AD5171

Rev. D | Page 14 of 24

Ignoring the effect of the wiper resistance, the transfer function

is simply

A

W

V

D

D

V

63

)

(

=

(3)

A more accurate calculation, which includes the wiper

resistance effect, yields

A

W

AB

W

AB

W

V

R

R

R

R

D

D

V

2

63

)

(

+

+

=

(4)

Unlike in rheostat mode where the absolute tolerance is high,

potentiometer mode yields an almost ratiometric function of

thus, the tolerance effect is almost cancelled. Although the thin

very different temperature coefficients, the ratiometric adjustment

also reduces the overall temperature coefficient effect to 5 ppm/°C,

Potentiometer mode includes other operations such as op amp

input, feedback resistor networks, and voltage scaling applications.

Terminal A, Terminal W, and Terminal B can, in fact, be input

POWER SUPPLY CONSIDERATIONS

To minimize the package pin count, both the OTP and normal

AD5171. The AD5171 employs fuse link technology that requires

4.75 V to 5.25 V for blowing the internal fuses to achieve a

and 5.5 V after the fuse programming process. As a result, dual

an on-board regulator or rack-mount power supply) must be

rated at 4.75 V to 5.25 V and able to provide a 100 mA current

for 400 ms for successful one-time programming. Once fuse

to allow normal operation at 2.7 V to 5.5 V; the device then

consumes current in the μA range.

2.7V

5V

P1

R1

10kΩ

P2

APPLY FOR OTP ONLY

AD5171

C2

0.1µF

C1

10µF

P1 = P2 = FDV302P, NDS0610

Figure 29. 5 V OTP Supply Isolated from the 2.7 V Normal Operating Supply;

When operating at 2.7 V, use of the bidirectional low threshold

P-Ch MOSFETs is recommended for the isolation of the supply.

As shown in Figure 29, this assumes that the 2.7 V system

voltage is applied first, and the P1 and P2 gates are pulled to

ground, thus turning on P1 and, subsequently, P2. As a result,

and the MOSFETs gates, thus turning off P1 and P2. The OTP

command should be executed at this time to program the

AD5171 while the 2.7 V source is protected. Once the fuse

the setting of the AD5171 is permanently fixed.

The AD5171 achieves the OTP function through blowing

internal fuses. Users should always apply the 4.75 V to

5.25 V one-time program voltage requirement at the first

fuse programming attempt. Failure to comply with this

requirement may lead to a change in the fuse structures,

rendering programming inoperable.

Care should be taken when SCL and SDA are driven from a low

voltage logic controller. Users must ensure that the logic high

for Different Voltage Operation section.

Poor PCB layout introduces parasitics that may affect the fuse

programming. Therefore, it is recommended that a 10 μF

tantalum capacitor be added in parallel with a 1 nF ceramic

chosen for both capacitors are important. This combination of

capacitor values provides both a fast response and larger supply

current handling with minimum supply droop during transients.

As a result, these capacitors increase the OTP programming

success by not inhibiting the proper energy needed to blow the

internal fuses. Additionally, C1 minimizes transient disturbance

and low frequency ripple, while C2 reduces high frequency

noise during normal operation.

ESD PROTECTION

Digital inputs SDA and SCL are protected with a series input

resistor and parallel Zener ESD structures (see Figure 30).

LOGIC

340Ω

GND

Figure 30. ESD Protection of Digital Pins