MCP6031/2/3/4
4.2
Rail-to-Rail Output
The output voltage range of the MCP6031/2/3/4 op
amps is VSS + 10 mV (minimum) and VDD – 10 mV
(maximum) when RL = 50 kΩ is connected to VDD/2
and VDD = 5.5V. Refer to Figures 2-25 and 2-26 for
more information.
–
RISO
VOUT
MCP603X
+
VIN
CL
4.3
Output Loads and Battery Life
FIGURE 4-3:
stabilizes large capacitive loads.
Output resistor, R
ISO
The MCP6031/2/3/4 op amp family has outstanding
quiescent current, which supports battery-powered
applications. There is minimal quiescent current glitch-
ing when Chip Select (CS) is raised or lowered. This
prevents excessive current draw, and reduced battery
life, when the part is turned off or on.
Figure 4-4 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 = +2 V/V).
Heavy resistive loads at the output can cause exces-
sive battery drain. Driving a DC voltage of 2.5V across
a 100 kΩ load resistor will cause the supply current to
increase by 25 µA, depleting the battery 28 times as
fast as IQ (0.9 µA, typical) alone.
1000010M0
High frequency signals (fast edge rate) across capaci-
tive loads will also significantly increase supply current.
For instance, a 0.1 µF capacitor at the output presents
an AC impedance of 15.9 kΩ (1/2πfC) to a 100 Hz
sinewave. It can be shown that the average power
drawn from the battery by a 5.0 Vp-p sinewave
(1.77 Vrms), under these conditions, is
100k
100000
GN:
1 V/V
2 V/V
≥ 5 V/V
10k
10000
1k
1000
10p
100p
1n
10n
100n
1µ
1.E-11 1.E-10 1.E-09 1.E-08 1.E-07 1.E-06
Normalized Load Capacitance; CL/GN (F)
EQUATION 4-1:
PSupply = (VDD - VSS) (IQ + VL(p-p) f CL )
= (5V)(0.9 µA + 5.0Vp-p · 100Hz · 0.1µF)
= 4.5 µW + 50 µW
FIGURE 4-4:
for Capacitive Loads.
Recommended R
values
ISO
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 simula-
tions with the MCP6031/2/3/4 SPICE macro model are
very helpful.
This will drain the battery about 12 times as fast as IQ
alone.
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. While a unity-gain buffer (G = +1) is the most
sensitive to capacitive loads, all gains show the same
general behavior.
4.5
MCP6033 Chip Select
The MCP6033 is a single op amp with Chip Select
(CS). When CS is pulled high, the supply current drops
to 0.4 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. If the CS pin is left floating, the amplifier will
not operate properly. Figure 1-1 shows the output
voltage and supply current response to a CS pulse.
When driving large capacitive loads with these op
amps (e.g., > 100 pF when G = +1), a small series
resistor at the output (RISO in Figure 4-3) 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 capacitance load.
© 2008 Microchip Technology Inc.
DS22041B-page 15
MICROCHIP [ MICROCHIP ]
