The LPV521 is internally compensated for stable unity gain operation, with a 6.2kHz, typical gain bandwidth. However, the unity gain follower is the most sensitive configuration to capacitive load. The combination of a capacitive load placed at the output of an amplifier along with the amplifier output impedance creates a phase lag, which reduces the phase margin of the amplifier. If the phase margin is significantly reduced, the response is underdamped, and causes peaking in the transfer. When there is too much peaking, the op amp can start oscillating.

Figure 7-1 Resistive Isolation of Capacitive Load

To drive heavy capacitive loads, use an isolation resistor, R_ISO, as in Figure 7-1. By using this isolation resistor, the capacitive load is isolated from the amplifier output. The larger the value of R_ISO, the more stable the amplifier. 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 result in reduced output swing and reduced output current drive.

Table 7-1 shows the recommended minimum R_ISO values for a 5V supply. Figure 7-2 shows the typical response obtained with the C_L = 50pF and R_ISO = 154kΩ. The other values of R_ISO in Table 7-1 are chosen to achieve similar dampening at the respective capacitive loads. Notice that for the LPV521 with larger a C_L, a smaller R_ISO can be used for stability. However, for a given C_L, a larger R_ISO provides a more damped response. For capacitive loads of 20pF and less, no isolation resistor is needed.

Figure 7-2 Step Response