AD5228BUJZ50-R2 datasheet(12/20 Pages) AD

12 / 20 page

AD5228

Rev. A | Page 12 of 20

If the PU button is held for longer than 1 second, continuously

holding it activates autoscan mode such that the AD5228

increments by four RWB steps per second (see

Figure 3

Whenever the maximum RWB (= RAB) is reached, RWB stops

incrementing regardless of the state of the PU pin. Any continu-

ous holding of the PU pin to logic-low simply elevates the supply

current.

When both PU and PD buttons are pressed, RWB decrements

until it stops at zero scale.

All the preceding descriptions apply to PD operation. Due to

the tolerance of the internal RC oscillator, all the timing

information given previously is based on the typical values,

which can vary ±30%.

The AD5228 debouncer is carefully designed to handle common

pushbutton tactile switches. Other switches that have excessive

bounces and duration are not suitable to use in conjunction

with the AD5228.

RS = RAB/32

RDAC

UP/DOWN

CTRL AND

DECODE

Figure 34. AD5228 Equivalent RDAC Circuit

PROGRAMMING THE DIGITAL POTENTIOMETERS

Rheostat Operation

If only the W-to-B or W-to-A terminals are used as variable

resistors, the unused terminal can be opened or shorted with W.

Such operation is called rheostat mode and is shown in Figure 35.

Figure 35. Rheostat Mode Configuration

The end-to-end resistance, RAB, has 32 contact points accessed

by the wiper terminal, plus the B terminal contact if RWB is used.

Pushing the PU pin discretely increments RWB by one step. The

total resistance becomes RS + RW as shown in

. The

change of RWB can be determined by the number of discrete

Figure 34

executions provided that its maximum setting is not reached

during operation. ∆RWB can, therefore, be approximated as

⎟

⎠

⎞

⎜

⎝

⎛

(1)

⎟

⎠

⎞

⎜

⎝

⎛

−

(2)

where:

PU is the number of push-up executions.

PD is the number of push-down executions.

RAB is the end-to-end resistance.

RW is the wiper resistance contributed by the on-resistance of

the internal switch.

Similar to the mechanical potentiometer, the resistance of the

RDAC between the Wiper W and Terminal A also produces a

complementary resistance, RWA. When these terminals are used,

the B terminal can be opened or shorted to W. RWA can also be

approximated if its maximum and minimum settings are not

reached.

(

)

⎟

⎠

⎞

⎜

⎝

⎛

−

(

)

⎟

⎠

⎞

⎜

⎝

⎛

−

(4)

Note that Equations 1 to 4 do not apply when PU and PD = 0

execution.

Because in the lowest end of the resistor string, a finite wiper

resistance is present, care should be taken to limit the current

flow between W and B in this state to a maximum pulse current

of no more than 20 mA. Otherwise, degradation or possible

destruction of the internal switches can occur.

The typical distribution of the resistance tolerance from device

to device is process lot dependent, and ±20% tolerance is possible.

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