APPLICATION NOTES

Driving Capacitive Loads

The FC358F can directly drive 250pF in unity-gain without oscillation. The unity-gain follower

(buffer) is the most sensitive configuration to capacitive loading. Direct capacitive loading reduces

the phase margin of amplifiers and this results in ringing or even oscillation.Applications that require

greater capacitive drive capability should use an isolation resistor between the output and the

capacitive load like the circuit in Figure 1. Theisolation resistor RISO and the load capacitor CL

Figure 1. Indirectly Driving Heavy Capacitive Load

An improvement circuit is shown in Figure 2, It provides DC accuracy as well as AC stability. RF

provides the DC accuracy by connecting the inverting signal with the output, CF and RIso serve to

counteract the loss of phase margin by feeding the high frequency component of the output signal

back to the amplifier’s inverting input, thereby preserving phase margin in the overall feedback loop.

Figure 2. Indirectly Driving Heavy Capacitive Load with DC Accuracy

For no-buffer configuration, there are two others ways to increase the phase margin: (a) by

increasing the amplifier’s gain or (b) by placing a capacitor in parallel with the feedback resistor to

counteract the parasitic capacitance associated with inverting node.

FC358F

2019. 11. 15

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Revision No : 0