Datasheet
AD5258 Data Sheet
Rev. D | Page 20 of 24
DISPLAY APPLICATIONS
CIRCUITRY
A special feature of the AD5258 is its unique separation of
the V
LOGIC
and V
DD
supply pins. The reason for doing this is
to provide greater flexibility in applications that do not always
provide the needed supply voltages.
In particular, LCD panels often require a V
COM
voltage in
the range of 3 V to 5 V. The circuit in Figure 43 is the rare
exception in which a 5 V supply is available to power the
digital potentiometer.
05029-045
A
B
W
R2
10kΩ
R1
70kΩ
R3
25kΩ
V
DD
V
LOGIC
SCL
SDA
GND
–
+
U1
AD8565
3.5V < V
COM
< 4.5V
14.4VVCC (~3.3V) 5V
AD5258
MCU
C1
1µF
R5
10kΩ
R6
10kΩ
Figure 43. V
COM
Adjustment Application
More commonly, only analog 14.4 V and digital logic 3.3 V sup-
plies are available (see Figure 44). By placing discrete resistors
above and below the digital potentiometer, V
DD
can be tapped
off the resistor string itself. Based on the chosen resistor values,
the voltage at V
DD
in this case equals 4.8 V, allowing the wiper to
be safely operated up to 4.8 V. The current draw of V
DD
will not
affect that node’s bias because it is only on the order of
microamps. V
LOGIC
is tied to the microcontroller’s (MCU) 3.3 V
digital supply because V
LOGIC
will draw the 35 mA that is needed
when writing to the EEPROM. It would be impractical to try to
source 35 mA through the 70 kΩ resistor; therefore, V
LOGIC
is
not connected to the same node as V
DD
.
For this reason, V
LOGIC
and V
DD
are provided as two separate
supply pins that can either be tied together or treated inde-
pendently; V
LOGIC
supplies the logic/EEPROM with power, and
V
DD
biases up the A, B, and W terminals for added flexibility.
05029-046
A
B
W
R2
10kΩ
R1
70kΩ
R3
25kΩ
V
DD
V
LOGIC
SCL
SDA
GND
–
+
U1
AD8565
3.5V < V
COM
< 4.5V
14.4V
VCC (~3.3V)
AD5258
MCU
C1
1µF
R5
10kΩ
R6
10kΩ
SUPPLIES POWER
TO BOTH THE MCU
AND THE LOGIC
SUPPLY OF THE
DIGITAL
POTENTIOMETER
Figure 44. Circuitry When a Separate Supply Is Not Available for V
DD
For a more detailed look at this application, refer to the article,
“Simple V
COM
Adjustment uses any Logic-Supply Voltage” in the
September 30, 2004, issue of EDN magazine.