Datasheet

© 2006 Microchip Technology Inc. DS21945E-page 27
MCP4021/2/3/4
4.0 GENERAL OVERVIEW
The MCP402X devices are general purpose digital
potentiometers intended to be used in applications
where a programmable resistance with moderate
bandwidth is desired.
Applications generally suited for the MCP402X devices
include:
• Set point or offset trimming
• Sensor calibration
• Selectable gain and offset amplifier designs
• Cost-sensitive mechanical trim pot replacement
The digital potentiometer is available in four nominal
resistances (R
AB
), where the nominal resistance is
defined as the resistance between terminal A and
terminal B. The four nominal resistances are 2.1 kΩ,
5kΩ, 10 kΩ and 50 kΩ.
There are 63 resistors in a string between terminal A
and terminal B. The wiper can be set to tap onto any of
these 63 resistors thus providing 64 possible settings
(including terminal A and terminal B).
Figure 4-1 shows a block diagram for the resistive
network of the device. Equation 4-1 shows the
calculation for the step resistance, while Equation 4-2
illustrates the calculation used to determine the
resistance between the wiper and terminal B.
FIGURE 4-1: Resistor Block Diagram.
EQUATION 4-1: R
S
CALCULATION
EQUATION 4-2: R
WB
CALCULATION
1 LSb is the ideal resistance difference between two
successive codes. If we use N = 1 and R
W
= 0 in
Equation 4-2, we can calculate the step size for each
increment or decrement command.
The MCP4021 device offers a voltage divider
(potentiometer) with all terminals available on pins.
The MCP4022 is a true rheostat, with terminal A and
the wiper (W) of the variable resistor available on pins.
The MCP4023 device offers a voltage divider (potenti-
ometer) with terminal B connected to ground.
The MCP4024 device is a rheostat device with terminal
A of the resistor floating, terminal B connected to
ground, and the wiper (W) available on pin.
The MCP4021 can be externally configured to
implement any of the MCP4022, MCP4023 or
MCP4024 configurations.
4.1 Serial Interface
A 2-wire synchronous serial protocol is used to
increment or decrement the digital potentiometer’s
wiper terminal. The Increment/Decrement (U/D)
protocol utilizes the CS
and U/D input pins. Both inputs
are tolerant of signals up to 12.5V without damaging
the device. The CS pin can differenciate between two
high-voltage levels, V
IH
and V
IHH
. This enables
additional commands without requiring additional input
pins. The high-voltage commands (V
IHH
on the CS pin)
are similar to the standard commands, except that they
control (enable, disable, ...) the state of the non-volatile
WiperLock technolgy feature.
The simple U/D
protocol uses the state of the U/D pin
at the falling edge of the CS
pin to determine if
Increment or Decrement mode is desired. Subsequent
rising edges of the U/D pin move the wiper.
The wiper value will not underflow or overflow. The new
wiper setting can be saved to EEPROM, if desired, by
selecting the state of the U/D
pin during the rising edge
of the CS
pin.
The non-volatile wiper enables the MCP4021/2/3/4 to
operate stand alone (without microcontroller control).
R
S
A
R
S
R
S
R
S
B
N = 63
N = 62
N = 61
N = 1
N = 0
R
W
(1)
W
01h
Analog
Mux
R
W
(1)
00h
R
W
(1)
3Dh
R
W
(1)
3Eh
R
W
(1)
3Fh
Note 1: The wiper resistance is tap dependent.
That is, each tap selection resistance
has a small variation. This variation
effects the smaller resistance devices
(2.1 kΩ) more.
R
S
R
AB
63
---------=
R
WB
R
AB
N
63
-------------- R
W
+=
N = 0 to 63 (decimal)