be set high enough for proper operation but low

enough to conserve power. The ADP1610’s 1.2 V band gap

reference is buffered to provide the reference level for the

voltage dividers set by the AD5254’s RDAC0 to RDAC2 and

resistors R2 to R4. For example, by adjusting the AD5254’s

RDAC0, the desirable voltage appears across the sense resistors,

RR. If U2’s output is set properly, op amp U3A and power

MOSFET N1 do whatever is necessary to regulate the current of

the loop. As a result, the current through the sense resistor and

the red LEDs is

I =

(8)

R8 is needed to prevent oscillation.

In addition to the 256 levels of adjustable current/brightness,

users may also apply a PWM signal at U3’s SD pin to achieve

finer brightness resolution or better power efficiency.

AD5253 датащи(PDF) 25 Page - Analog Devices