MCP4021 datasheet(27/64 Pages) MICROCHIP

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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 (RAB), where the nominal resistance is

defined as the resistance between terminal A and

terminal B. The four nominal resistances are 2.1 k

Ω,

Ω, 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:

RS CALCULATION

EQUATION 4-2:

RWB CALCULATION

1 LSb is the ideal resistance difference between two

successive codes. If we use N = 1 and RW = 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, VIH and VIHH. This enables

additional commands without requiring additional input

pins. The high-voltage commands (VIHH 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).

N = 63

N = 62

N = 61

N = 1

N = 0

RW (1)

01h

Analog

Mux

RW (1)

00h

RW (1)

3Dh

RW (1)

3Eh

RW (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 k

Ω) more.

RAB

---------

RWB

RABN

-------------- RW

N = 0 to 63 (decimal)

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