Datasheet

 2009-2012 Microchip Technology Inc. DS22135C-page 11
MCP6L1/1R/2/4
4.0 APPLICATION INFORMATION
The MCP6L1/1R/2/4 family of op amps is
manufactured using Microchip’s state of the art CMOS
process. They are unity gain stable and suitable for a
wide range of general purpose applications.
4.1 Inputs
4.1.1 PHASE REVERSAL
The MCP6L1/1R/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 pro-
tecting these inputs. The internal ESD diodes prevent
the input pins (V
IN
+ and V
IN
-) from going too far below
ground, and the resistors, R
1
and R
2
, limit the possible
current drawn out of the input pins. Diodes, D
1
and D
2
,
prevent the input pins (V
IN
+ and V
IN
-) from going too far
above V
DD
, and dump any currents onto V
DD
.
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 (V
CM
) is below ground (V
SS
); see
Figure 2-7. Applications that are high-impedance may
need to limit the usable voltage range.
4.1.3 NORMAL OPERATION
The Common-Mode Input Voltage Range (V
CMR
)
includes ground in single-supply systems (V
SS
), but
does not include V
DD
. This means that the amplifier
input behaves linearly as long as the Common-Mode
Input Voltage (V
CM
) is kept within the V
CMR
limits
(typically V
SS
– 0.3V to V
DD
– 1.3V at +25°C).
Figure 4-3 shows a unity gain buffer. Since V
OUT
is the
same voltage as the inverting input, V
OUT
must be kept
below V
DD
– 1.2V (typically) for correct operation.
FIGURE 4-2: Unity Gain Buffer has a
Limited V
OUT
Range.
4.2 Rail-to-Rail Output
The output voltage range of the MCP6L1/1R/2/4 op
amps is V
DD
– 35 mV (minimum) and V
SS
+35mV
(maximum) when R
L
=10k is connected to V
DD
/2 and
V
DD
= 5.0V. Refer to Figure 2-13 for more information.
4.3 Capacitive Loads
Driving large capacitive loads can cause stability prob-
lems for voltage feedback op amps. As the load capac-
itance 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 (R
ISO
in Figure 4-3) improves the
feedback loop’s stability by making the output load
resistive at higher frequencies; the bandwidth will
usually be decreased.
FIGURE 4-3: Output Resistor, R
ISO,
Stabilizes Large Capacitive Loads.
Bench measurements are helpful in choosing R
ISO
.
Adjust R
ISO
so that a small signal step response (see
Figure 2-14) has reasonable overshoot (e.g., 4%).
V
1
MCP6LX
R
1
V
DD
D
1
R
1
>
V
SS
– (minimum expected V
1
)
2mA
R
2
>
V
SS
– (minimum expected V
2
)
2mA
V
2
R
2
D
2
R
3
V
1
MCP6LX
V
2
R
ISO
V
OUT
C
L
MCP6LX
R
F
R
G
R
N