AD5259 datasheet(20/24 Pages) AD

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AD5259

Data Sheet

Rev. C | Page 20 of 24

DISPLAY APPLICATIONS

CIRCUITRY

A special feature of the AD5259 is its unique separation of the

VLOGIC and VDD supply pins. The separation provides greater

flexibility in applications that do not always provide 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 46 is the rare excep-

tion in which a 5 V supply is available to power the digital

potentiometer.

10k

Ω

70k

Ω

25k

Ω

VDD

VLOGIC

SCL

SDA

GND

–

AD8565

3.5V < VCOM < 4.5V

14.4V

VCC (~3.3V)

AD5259

MCU

µF

10k

Ω

10k

Ω

Figure 46. VCOM Adjustment Application

In the more common case shown in Figure 47, only analog

14.4 V and digital logic 3.3 V supplies are available. By placing

discrete resistors above and below the digital potentiometer,

VDD can now 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 all the way 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 MCU’s 3.3 V digital supply because VLOGIC 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, 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 supplying the logic/EEPROM with power,

and VDD biasing up the A, B, and W terminals for added

flexibility.

10k

Ω

70k

Ω

25k

Ω

VDD

VLOGIC

SCL

SDA

GND

–

AD8565

3.5V < VCOM < 4.5V

14.4V

VCC (~3.3V)

AD5259

MCU

µF

10k

Ω

10k

Ω

SUPPLIES POWER

TO BOTH THE

MICRO AND THE

LOGIC SUPPLY OF

THE DIGITAL POT

Figure 47. 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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