MCP6021 datasheet(17/42 Pages) MICROCHIP

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DS21685D-page 17

MCP6021/1R/2/3/4

4.0

APPLICATIONS INFORMATION

The MCP6021/1R/2/3/4 family of operational amplifiers

are fabricated on Microchip’s state-of-the-art CMOS

process. They are 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 MCP6021/1R/2/3/4 op amp is designed to prevent

phase reversal when the input pins exceed the supply

voltages. Figure 2-42 shows the input voltage exceed-

ing the supply voltage without any phase reversal.

4.1.2

INPUT VOLTAGE AND CURRENT

LIMITS

The ESD protection on the inputs can be depicted as

shown in Figure 4-1. This structure was chosen to

protect the input transistors, and to minimize input bias

current (IB). 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 too far

above VDD; their breakdown voltage is high enough to

allow normal operation, and low enough to bypass

quick ESD events within the specified limits.

FIGURE 4-1:

Simplified Analog Input ESD

Structures.

In order to prevent damage and/or improper operation

of these op amps, the circuit they are in must limit the

currents and voltages at the VIN+ and VIN– pins (see

Absolute Maximum Ratings † at the beginning of

Section 1.0 “Electrical Characteristics”). Figure 4-2

shows the recommended approach to protecting these

inputs. The internal ESD diodes prevent the input pins

(VIN+ and VIN–) from going too far below ground, and

the resistors R1 and R2 limit the possible current drawn

out of the input pins. Diodes D1 and D2 prevent the

input pins (VIN+ and VIN–) from going too far above

VDD, and dump any currents onto VDD. When

implemented as shown, resistors R1 and R2 also limit

the current through D1 and D2.

FIGURE 4-2:

Protecting the Analog

Inputs.

It is also possible to connect the diodes to the left of

resistors R1 and R2. In this case, current through the

diodes D1 and D2 needs to be limited by some other

mechanism. The resistors then serve as in-rush current

limiters; the DC current into the input pins (VIN+ and

VIN–) should be very small.

A significant amount of current can flow out of the

inputs when the common mode voltage (VCM) is below

ground (VSS); see Figure 2-42. Applications that are

high impedance may need to limit the useable voltage

range.

4.1.3

NORMAL OPERATION

The input stage of the MCP6021/1R/2/3/4 op amps use

two differential CMOS input stages in parallel. One

operates at low common mode input voltage (VCM),

while the other operates at high VCM. WIth this topol-

ogy, the device operates with Vcm up to 0.3V above

VDD and 0.3V below VSS.

4.2

Rail-to-Rail Output

The Maximum Output Voltage Swing is the maximum

swing

possible

under

particular

output

load.

According to the specification table, the output can

reach within 20 mV of either supply rail when

RL =10kΩ. See Figure 2-31 and Figure 2-34 for more

information concerning typical performance.

4.3

Capacitive Loads

Driving large capacitive loads can cause stability

problems for voltage feedback op amps. As the load

capacitance increases, the feedback loop’s phase

margin decreases, and the closed loop bandwidth is

reduced. This produces gain-peaking in the frequency

response, with overshoot and ringing in the step

response.

Bond

Pad

Bond

Pad

Bond

Pad

VDD

VIN+

VSS

Input

Stage

Bond

Pad

VIN–

MCP602X

VDD

R1 >

VSS – (minimum expected V1)

2mA

R2 >

VSS – (minimum expected V2)

2mA

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