AD5172 datasheet(14/22 Pages) AD

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Data Sheet

AD5172/AD5173

THEORY OF OPERATION

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Rev. J | 14 of 22

Figure 38. Detailed Functional Block Diagram

The AD5172/AD5173 are 256-position, digitally controlled variable

resistors (VRs) that employ fuse link technology to achieve memory

retention of the resistance setting.

An internal power-on preset places the wiper at midscale during

power-on. If the OTP function is activated, the device powers up at

the user-defined permanent setting.

ONE-TIME PROGRAMMING (OTP)

Prior to OTP activation, the AD5172/AD5173 presets to midscale

during initial power-on. After the wiper is set to the desired position,

the resistance can be permanently set by programming the T

bit high, with the proper coding (see Table 8 and Table 9), and

one-time VDD_OTP. The fuse link technology of the AD517x family

of digital potentiometers requires VDD_OTP to be between 5.6 V

and 5.8 V to blow the fuses to achieve a given nonvolatile setting.

However, during operation, VDD can be 2.7 V to 5.5 V. As a result,

an external supply is required for one-time programming. The user

is allowed only one attempt to blow the fuses. If the user fails

to blow the fuses during this attempt, the structure of the fuses

can change such that they may never be blown, regardless of

the energy applied during subsequent events. For details, see the

Power Supply Considerations section.

The device control circuit has two validation bits, E1 and E0, that

can be read back to check the programming status (see Table

7). Users should always read back the validation bits to ensure

that the fuses are properly blown. After the fuses are blown, all

fuse latches are enabled upon subsequent power-on; therefore,

the output corresponds to the stored setting. Figure 38 shows a

detailed functional block diagram.

Table 7. Validation Status

Status

Ready for programming.

Fatal error. Some fuses are not blown. Do not retry. Discard this

unit.

Successful. No further programming is possible.

PROGRAMMING THE VARIABLE RESISTOR

AND VOLTAGE

Rheostat Operation

The nominal resistance of the RDAC between Terminal A and

Terminal B is available in 2.5 kΩ, 10 kΩ, and 100 kΩ. The nominal

resistance (RAB) of the VR has 256 contact points accessed by

the wiper terminal and the B terminal contact. The 8-bit data in the

RDAC latch is decoded to select one of the 256 possible settings.

Figure 39. Rheostat Mode Configuration

Assuming a 10 kΩ part is used, the first connection of the wiper

starts at the B terminal for Data 0x00. Because there is a 160 Ω

wiper contact resistance, such a connection yields a minimum of

320 Ω (2 × 160 Ω) resistance between Terminal W and Terminal B.

The second connection is the first tap point, which corresponds

to 359 Ω (RWB = RAB/256 + 2 × RW = 39 Ω + 2 × 160 Ω) for

Data 0x01. The third connection is the next tap point, representing

398 Ω (2 × 39 Ω + 2 × 160 Ω) for Data 0x02, and so on. Each LSB

data value increase moves the wiper up the resistor ladder until the

last tap point is reached at 10281 Ω (RAB − 1 LSB + 2 × RW).

Figure 40. AD5172/AD5173 Equivalent RDAC Circuit

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