MIC7111

Micrel

MIC7111

6

June 1998

Application Information

Input Common-Mode Voltage

The MIC7111 tolerates input overdrive by at least 300mV

beyond either rail without producing phase inversion.

If the absolute maximum input voltage is exceeded, the input

current should be limited to

±5mA maximum to prevent

reducing reliability. A 10k

Ω series input resistor, used as a

current limiter, will protect the input structure from voltages as

large as 50V above the supply or below ground. See Figure

1.

10k

Ω

Figure 1. Input Current-Limit Protection

Output Voltage Swing

Sink and source output resistances of the MIC7111 are

equal. Maximum output voltage swing is determined by the

load and the approximate output resistance. The output

resistance is:

R

V

I

OUT

DROP

LOAD

=

V

stage. V

(output swing) portion of the appropriate Electrical Character-

istics table. I

minus V+/2 and divided by R

Electrical Characteristics DC (5V) table, the typical output

voltage drop using a 2k

Ω load (connected to V+/2) is 0.015V,

which produces an I

2.5V

V

2k

1.243mA

−

Ω

=

0 015

.

then:

R

15mV

1.243mA

1

≈

12 1

2

.

Ω

Driving Capacitive Loads

Driving a capacitive load introduces phase-lag into the output

signal, and this in turn reduces op-amp system phase margin.

The application that is least forgiving of reduced phase

margin is a unity gain amplifier. The MIC7111 can typically

drive a 500pF capacitive load connected directly to the output

when configured as a unity-gain amplifier.

Using Large-Value Feedback Resistors

A large-value feedback resistor (> 500k

Ω) can reduce the

phase margin of a system. This occurs when the feedback

resistor acts in conjunction with input capacitance to create

phase lag in the fedback signal. Input capacitance is usually

a combination of input circuit components and other parasitic

capacitance, such as amplifier input capacitance and stray

printed circuit board capacitance.

Figure 2 illustrates a method of compensating phase lag

caused by using a large-value feedback resistor. Feedback

capacitor C

the phase lag caused by feedback resistor R

capacitance C

estimating C

RC

R

C

IN

IN

FB

FB

×≤

×

Figure 2. Cancelling Feedback Phase Lag

Since a significant percentage of C

layout, it is important to note that the correct value of C