AD5258BRMZ1-R71 datasheet(20/24 Pages) AD

20 / 24 page

AD5258

Rev. 0 | 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 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 45 is the rare exception

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)

AD5258

MCU

µF

10k

Ω

10k

Ω

Figure 45. VCOM 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, 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)

AD5258

MCU

µF

10k

Ω

10k

Ω

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 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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