MCP6421 datasheet(17/34 Pages) MICROCHIP

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DS25165A-page 17

MCP6421

4.0

APPLICATION INFORMATION

The

MCP6421 op amp is manufactured using

Microchip’s state-of-the-art CMOS process. This op

amp is unity gain stable and suitable for a wide range

of general purpose applications.

4.1

Rail-to-Rail Input

4.1.1

PHASE REVERSAL

The MCP6421 op amp is designed to prevent phase

reversal, when the input pins exceed the supply

voltages.

Figure 2-33

shows

the

input

voltage

exceeding the supply voltage with no phase reversal.

4.1.2

INPUT VOLTAGE LIMITS

In order to prevent damage and/or improper operation

of the amplifier, the circuit must limit the voltages at the

input pins (see Section 1.1, Absolute Maximum

Ratings †).

The Electrostatic Discharge (ESD) protection on the

inputs can be depicted as shown in Figure 4-1. This

structure was chosen to protect the input transistors

against many, but not all, over-voltage conditions, and

to minimize the input bias current (IB).

FIGURE 4-1:

Simplified Analog Input ESD

Structures.

The input ESD diodes clamp the inputs when they try

to go more than one diode drop below VSS. They also

clamp any voltages that go well above VDD; their

breakdown voltage is high enough to allow normal

operation, but not low enough to protect against slow

over-voltage (beyond VDD) events. Very fast ESD

events that meet the spec are limited so that damage

does not occur.

In some applications, it may be necessary to prevent

excessive voltages from reaching the op amp inputs;

Figure 4-2 shows one approach to protecting these

inputs.

FIGURE 4-2:

Protecting the Analog

Inputs.

A significant amount of current can flow out of the

inputs when the Common mode voltage (VCM) is below

ground (VSS); see Figure 2-35.

4.1.3

INPUT CURRENT LIMITS

In order to prevent damage and/or improper operation

of the amplifier, the circuit must limit the currents into

the input pins (see Section 1.1, Absolute Maximum

Ratings †).

Figure 4-3 shows one approach to protecting these

inputs. The resistors R1 and R2 limit the possible

currents in or out of the input pins (and the ESD diodes,

D1 and D2). The diode currents will go through either

VDD or VSS.

FIGURE 4-3:

Protecting the Analog

Inputs.

Bond

Pad

Bond

Pad

Bond

Pad

VDD

VIN+

VSS

Input

Stage

Bond

Pad

VIN–

VDD

MCP6421

VOUT

VDD

min(R1,R2)>

VSS –min(V1, V2)

2mA

MCP6421

VOUT

min(R1,R2)>

max(V1,V2)–VDD

2mA

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