AD5258BRMZ1-R7 datasheet(20/24 Pages) AD

20 / 24 page

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.

10kΩ

70kΩ

25kΩ

VDD

VLOGIC

SCL

SDA

GND

–

AD8565

3.5V < VCOM < 4.5V

14.4V

VCC (~3.3V)

AD5258

MCU

1µF

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.

10kΩ

70kΩ

25kΩ

VDD

VLOGIC

SCL

SDA

GND

–

AD8565

3.5V < VCOM < 4.5V

14.4V

VCC (~3.3V)

AD5258

MCU

1µF

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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