8.4.7 Driving Capacitive Load

The TLV854x 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.

In order to drive heavy (> 50 pF) capacitive loads, use an isolation resistor, R_ISO, as shown in Figure 31. By using this isolation resistor, the capacitive load is isolated from the output of the amplifier. The larger the value of R_ISO, the more stable the amplifier will be. If the value of R_ISO is sufficiently large, the feedback loop is stable, independent of the value of C_L. However, larger values of R_ISO (e.g. 50 kΩ) result in reduced output swing and reduced output current drive.

Figure 31. Resistive Isolation of Capacitive Load