AD5253 datasheet(26/32 Pages) AD

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AD5253/AD5254

Data Sheet

Rev. C | Page 26 of 32

APPLICATIONS INFORMATION

RGB LED BACKLIGHT CONTROLLER

FOR LCD PANELS

Because high power (>1 W) RGB LEDs offer superior color

quality compared with cold cathode florescent lamps (CCFLs)

as backlighting sources, it is likely that high-end LCD panels

will employ RGB LEDs as backlight 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 of the forward voltage. To increase

the 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 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

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

Resistor R2 to Resistor 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 can also apply a PWM signal at U3’s SD pin to achieve

finer brightness resolution or better power efficiency.

AD5253 Datasheet(PDF) 26 Page - Analog Devices