MCP6L91RTESLASHMS datasheet(13/40 Pages) MICROCHIP

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DS20002141C-page 13

MCP6L91/1R/2/4

4.0

APPLICATION INFORMATION

The MCP6L91/1R/2/4 family of op amps is manufac-

tured using Microchip’s state-of-the-art CMOS process.

It is designed for low-cost, low-power and general

purpose applications. The low supply voltage, low

quiescent current and wide bandwidth makes the

MCP6L91/1R/2/4 ideal for battery-powered applications.

4.1

Rail-to-Rail Inputs

4.1.1

PHASE REVERSAL

The MCP6L91/1R/2/4 op amps are designed to

prevent phase inversion when the input pins exceed

the supply voltages. Figure 2-10 shows an input

voltage exceeding both supplies without any phase

reversal.

4.1.2

INPUT VOLTAGE AND CURRENT

LIMITS

In order to prevent damage and/or improper operation

of these amplifiers, the circuit they are in must limit the

currents (and voltages) at the input pins (see

Section 1.1

“Absolute

Maximum

Ratings†”).

Figure 4-1 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.

FIGURE 4-1:

Protecting the Analog Inputs.

A significant amount of current can flow out of the

inputs (through the ESD diodes) when the Common-

mode voltage (VCM) is below ground (VSS); see

Figure 2-7. Applications that are high-impedance may

need to limit the usable voltage range.

4.1.3

NORMAL OPERATION

The input stage of the MCP6L91/1R/2/4 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

topology, and at room temperature, the device

operates with VCM up to 0.3V above VDD and 0.3V

below VSS (typical at +25°C).

The transition between the two input stages occurs

when VCM = VDD – 1.1V. For the best distortion and

gain linearity, with noninverting gains, avoid this region

of operation.

4.2

Rail-to-Rail Output

The output voltage range of the MCP6L91/1R/2/4

op amps is VDD – 20 mV (minimum) and VSS + 20 mV

(maximum) when RL = 10 k, and is connected to VDD/2

and VDD = 5.0V. Refer to Figure 2-13 for more

information.

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.

When driving large capacitive loads with these

op amps (e.g., >100 pF when G = +1), a small series

resistor at the output (RISO in Figure 4-2) improves the

feedback loop’s stability by making the output load

resistive at higher frequencies; the bandwidth will

usually be decreased.

FIGURE 4-2:

Output Resistor, RISO,

Stabilizes Large Capacitive Loads.

Bench measurements are helpful in choosing RISO.

Adjust RISO so that a small signal step response (see

Figure 2-14) has reasonable overshoot (e.g., 4%).

MCP6L9X

VDD

R1 >

VSS – (minimum expected V1)

2mA

R2 >

VSS – (minimum expected V2)

2mA

–

RISO

VOUT

MCP6L9X

–

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