ad5227 Analog Devices, Inc., ad5227 Datasheet - Page 13

• Manufacturer Part Number: ad5227
• Description: 64-position Up/down Control Digital Potentiometer
• Manufacturer: Analog Devices, Inc.

The U1 ADP3333ARM-1.5 is a 1.5 V LDO that is lifted above or
lowered below 0 V. When V
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
AD5227 is at its maximum, V
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
gap reference voltage. As the loop is set, the voltage across R
regulated around 0.1 V and adjusted by the digital
potentiometer.
R
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.
5V
SET
L1–SLF6025-100M1R0
D1–MBR0520LT1
10µF
should be small enough to conserve power but large
C2
I
LED
Figure 35. Adjustable Current Source for LEDs in Series
=
13.5kΩ
V
R
R
SET
R4
SET
SET
390pF
voltage and ADP1610’s FB pin 1.2 V band
PWM
1.2V
100kΩ
C
R
C
C
1.1kΩ
0.1µF
R2
U1
C8
WB
AD8541
SD
FB
COMP
SS
V+
V–
ADP1610
OUT
10nF
C
5V
of the AD5227 is at minimum,
SS
U3
U2
RT GND
becomes V
+
–
IN
SW
B
W
10kΩ
200Ω
R3
AD5227
L1
10µF
U1
A
D1
L
+ V
100Ω
AB
WB
R1
0.25Ω
C3
10µF
R
, so the U1
SET
of the
D2
D3
D4
V
SET
OUT
Rev. 0 | Page 13 of 16
(6)
is
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.
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 Figure 36, it is self-contained, very easy to
design, and can adapt to various applications.
C1
1µF
Figure 36. Automatic LCD Panel Backlight Control
C2
0.1µF
5V
COMPARATOR
CS
CLK
U/D
U2
V
DD
CLK
U/D
V
5V
DD
–
+
Figure 37. 6-Bit Controller
CS
AD5227
A
10kΩ
B
GND
U1
W
B
AD5227
GND
SAMPLING_OUTPUT
REF
1kΩ
R1
U1
+
PHOTOCELL
–
AD8591
–
+
AD8531
5V
U2
V+
V–
U3
OP AMP
PWM
0.1µF
SD
C3
R2
5V
N1
OUTPUT
AD5227
5V
WHITE
LED
2N7002
D1