7.3.5 Capacitive Load and Stability

The TLVx314-Q1 is designed for applications where driving a capacitive load is required. As with all operational amplifiers, there may be specific instances where the TLVx314-Q1 can become unstable. The particular operational amplifier circuit configuration, layout, gain, and output loading are some of the factors to consider when establishing whether or not an amplifier is stable in operation. An operational amplifier in the unity-gain
(1 V/V) buffer configuration that drives a capacitive load exhibits a greater tendency to be unstable than an amplifier operated at a higher noise gain. The capacitive load, in conjunction with the operational amplifier output resistance, creates a pole within the feedback loop that degrades the phase margin. The degradation of the phase margin increases when capacitive loading increases. When operating in the unity-gain configuration, the TLVx314-Q1 remains stable with a pure capacitive load up to approximately 1 nF. The equivalent series resistance (ESR) of some very large capacitors (C_L capacitors with a value greater than 1 μF) is sufficient to alter the phase characteristics in the feedback loop so the amplifier remains stable. Increasing the amplifier closed-loop gain allows the amplifier to drive increasingly larger capacitance. This increased capability is evident when measuring the overshoot response of the amplifier at higher voltage gains, as shown in Figure 8.

Inserting a small resistor (typically 10-Ω to 20-Ω) can increase the capacitive load drive of the amplifier in a unity-gain configuration, as shown in Figure 14. This resistor significantly reduces the overshoot and ringing associated with large capacitive loads. One possible problem with this technique, however, is that a voltage divider is created with the added series resistor and any resistor connected in parallel with the capacitive load. The voltage divider introduces a gain error at the output that reduces the output swing.

Figure 14. Improving Capacitive Load Drive