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AD5258 Datasheet(PDF) 20 Page - Analog Devices

Part No. AD5258
Description  Nonvolatile, IC-Compatible 64-Position, Digital Potentiometer
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

AD5258 Datasheet(HTML) 20 Page - Analog Devices

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AD5258
Data Sheet
Rev. D | Page 20 of 24
DISPLAY APPLICATIONS
CIRCUITRY
A special feature of the AD5258 is its unique separation of
the VLOGIC and VDD 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 VCOM 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.
A
B
W
R2
10kΩ
R1
70kΩ
R3
25kΩ
VDD
VLOGIC
SCL
SDA
GND
+
U1
AD8565
3.5V < VCOM < 4.5V
14.4V
VCC (~3.3V)
5V
AD5258
MCU
C1
1µF
R5
10kΩ
R6
10kΩ
Figure 43. VCOM 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, VDD can be tapped
off the resistor string itself. Based on the chosen resistor values,
the voltage at VDD in this case equals 4.8 V, allowing the wiper to
be safely operated up to 4.8 V. The current draw of VDD will not
affect that node’s bias because it is only on the order of
microamps. VLOGIC is tied to the microcontroller’s (MCU) 3.3 V
digital supply because VLOGIC 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, VLOGIC is
not connected to the same node as VDD.
For this reason, VLOGIC and VDD are provided as two separate
supply pins that can either be tied together or treated inde-
pendently; VLOGIC supplies the logic/EEPROM with power, and
VDD biases up the A, B, and W terminals for added flexibility.
A
B
W
R2
10kΩ
R1
70kΩ
R3
25kΩ
VDD
VLOGIC
SCL
SDA
GND
+
U1
AD8565
3.5V < VCOM < 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 VDD
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.


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