LM2937

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SNVS100E – MARCH 2000 – REVISED APRIL 2013

Typical Application

* Required if the regulator is located more than 3 inches from the power supply filter capacitors.

as close as possible to the regulator. The equivalent series resistance, ESR, of this capacitor may be as high as 3

Ω.

APPLICATION HINTS

EXTERNAL CAPACITORS

The output capacitor is critical to maintaining regulator stability, and must meet the required conditions for both

ESR (Equivalent Series Resistance) and minimum amount of capacitance.

MINIMUM CAPACITANCE:

The minimum output capacitance required to maintain stability is 10

μF (this value may be increased without

limit). Larger values of output capacitance will give improved transient response.

ESR LIMITS:

The ESR of the output capacitor will cause loop instability if it is too high or too low. The acceptable range of

ESR plotted versus load current is shown in the graph below. It is essential that the output capacitor meet

these requirements, or oscillations can result.

Output Capacitor ESR

Figure 24. ESR Limits

It is important to note that for most capacitors, ESR is specified only at room temperature. However, the designer

must ensure that the ESR will stay inside the limits shown over the entire operating temperature range for the

design.

For aluminum electrolytic capacitors, ESR will increase by about 30X as the temperature is reduced from 25°C to

−40°C. This type of capacitor is not well-suited for low temperature operation.

Solid tantalum capacitors have a more stable ESR over temperature, but are more expensive than aluminum

electrolytics. A cost-effective approach sometimes used is to parallel an aluminum electrolytic with a solid

Tantalum, with the total capacitance split about 75/25% with the Aluminum being the larger value.

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