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Data Sheet

AD5253/AD5254

Rev. C | Page 21 of 32

THEORY OF OPERATION

The AD5253/AD5254 are quad-channel digital potentiometers

in 1 kΩ, 10 kΩ, 50 kΩ, or 100 kΩ that allow 64/256 linear resis-

tance step adjustments. The AD5253/AD5254 employ double-

gate CMOS EEPROM technology, which allows resistance

settings and user-defined data to be stored in the EEMEM

registers. The EEMEM is nonvolatile, such that settings remain

when power is removed. The RDAC wiper settings are restored

from the nonvolatile memory settings during device power-up

and can also be restored at any time during operation.

The AD5253/AD5254 resistor wiper positions are determined

by the RDAC register contents. The RDAC register acts like a

scratch-pad register, allowing unlimited changes of resistance

settings. RDAC register contents can be changed using the

device’s serial I2C interface. The format of the data-words and

the commands to program the RDAC registers are discussed in

the I2C Interface section.

The four RDAC registers have corresponding EEMEM memory

locations that provide nonvolatile storage of resistor wiper

position settings. The AD5253/AD5254 provide commands to

store the RDAC register contents to their respective EEMEM

memory locations. During subsequent power-on sequences, the

RDAC registers are automatically loaded with the stored value.

Whenever the EEMEM write operation is enabled, the device

activates the internal charge pump and raises the EEMEM cell

gate bias voltage to a high level; this essentially erases the

current content in the EEMEM register and allows subsequent

storage of the new content. Saving data to an EEMEM register

consumes about 35 mA of current and lasts approximately

26 ms. Because of charge-pump operation, all RDAC channels

may experience noise coupling during the EEMEM writing

operation.

The EEMEM restore time in power-up or during operation is

about 300 µs. Note that the power-up EEMEM refresh time

depends on how fast VDD reaches its final value. As a result, any

supply voltage decoupling capacitors limit the EEMEM restore

time during power-up. For example, Figure 20 shows the

power-up profile of the VDD where there is no decoupling

capacitors and the applied power is a digital signal. The device

initially resets the RDACs to midscale before restoring the

EEMEM contents. The omission of the decoupling capacitors

should only be considered when the fast restoring time is

absolutely needed in the application. In addition, users should

issue a NOP Command 0 immediately after using Command 1

to restore the EEMEM setting to RDAC, thereby minimizing

supply current dissipation. Reading user data directly from

EEMEM does not require a similar NOP command execution.

In addition to the movement of data between RDAC and

EEMEM registers, the AD5253/AD5254 provide other shortcut

commands that facilitate programming, as shown in Table 11.

Table 11. Quick Commands

Command

Description

NOP.

Restore EEMEM content to RDAC. User should

issue NOP immediately after this command to

conserve power.

Store RDAC register setting to EEMEM.

Decrement RDAC 6 dB (shift data bits right).

Decrement all RDACs 6 dB (shift all data bits right).

Decrement RDAC one step.

Decrement all RDACs one step.

Reset EEMEM contents to all RDACs.

Increment RDAC 6 dB (shift data bits left).

Increment all RDACs 6 dB (shift all data bits left).

Increment RDAC one step.

Increment all RDACs one step.

12 to 15

Reserved.

LINEAR INCREMENT/DECREMENT COMMANDS

The increment and decrement commands (10, 11, 5, and 6) are

useful for linear step-adjustment applications. These commands

simplify microcontroller software coding by allowing the

controller to send just an increment or decrement command to

the AD5253/AD5254. The adjustments can be directed to a

single RDAC or to all four RDACs.

±6 dB ADJUSTMENTS

(DOUBLING/HALVING WIPER SETTING)

The AD5253/AD5254 accommodate ±6 dB adjustments of the

RDAC wiper positions by shifting the register contents to left/

right for increment/decrement operations, respectively. Com-

mand 3, Command 4, Command 8, and Command 9 can be

used to increment or decrement the wiper positions in 6 dB

steps synchronously or asynchronously.

Incrementing the wiper position by +6 dB essentially doubles

the RDAC register value, whereas decrementing the wiper

position by –6 dB halves the register content. Internally, the

AD5253/AD5254 use shift registers to shift the bits left and

right to achieve a ±6 dB increment or decrement. The

maximum number of adjustments is nine and eight steps for

incrementing from zero scale and decrementing from full scale,

respectively. These functions are useful for various audio/video

level adjustments, especially for white LED brightness settings

in which human visual responses are more sensitive to large

adjustments than to small adjustments.

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