Rev. 10/11/00

AS2940

APPLICATION HINTS

External Capacitors

A minimum capacitance of 22

µF and conditions on ESR (Equivalent Series Resistance) must be met. The minimum value for the capacitance is

22

µF and can be increased without limit. However the ESR may cause loop instability if it is too high or too low. The following graph shows the

acceptable range for the ESR.

If the capacitor does not meet these requirements oscillation can result.

ESR is specified only at room temperature. Therefore the designer must ensure the proper behavior of the ESR over the temperature range. ESR, for

electrolytic capacitor, will increase by about 30X as the temperature is reduced from 25

°C to -40°C. Aluminum electrolytic capacitors are not well

suited for low temperature operation.

Solid tantalum capacitors’ ESR are more stable over temperature, but expensive. A cost-effective approach is then to put in parallel a solid tantalum

and a aluminum electrolytic capacitors in the ratio25/75%.

Thermal Consideration

Although the AS2940 offers some limiting circuitry for overload conditions, it is necessary not to exceed the maximum junction temperature, and

therefore to be careful about thermal resistance. The heat flow will follow the lowest resistance path, which is the Junction-to-case thermal

resistance. In order to insure the best

thermal flow of the component, a proper mounting is required. Note that the case of the device is electrically connected to the output. In case the

case has to be electrically isolated, a thermally conductive spacer can be used. However do not forget to consider its contribution to thermal

resistance.

Formulas for calculating the power dissipated in the regulator are the following:

output voltage.

Output Capacitor ESR

OUTPUT CURRENT (mA)

1000

800

600

400

0

200

0.01

0.1

1

10

100

C

V

STABLE

REGION