MCP433X datasheet(83/88 Pages) MICROCHIP

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DS22242A-page 83

MCP433X/435X

B.2

Optimizing Circuit Design for Low-

Voltage Characteristics

The low-voltage nonlinear characteristics can be

minimized by application design. The section will show

two application circuits that can be used to control a

programmable reference voltage (VOUT).

Minimizing the low-voltage nonlinear characteristics is

done by keeping the voltages on the wiper switch

nodes at a voltage where either the NMOS or PMOS

transistor is turned on.

An example of this is if we are using a digital potentiom-

eter for a voltage reference (VOUT). Lets say that we

want VOUT to range from 0.5 * VDD to 0.6 * VDD.

In example implementation #1 (Figure B-15) we

window the digital potentiometer using resistors R1 and

R2. When the wiper code is at full scale the VOUT

voltage will be

 0.6 * VDD, and when the wiper code is

at zero scale the VOUT voltage will be  0.5 * VDD.

Remember that the digital potentiometers RAB variation

must be included. Table B-1 shows that the VOUT volt-

age can be selected to be between 0.455 * VDD and

0.727 * VDD, which includes the desired range. With

respect to the voltages on the resistor network node, at

1.8V the VA voltage would range from 1.29V to 1.31V

while the VB voltage would range from 0.82V to 0.86V.

These voltages cause the wiper resistance to be in the

nonlinear region (see Figure B-12). In Potentiometer

mode, the variation of the wiper resistance is typically

not an issue, as shown by the INL/DNL graph

(Figure 2-7).

In example implementation #2 (Figure B-16) we use

the digital potentiometer in Rheostat mode. The resis-

tor ladder uses resistors R1 and R2 with RBW at the

bottom of the ladder. When the wiper code is at full

scale, the VOUT voltage will be  0.6 * VDD and when

the wiper code is at full scale the VOUT voltage will be

 0.5 * VDD. Remember that the digital potentiometers

RAB variation must be included. Table B-2 shows that

the VOUT voltage can be selected to be between 0.50 *

VDD and 0.687 * VDD, which includes the desired

range. With respect to the voltages on the resistor net-

work node, at 1.8V the VW voltage would range from

0.29V to 0.38V. These voltages cause the wiper

resistance to be in the linear region (see Figure B-12).

FIGURE B-15:

Example Implementation #1.

TABLE B-1:

EXAMPLE #1 VOLTAGE

CALCULATIONS

Variation

Min

Typ

Max

12,000

20,000

RAB

8,000

10,000

12,000

VOUT (@ FS) 0.714 VDD

0.70 VDD

0.727 VDD

VOUT (@ ZS) 0.476 VDD

0.50 VDD

0.455 VDD

0.714 VDD

0.70 VDD

0.727 VDD

0.476 VDD

0.50 VDD

0.455 VDD

Legend: FS – Full Scale, ZS – Zero Scale

VOUT

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