AD5227
Rev. B | Page 13 of 16
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 AD5227 is at minimum,
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
until the U1 GND pin is lifted above 0 V. When V
WB
of the
AD5227 is at its maximum, V
OUT
becomes V
L
+ V
AB
, so the U1
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
that servos the R
SET
voltage and ADP1610’s 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
=
(6)
R
SET
should be small enough to conserve power but large
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.
04419-0-037
SS RT GND
IN
U2
ADP1610
SW
FB
PWM
1.2V
SD
COMP
C
SS
10nF
C
C
390pF
R
C
100kΩ
L1
10μF
D1
C3
10μF
R4
13.5kΩ
D2
D3
D4
V
OUT
C2
10μF
5V
+
–
AD8541
U1
L1–SLF6025-100M1R0
D1–MBR0520LT1
U3
V+
V–
5V
C8
0.1μF
U1
AD5227
BA
10kΩ
R3
200Ω
W
R1
100Ω
R2
1.1kΩ
R
SET
0.25Ω
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.
04419-0-038
CS
CLK
U/D
A
W
B
10kΩ
V
DD
GND
U1
AD5227
C1
1μF
C2
0.1μF
5V
U2
AD8591
+
–
V+
V–
5V
C3
0.1μF
R1
1kΩ
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
04419-0-039
CLK
U/D
B
5V
V
DD
GNDCS
U1
AD5227
+
–
U2
COMPARATOR
SAMPLING_OUTPUT
REF
U3
AD8531
+
–
OUTPUT
OP AMP
Figure 37. 6-Bit Controller
