Datasheet MIC7111 (Microchip) - 9

Micrel, Inc. MIC7111
Then, Application Information
Input Common Mode Voltage
The MIC7111 tolerates input overdrive by at least
300mV beyond either rail without producing phase
inversion. R If the absolute maximum input voltage is exceeded, the
input current should be limited to ±5mA maximum to
prevent reducing reliability. A 10kΩ 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. OUT 1.243mA = 12.1 = 12Ω Eq. 3 Figure 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: V DROP 15mV Using Large-Value Feedback Resistors
A large-value feedback resistor (> 500kΩ) 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. 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 1: = = 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. Figure 1. Input Current-Limit Protection R OUT Eq. 1 I RIN × CIN ≤ RFB × CFB LOAD Eq. 4 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 (5V)
table, the typical output voltage drop using a 2kΩ load
(connected to V+/2) is 0.015V, which produces an ILOAD
of: 2.5V − 0.015V
2kΩ February 11, 2013 = 1.243mA Figure 2. Cancelling Feedback Phase Lag Eq. 2 9 Revision 2.0