MIC7111 datasheet(10/20 Pages) MICROCHIP

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MIC7111

DS20006316A-page 10

 2020 Microchip Technology Inc.

3.0

APPLICATION INFORMATION

3.1

Input Common Mode Voltage

The MIC7111 tolerates input overdrive by at least

300 mV beyond either rail without producing phase

inversion.

If the absolute maximum input voltage is exceeded, the

input current should be limited to ±5 mA maximum to

prevent reducing reliability. A 10 kΩ 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 3-1.

10k

OUT

FIGURE 3-1:

Input Current-Limit

Protection.

3.2

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 presented in Equation 3-1:

EQUATION 3-1:

ROUT

V DROP

I LOAD

------------------

VDROP is the voltage dropped within the amplifier

output stage. VDROP and ILOAD can be determined from

the VO (output swing) portion of the appropriate

electrical characteristics table. ILOAD is equal to the

typical output high voltage minus V+/2 and divided by

RLOAD. For example, using the DC Electrical

Characteristics (5.0V) table, the typical output voltage

drop using a 2 kΩ load (connected to V+/2) is 0.015V,

which produces an ILOAD of:

EQUATION 3-2:

2.5V

0.015V

–



------------------------------------

1.243mA

Then:

EQUATION 3-3:

ROUT

15mV

1.243mA

----------------------

12.1



3.3

Driving Capacitative 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 500 pF capacitive load

connected directly to the output when configured as a

unity-gain amplifier.

3.4

Using Large-Value Feedback

Resistors

A large-value feedback resistor (>500 kΩ) 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 feedback 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 3-2 illustrates a method of compensating phase

lag caused by using a large-value feedback resistor.

Feedback capacitor CFB introduces sufficient phase

lead to overcome the phase lag caused by feedback

resistor RFB and input capacitance CIN. The value of

CFB is determined by first estimating CIN and then

applying the following formula:

EQUATION 3-4:

RIN CIN



RFB



CFB



OUT

FIGURE 3-2:

Canceling Feedback Phase

Lag.

Because a significant percentage of CIN may be

caused by board layout, it is important to note that the

correct value of CFB may change when changing from

a breadboard to the final circuit layout.

3.5

Typical Circuits

Some single-supply, rail-to-rail applications for which

the MIC7111 is well suited are shown in the circuit

diagrams of Figure 3-3 through Figure 3-8.

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