MIC7111_05 Datasheet, PDF (6/8 Pages) Micrel Semiconductor – 1.8V IttyBitty Rail-to-Rail Input/Output Op Amp

MIC7111
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 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.
R_IN
V_IN
10kΩ
V_OUT
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_OUT
=
V_DROP
I_LOAD
V_DROP is the voltage dropped within the amplifier output
stage. V_DROP and I_LOAD can be determined from the V_O
(output swing) portion of the appropriate Electrical Character-
istics table. I_LOAD is equal to the typical output high voltage
minus V+/2 and divided by R_LOAD. For example, using the
Electrical Characteristics DC (5V) table, the typical output
voltage drop using a 2kΩ load (connected to V+/2) is 0.015V,
which produces an I_LOAD of:
2.5V − 0.015V _{= 1.243mA}
2kΩ
then:
R_OUT
=
15mV
1.243mA
=
12.1≈ 12Ω
Micrel
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 (> 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 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_FB introduces sufficient phase lead to overcome
the phase lag caused by feedback resistor R_FB and input
capacitance C_IN. The value of C_FB is determined by first
estimating C_IN and then applying the following formula:
R_IN × C_IN ≤ R_FB × C_FB
R_IN
V_IN
C_IN
C_FB
R_FB
V_OUT
Figure 2. Cancelling Feedback Phase Lag
Since a significant percentage of C_IN may be caused by board
layout, it is important to note that the correct value of C_FB may
MIC7111
6
January 2005