LM2940 datasheet(13/19 Pages) NSC

13 / 19 page

Equivalent Schematic Diagram

00882201

Application Hints

EXTERNAL CAPACITORS

The output capacitor is critical to maintaining regulator sta-

bility, and must meet the required conditions for both ESR

(Equivalent Series Resistance) and minimum amount of ca-

pacitance.

MINIMUM CAPACITANCE:

The minimum output capacitance required to maintain sta-

bility is 22 µF (this value may be increased without limit).

Larger values of output capacitance will give improved tran-

sient 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 essen-

tial that the output capacitor meet these requirements,

or oscillations can result.

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 tem-

perature operation.

Solid tantalum capacitors have a more stable ESR over

temperature, but are more expensive than aluminum elec-

trolytics. A cost-effective approach sometimes used is to

Output Capacitor ESR

00882206

FIGURE 1. ESR Limits

LM2940 Datasheet(PDF) 13 Page - National Semiconductor (TI)