Datasheet

AD5258
Rev. 0 | 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 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 45 is the rare exception
in which a 5 V supply is available to power the digital
potentiometer.
05029-006
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 45. V
COM
Adjustment Application
In the more common case shown in Figure 46, only analog 14.4 V
and digital logic 3.3 V supplies are available. By placing discrete
resistors above and below the digital pot, V
DD
can now 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 all the way 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 MCU’s 3.3 V digital
supply because V
LOGIC
will draw the 35 mA which is needed
when writing to the EEPROM. It would be impractical to try
and 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
supplying the logic/EEPROM with power,
and V
DD
biasing up the A, B, and W terminals for added
flexibility.
05029-007
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)
AD5258
MCU
C1
1µF
R5
10kΩ
R6
10kΩ
SUPPLIES POWER
TO BOTH THE
MICRO AND THE
LOGIC SUPPLY OF
THE DIGITAL POT
Figure 46. Circuitry When a Separate Supply is Not Available for V
DD
For a more detailed look at this application, refer to the article,
“Simple VCOM Adjustment uses any Logic Supply Voltage” in
the September 30, 2004, issue of EDN magazine.