VIN

VOUT

VIN

+

VOUT

RISO

CL

Vcc

14

LPV821

SNOSD36A – AUGUST 2017 – REVISED DECEMBER 2017

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Device Functional Modes (continued)

8.4.2 Driving Capacitive Load

The LPV821 is internally compensated for stable unity-gain operation, with a 8-kHz typical gain bandwidth.

However, the unity-gain follower is the most sensitive configuration-to-capacitive load. The combination of a

capacitive load placed directly on the output of an amplifier along with the output impedance of the amplifier

creates a phase lag, which reduces the phase margin of the amplifier. If the phase margin is significantly

reduced, the response is under-damped, which causes peaking in the transfer and, when there is too much

peaking, the op amp might start oscillating.

Figure 38. Resistive Isolation of Capacitive Load

In order to drive heavy (> 50 pF) capacitive loads, use an isolation resistor, RISO, as shown in Figure 38. The

value of the RISO to be used should be decided depending on the size of the CLand the level of performance

desired. Recommended minimum values for RISO are given in the following table, for 3.3V supply. Figure 39

shows the typical response obtained with the CL = 50 pF RISO = 160 kΩ. By using the isolation resistor, the

capacitive load is isolated from the output of the amplifier. The larger the value of RISO, the more stable the

amplifier will be. If the value of RISO is sufficiently large, the feedback loop is stable, independent of the value of

CL. However, larger values of RISO (e.g. 50 kΩ) result in reduced output swing and reduced output current drive.

Table 1. Capacitive Loads vs. Needed Isolation Resistors

CL

RISO

0 – 20 pF

not needed

50 pF

160 k

Ω

100 pF

140 k

Ω

500 pF

54.9 k

Ω

1 nF

33 k

Ω

5 nF

15 k

Ω

10 nF

5.62 k

Ω

Figure 39. Typical Step Response