OPA4191IPWT datasheet(27/65 Pages) TI2

27 / 65 page

For additional drive capability in unity-gain configurations, improve capacitive load drive by inserting a small,

10-Ω to 20-Ω resistor (RISO) in series with the output, as shown in Figure 8-7. This resistor significantly reduces

ringing while maintaining dc performance for purely capacitive loads. However, if there is a resistive load in

parallel with the capacitive load, a voltage divider is created, introducing a gain error at the output and slightly

reducing the output swing. The error introduced is proportional to the ratio RISO / RL, and is generally negligible

at low output levels. A high capacitive load drive makes the OPA191 a great choice for applications such as

reference buffers, MOSFET gate drives, and cable-shield drives. The circuit shown in Figure 8-7 uses RISO to

stabilize the output of an op amp. RISO modifies the open-loop gain of the system for increased phase margin.

Results using the OPA191 are summarized in Table 8-2. For additional information on techniques to optimize

and design using this circuit, TI Precision Design TIPD128, Capacitive Load Drive Verified Reference Design

Using an Isolation Resistor, details complete design goals, simulation, and test results.

load

out

iso

-V

Figure 8-7. Extending Capacitive Load Drive With the OPA191

Table 8-2. OPA191 Capacitive Load Drive Solution Using Isolation Resistor Comparison of Calculated

and Measured Results

PARAMETER

VALUE

Capacitive Load

100 pF

1000 pF

0.01 µF

0.1 µF

1 µF

Phase Margin

45°

60°

45°

60°

45°

60°

45°

60°

RISO (Ω)

280

113

432

210

17.8

53.6

3.6

Measured

Overshoot (%)

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OPA191, OPA2191, OPA4191

SBOS701D – DECEMBER 2015 – REVISED AUGUST 2021

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