LDO stability vs ESR of output cap

This is a behavioral model of an LDO voltage regulator that demonstrates how its open- and closed-loop transfer function changes versus the ESR of the output capacitor (toggle the switch to change between open- and close-loop modes). To see this effect, adjust the value of the resistor that is in series with the 100 uF capacitor. You can also see what happens when the load resistance changes (300 Ohm resistor on the right). On the Bode plot, observe several characteristic knee points, where the slope changes by ±20 dB/decade. These are due to "poles" (-20 dB/decade change) and "zeros" (+20 dB/decade change) of the transfer function. By moving the knee point created by the branch with the ESR and 100 uF cap in series (corresponds to a "zero") to a frequency somewhere between the unit gain frequency and the knee point due to the internal pole of the amplifier (a "pole" determined by the 100 kOm resistor and 1 nF cap) the responce of the LDO regulator can be made stable. Finally, the 8 kOhm resistor in the PMOS gate defines the position of the high-frequency pole due to the gate capacitance of the PMOS.

published 8 months ago