3.1.2 Output Capacitor ESR Effects

The primary function of the output capacitor in a switching regulator is filtering. As the converter operates, current must flow into and out of the output filter capacitor.

The ESR of the output capacitor directly affects the performance of the switching regulator. ESR is specified by the manufacturer on good quality capacitors, but be certain that it is specified at the frequency of intended operation.

General-purpose electrolytics usually only specify ESR at 120 Hz, but capacitors intended for high-frequency switching applications will have the ESR guaranteed at high frequency (like 20 kHz to 100 kHz).

Some ESR dependent parameters are:

• Ripple Voltage: In most cases, the majority of the output ripple voltage results from the ESR of the output capacitor. If the ESR increases (as it will at low operating temperatures) the output ripple voltage will increase accordingly.
• Efficiency: As the switching current flows into and out of the capacitor (through the ESR), power is dissipated internally. This wasted power reduces overall regulator efficiency, and can also cause the capacitor to fail if the ripple current exceeds the maximum allowable specification for the capacitor.
• Loop Stability: The ESR of the output capacitor can affect regulator loop stability. Products such as the LM2575 and LM2577 are compensated for stability assuming the ESR of the output capacitor will stay within a specified range.

Keeping the ESR within the stable range is not always simple in designs that must operate over a wide temperature range. The ESR of a typical aluminum electrolytic may increase by 40X as the temperature drops from 25°C to –40°C.

In these cases, an aluminum electrolytic must be paralleled by another type of capacitor with a flatter ESR curve (like Tantalum or Film) so that the effective ESR (which is the parallel value of the two ESR's) stays within the allowable range.

NOTE

If operation below –40°C is necessary, aluminum electrolytics are probably not feasible for use.