OPA170-Q1 Datasheet, PDF (22/35 Pages) Texas Instruments – 36-V, Single-Supply, Low-Power, Automotive Grade Operational Amplifiers

OPA170-Q1, OPA2170-Q1, OPA4170-Q1
SBOS834 – DECEMBER 2016
Typical Application (continued)
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120
AOL
100
1
f_p
80
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 42. Unity-Gain Amplifier With R_ISO Compensation
ROC stability analysis is typically simulated. The validity of the analysis depends on multiple factors, especially
the accurate modeling of R _O . In addition to simulating the ROC, a robust stability analysis includes a
measurement of overshoot percentage and ac gain peaking of the circuit using a function generator,
oscilloscope, and gain and phase analyzer. Phase margin is then calculated from these measurements. Table 3
shows the overshoot percentage and ac gain peaking that correspond to 45° and 60° phase margins. For more
details on this design and other alternative devices that can be used in place of the OPAx170-Q1, see Capacitive
Load Drive Solution Using an Isolation Resistor (TIPD128).
Table 3. Phase Margin versus Overshoot and AC Gain
Peaking
PHASE
MARGIN
45°
60°
OVERSHOOT
23.3%
8.8%
AC GAIN PEAKING
2.35 dB
0.28 dB
8.2.3 Application Curve
Using the described methodology, the values of R_ISO that yield phase margins of 45º and 60º for various
capacitive loads were determined. Figure 43 shows the results.
10000
45°Phase Margin
60°Phase Margin
1000
100
10
0.1
1
10
100
Capacitive Load (nF)
1000
C002
Figure 43. Isolation Resistor Required for Various Capacitive Loads to Achieve a Target Phase Margin
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