OPA172-Q1 Datasheet, PDF (23/36 Pages) Texas Instruments – 36-V, Single-Supply, 10-MHz, Rail-to-Rail Output Automotive Grade Operational Amplifiers

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OPA172-Q1, OPA2172-Q1, OPA4172-Q1
SBOS809 – NOVEMBER 2016
Typical Applications (continued)
9.2.1.2 Detailed Design Procedure
Figure 47 depicts a unity-gain buffer driving a capacitive load. Equation 1 shows the transfer function for the
circuit in Figure 47. Not depicted in Figure 47 is the open-loop output resistance of the op amp, R_o.
T(s) =
1 + C_LOAD × R_ISO × s
1 + R_o + R_ISO × 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 per decade. Figure 48 shows the concept. Note that the 1 / β curve for a unity-gain buffer is 0 dB.
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 48. 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 5
shows the overshoot percentage and ac gain peaking that correspond to phase margins of 45° and 60°. For
more details on this design and other alternative devices that can be used in place of the OPA172-Q1, see the
Capacitive Load Drive Solution using an Isolation Resistorprecision design (TIPD128).
Table 5. 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
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