Why is your LDO output unstable?

A while ago, a friend told me that when he used a certain model of LDO, he found that the output was abnormal. After carefully reading the datasheet, he replaced the output capacitor to solve the problem.

The output capacitor of the LDO is crucial to its performance. In addition to improving the power supply rejection ratio (PSRR) to suppress noise, it is also crucial to loop stability. In addition to the capacitance parameter, the capacitor also has the ESR (Equivalent Series Resistance) equivalent series resistance parameter. Both must be carefully considered during the selection and design.

We take PMOS LDO as an example to simulate the effect of ESR on LDO output. The LDO output voltage is 3.2V, the output capacitance is 2.2uF, the ESR is R4 which we choose as 0.1Ω, the load is 50Ω (the load current is 3.2/50=70mA), when the switch S1 is closed, the load is the parallel connection of R6 and R5, at this time the load current is about 700mA. Our simulation method is to change the ESR resistor R4, switch the load current, and observe the change of the output voltage.

How to obtain the simulation file, reply to the WeChat public account backend: LDO stable simulation

The figure below shows the output result when R4 ESR is 0.1Ω. The black curve is the current waveform of repeatedly switching the load current from 70mA to 700mA, and the red curve is the output voltage waveform. It can be seen that when the current changes, the output voltage has only a slight fluctuation, and the overall voltage is still stable at 3.2V.

The figure below shows the result after changing the ESR to 0.001Ω. The output is stable at the beginning, but becomes abnormal when the load current is switched.

The figure below shows the result after changing the ESR to 100Ω. The output is stable at the beginning, but when switching the load current, the output is prone to abnormality.

In summary, the output capacitor of the LDO is crucial to maintaining the stability of the regulator and must meet the minimum capacitance and equivalent series resistance (ESR) requirements. The increase in output capacitance will affect loop stability and transient response. The capacitance and ESR of the capacitor are not good if they are too large or too small, and they are likely to cause loop oscillation.