Rainbow Electronics DS3903 - Manual (Page 11)

DS3903
Triple 128-Position Nonvolatile
Digital Potentiometer
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the appropriate device address bit, and the read/write
bit, the slave device outputs an acknowledge signal on
the SDA line.
Applications Information
Power-Supply Decoupling
To achieve the best results when using the DS3903,
decouple the power supply with a 0.01µF or 0.1µF
capacitor. Use a high-quality ceramic surface-mount
capacitor if possible. Surface-mount components mini-
mize lead inductance, which improves performance,
and ceramic capacitors tend to have adequate high-
frequency response for decoupling applications.
Write Protection
The write-protect pin has an internal pullup resistor. To
be able to adjust the potentiometers’ position, this pin
must be grounded. This pin can be left floating or con-
nected to V
CC to write protect the potentiometer posi-
tions.
Wiper Resistance and Wiper Current Limit
Two substantial differences between digital poten-
tiometers and mechanical potentiometers are the wiper
resistance and the wiper current limit. The wiper resis-
tance (R
W ) is a result of the interconnecting materials
on the IC between the internal resistive elements and
the wiper pin. This can be modelled by using an ideal
potentiometer, with a resistance of R
W connected
between the ideal wiper and wiper terminal of the digi-
tal potentiometer. One final note about the wiper resis-
tance is that it has a high temperature coefficient
(approximately +3000PPM), which can be noticeable in
certain circuit configurations.
The wiper current limit (I W ) is also due to the intercon-
necting materials between the internal resistive ele-
ments and the wiper terminal. While it may be possible
to exceed this value for a short period without prob-
lems, exceeding the wiper current limit is a long-term
reliability problem.
Both characteristics can be minimized in designs by
connecting the wiper terminal to high-impedance
loads. This reduces both the current through the wiper
and the voltage drop across the wiper resistance.
Using a Potentiometer as a
Variable Resistor
There are two ways to make a digital potentiometer into
a variable resistor. The first is to short the wiper terminal
to the high- or low-side terminal. This places wiper
resistance in parallel with the resistance from the wiper
to the high or low side of the potentiometer. The advan-
tage of this method is that it reduces the current
through the wiper, which is advantageous if the current
is approaching the wiper current limit. The disadvan-
tage is that the wiper resistance makes the resistance
versus position nonlinear, particularly for low-resistance
values.
The second way is to attach the wiper terminal, and
either the low- or high-side terminal. The unattached
terminal is connected to the wiper by the resistance
internal to the part, and stays at the same voltage as
the wiper. This method provides a linear resistance ver-
sus position function, but it limits the current through
the resistance to I
W since there is no current load shar-
ing between the wiper resistance and the paralleled
resistive elements.
Both configurations are heavily influenced by the wiper
resistance, particularly over temperature, where its tem-
perature coefficient noticeably affects the resistor’s value.
Chip Information
TRANSISTOR COUNT: 10,793
SUBSTRATE CONNECTED TO GROUND
Package Information
For the latest package outline information, go to www.maxim-ic.
com/packages.