8.2.2.6 Driving Capacitive Loads and Loop Compensation

For normal operation, the driver OPA and the IA are connected in a closed loop for voltage output. In current output mode, the current copy closes the loop directly.

In current output mode, loop compensation is not critical, even for large capacitive loads. However, in voltage output mode, the capacitive load, together with the source impedance and the impedance of the protection circuit, generates additional phase lag. The IA input might also be protected by a low-pass filter that influences phase in the closed loop.

The loop compensation low-pass filter consists of C_C and the parallel resistance of R_OS and R_SET. For loop stability with large capacitive load, the external phase shift has to be added to the OPA phase. With C_C, the voltage gain of the OPA has to approach zero at the frequency where the total phase approaches 180° + 135°.

The best stability for large capacitive loads is provided by adding a small resistor, R_C (15 Ω). See the Output Protection section.

An empirical method of evaluation is using a square wave input signal and observing the settling after transients. Use small signal amplitudes only—steep signal edges cause excessive current to flow into the capacitive load and may activate the current limit, which hides or prevents oscillation. A small-signal oscillation can be hidden from large capacitive loads, but observing the I_MON output on an appropriate resistor (use a similar value like R_SET||R_OS) would indicate stability issues. Note that noise pulses at I_MON during overload (EF_LD active) are normal and are caused by cycling of the current mirror.

The voltage output mode includes the IA in the loop. An additional low-pass filter in the input reverses the phase and therefore increases the signal bandwidth of the loop, but also increases the delay. Again, loop stability has to be observed. Overloading the IA disconnects the closed loop and the output voltage rails.