AD5227

Rev. B | Page 13 of 1

6

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

there is no current through D1, so the GND pin of U1 would be

at −1.5 V if U3 were biased with the dual supplies. As a result,

some of the U2 low resistance steps have no effect on the output

supply voltage must be biased with adequate headroom.

Similarly, a PWM signal can be applied at the U1 shutdown pin

for power efficiency. This circuit works well for a single LED.

ADJUSTABLE HIGH POWER LED DRIVER

Figure 35 shows a circuit that can drive three to four high power

LEDs. ADP1610 is an adjustable boost regulator that provides

the voltage headroom and current for the LEDs. The AD5227

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

is regulated around 0.1 V and adjusted by the digital

potentiometer.

SET

R

LED

R

V

I

SET

=

(6)

enough to limit maximum LED current. R3 should also be used

in parallel with AD5227 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.

SS

RT GND

IN

U2

ADP1610

SW

FB

PWM

1.2V

SD

COMP

10nF

390pF

100k

Ω

L1

10

μF

D1

C3

10

μF

R4

13.5k

Ω

D2

D3

D4

C2

10

μF

5V

+

–

AD8541

U1

L1–SLF6025-100M1R0

D1–MBR0520LT1

U3

V+

V–

5V

C8

0.1

μF

U1

AD5227

BA

10k

Ω

R3

200

Ω

W

R1

100

Ω

R2

1.1k

Ω

0.25

Ω

Figure 35. Adjustable Current Source for LEDs in Series

AUTOMATIC LCD PANEL BACKLIGHT CONTROL

With the addition of a photocell sensor, an automatic brightness

control can be achieved. As shown in Figure 36, the resistance

of the photocell changes linearly but inversely with the light

output. The brighter the light output, the lower the photocell

resistance and vice versa. The AD5227 sets the voltage level that

is gained up by U2 to drive N1 to a desirable brightness. With the

photocell acting as the variable feedback resistor, the change in

the light output changes the R2 resistance, therefore causing U2

to drive N1 accordingly to regulate the output. This simple low

cost implementation of the LED controller can compensate for

the temperature and aging effects typically found in high power

LEDs. Similarly, for power efficiency, a PWM signal can be

applied at the gate of N2 to switch the LED on and off without

any noticeable effect.

CS

CLK

U/D

A

W

B

10k

Ω

GND

U1

AD5227

C1

1

μF

C2

0.1

μF

5V

U2

AD8591

+

–

V+

V–

5V

C3

0.1

μF

R1

1k

Ω

R2

5V

PWM

N1

2N7002

WHITE

LED

D1

5V

PHOTOCELL

SD

Figure 36. Automatic LCD Panel Backlight Control

6-BIT CONTROLLER

The AD5227 can form a simple 6-bit controller with a clock

generator, a comparator, and some output components. Figure 37

shows a generic 6-bit controller with a comparator that first

compares the sampling output with the reference level and

outputs either a high or low level to the AD5227 U/D pin. The

AD5227 then changes step at every clock cycle in the direction

indicated by the U/D state. Although this circuit is not as elegant

as the one shown in

, it is self-contained, very easy to

design, and can adapt to various applications.

Figure 36

CLK

U/D

B

5V

GND

CS

U1

AD5227

+

–

U2

COMPARATOR

SAMPLING_OUTPUT

REF

U3

AD8531

+

–

OUTPUT

OP AMP

Figure 37. 6-Bit Controller