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MCP616 Datasheet(PDF) 17 Page - Microchip Technology

Part No. MCP616
Description  2.3V to 5.5V Micropower Bi-CMOS Op Amps
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Maker  MICROCHIP [Microchip Technology]
Homepage  http://www.microchip.com
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MCP616 Datasheet(HTML) 17 Page - Microchip Technology

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© 2008 Microchip Technology Inc.
DS21613C-page 17
MCP616/7/8/9
4.4
Capacitive Loads
Driving large capacitive loads can cause stability
problems for voltage feedback op amps. As the load
capacitance 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. A unity-gain buffer (G = +1) is the most
sensitive to capacitive loads, though all gains show the
same general behavior.
When driving large capacitive loads with these op
amps (e.g., > 60 pF when G = +1), a small series
resistor at the output (RISO in Figure 4-5) improves the
feedback loop’s phase margin (stability) by making the
output load resistive at higher frequencies. The
bandwidth will be generally lower than the bandwidth
with no capacitive load.
FIGURE 4-5:
Output Resistor, RISO
stabilizes large capacitive loads.
Figure 4-6 gives recommended RISO values for
different capacitive loads and gains. The x-axis is the
normalized load capacitance (CL/GN), where GN is the
circuit’s noise gain. For non-inverting gains, GN and the
Signal Gain are equal. For inverting gains, GN is
1+|Signal Gain| (e.g., -1 V/V gives GN =+2V/V).
FIGURE 4-6:
Recommended RISO Values
for Capacitive Loads.
After selecting RISO for your circuit, double-check the
resulting frequency response peaking and step
response overshoot. Modify RISO’s value until the
response is reasonable. Bench evaluation and
simulations with the MCP616/7/8/9 SPICE macro
model are helpful.
4.5
MCP618 Chip Select (CS)
The MCP618 is a single op amp with Chip Select (CS).
When CS is pulled high, the supply current drops to
50 nA (typical) and flows through the CS pin to VSS.
When this happens, the amplifier output is put into a
high-impedance state. By pulling CS low, the amplifier
is enabled. The CS pin has an internal 5 M
Ω (typical)
pull-down resistor connected to VSS, so it will go low if
the CS pins is left floating. Figure 1-1 shows the output
voltage and supply current response to a CS pulse.
4.6
Supply Bypass
With this family of operational amplifiers, the power
supply pin (VDD for single supply) should have a local
bypass capacitor (i.e., 0.01 µF to 0.1 µF) within 2 mm
for good high-frequency performance. It may use a bulk
capacitor (i.e., 1 µF or larger) within 100 mm to provide
large, slow currents. This bulk capacitor is not required
and can be shared with other analog parts.
4.7
Unused Op Amps
An unused op amp in a quad package (MCP619)
should be configured as shown in Figure 4-7. These
circuits prevent the output from toggling and causing
crosstalk. Circuits A sets the op amp at its minimum
noise gain. The resistor divider produces any desired
reference voltage within the output voltage range of the
op amp; the op amp buffers that reference voltage.
Circuit B uses the minimum number of components
and operates as a comparator, but it may draw more
current.
FIGURE 4-7:
Unused Op Amps.
VIN
MCP61X
RISO
VOUT
CL
100
1,000
10,000
1.E-11
1.E-10
1.E-09
1.E-08
Normalized Load Capacitance; CL/GN (F)
10p
1n
100
10k
100p
1k
GN = +1
GN
t +2
10n
VDD
VDD
¼ MCP619 (A)
¼ MCP619 (B)
R1
R2
VDD
VREF
V
REF
V
DD
R
2
R
1
R
2
+
-------------------
=


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