AD5228BUJZ100-R2 datasheet(15/20 Pages) AD

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AD5228

Rev. 0 | Page 15 of 20

APPLICATIONS

MANUAL ADJUSTABLE LED DRIVER

The AD5228 can be used in many electronics-level adjustments

such as LED drivers for LCD panel backlight controls. Figure 41

shows a manually adjustable LED driver. The AD5228 sets the

voltage across the white LED D1 for the brightness control.

Since U2 handles up to 250 mA, a typical white LED with VF of

3.5 V requires a resistor, R1, to limit U2 current. This circuit is

simple but not power efficient. The U2 shutdown pin can be

toggled with a PWM signal to conserve power.

PUSH-UP

BUTTON

PUSH-DOWN

BUTTON

10k

Ω

VDD

GND

PRE

AD5228

µF

0.1

µF

AD8591

–

V+

V–

0.1

µF

Ω

WHITE

LED

PWM

Figure 41. Low Cost Adjustable LED Driver

ADJUSTABLE CURRENT SOURCE FOR LED DRIVER

Because LED brightness is a function of current rather than of

forward voltage, an adjustable current source is preferred as

shown in Figure 42. The load current can be found as the VWB of

the AD5228 divided by RSET.

SET

(7)

The U1 ADP3333ARM-1.5 is a 1.5 V LDO that is lifted above or

lowered below 0 V. When VWB of the AD5228 is at its minimum,

there is no current through D1, so the GND pin of U1 is at –1.5 V

if U3 is biased with the dual supplies. As a result, some of the U2

low resistance steps have no effect on the output until the U1

GND pin is lifted above 0 V. When VWB of the AD5228 is at its

maximum, VOUT becomes VL + VAB, so the U1 supply voltage

must be biased with adequate headroom. Similarly, PWM signal

can be applied at the U1 shutdown pin for power efficiency.

PUSH-UP

BUTTON

PUSH-DOWN

BUTTON

10k

Ω

418k

Ω

VDD

GND

PRE

AD5228

AD8591

–

V+

V–

PWM

VIN

VOUT

GND

ADP3333

ARM-1.5

RSET

0.1

Ω

Figure 42. Adjustable Current Source for LED Driver

ADJUSTABLE HIGH POWER LED DRIVER

The previous circuit works well for a single LED. Figure 43

shows a circuit that can drive three to four high power LEDs.

The ADP1610 is an adjustable boost regulator that provides the

voltage headroom and current for the LEDs. The AD5228 and

the op amp form an average gain of 12 feedback network that

servos the RSET voltage and the ADP1610 FB pin 1.2 V band gap

reference voltage. As the loop is set, the voltage across RSET is

regulated around 0.1 V and adjusted by the digital

potentiometer.

SET

LED

SET

(8)

RSET should be small enough to conserve power but large enough

to limit maximum LED current. R3 should also be used in par-

allel with AD5228 to limit the LED current within an achievable

range. A wider current adjustment range is possible by lowering

the R2 to R1 ratio as well as changing R3 accordingly.

RT GND

ADP1610

PWM

1.2V

COMP

CSS

10nF

390pF

100k

Ω

µF

13.5k

Ω

VOUT

µF

–

AD8541

L1–SLF6025-100M1R0

D1–MBR0520LT1

V+

V–

0.1

µF

AD5228

10k

Ω

200

Ω

100

Ω

1.1k

Ω

RSET

0.25

Ω

Figure 43. Adjustable Current Source for LEDs in Series

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