OPA2171-EP_15 Datasheet, PDF (17/28 Pages) Texas Instruments – OPA2171-EP 36-V, Single-Supply, SOT553, General-Purpose Operational Amplifiers

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OPA2171-EP
SBOS735 – SEPTEMBER 2015
8.2 Typical Application
Figure 39 shows a capacitive load drive solution using an isolation resistor. The OPA2171-EP device can be
used capacitive loads such as cable shields, reference buffers, MOSFET gates, and diodes. The circuit uses an
isolation resistor (R_ISO) to stabilize the output of an op amp. R_ISO modifies the open loop gain of the system to
ensure the circuit has sufficient phase margin.
+V_S
R_ISO
V_OUT
+
V_{IN ±}
-V_S
C_LOAD
Figure 39. Unity-Gain Buffer with R_ISO Stability Compensation
8.2.1 Design Requirements
The design requirements are:
• Supply voltage: 30 V (±15 V)
• Capacitive loads: 100 pF, 1000 pF, 0.01 μF, 0.1 μF, and 1 μF
• Phase margin: 45° and 60°
8.2.2 Detailed Design Procedure
Figure 39 shows a unity-gain buffer driving a capacitive load. Equation 1 shows the transfer function for the
circuit in Figure 39. Not shown in Figure 39 is the open-loop output resistance of the op amp, R_o.
T(s)
=
1
1 + C_LOAD ×
+ R_o + R_ISO 
R_ISO × s
× C_LOAD
×
s
(1)
The transfer function in Equation 1 has a pole and a zero. The frequency of the pole (f_p) is determined by (R_o +
R_ISO) and C_LOAD. Components R_ISO and C_LOAD determine the frequency of the zero (f_z). A stable system is
obtained by selecting R_ISO such that the rate of closure (ROC) between the open-loop gain (A_OL) and 1/β is 20
dB/decade. Figure 40 depicts the concept. The 1/β curve for a unity-gain buffer is 0 dB.
120
AOL
100
1
80
f_p
  2 u Πu R_ISO  R_o u C_LOAD
60
40
20
0
10
40 dB
1/
1 dec
1
f_z
2 u Πu R_ISO u C_LOAD
ROC ^{20 dB}
dec
100
1k
10k 100k 1M 10M 100M
Frequency (Hz)
Figure 40. Unity-Gain Amplifier with R_ISO Compensation
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