Data Sheet AD5228
Rev. B | Page 15 of 18
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 V
F
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.
04422-0-042
PUSH-UP
BUTTON
PUSH-DOWN
BUTTON
A
W
B
10k
V
DD
GNDPRE
PU
PD
U1
AD5228
C1
1F
C2
0.1F
5V
U2
AD8591
+
–
V+
V–
SD
5V
C3
0.1F
R1
6
WHITE
LED
D1
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 V
WB
of the AD5228 divided by R
SET
.
SET
WB
D1
R
V
I
(7)
The U1 ADP3333ARM-1.5 is a 1.5 V LDO that is lifted above
or lowered below 0 V. When V
WB
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 V
WB
of the
AD5228 is at its maximum, V
OUT
becomes VL + V
AB
, 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.
04422-0-043
PUSH-UP
BUTTON
PUSH-DOWN
BUTTON
A
W
B
10k
R1
418k
V
DD
GND
PRE
PU
PD
U2
AD5228
5V
5V
U3
AD8591
+
–
V+
V–
5V
PWM
V
IN
V
OUT
GND
U1
ADP3333
ARM-1.5
SD
R
SET
0.1
VL
ID
D1
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 R
SET
voltage and the ADP1610 FB pin 1.2 V band gap
reference voltage. As the loop is set, the voltage across R
SET
is
regulated around 0.1 V and adjusted by the digital
potentiometer.
SET
R
LED
R
V
I
SET
(8)
R
SET
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.
04422-0-044
SS RT GND
IN
U2
ADP1610
SW
FB
PWM
1.2V
SD
COMP
C
SS
10nF
C
C
390pF
R
C
100k
L1
10F
D1
C3
10F
R4
13.5k
D2
D3
D4
V
OUT
C2
10F
5V
+
–
AD8541
U1
L1–SLF6025-100M1R0
D1–MBR0520LT1
U3
V+
V–
5V
C8
0.1F
U1
AD5228
BA
10k
R3
200
W
R1
100
R2
1.1k
R
SET
0.25
Figure 43. Adjustable Current Source for LEDs in Series
