Application Hints

EXTERNAL CAPACITORS

The LM2991 regulator requires an output capacitor to main-

tain stability. The capacitor must be at least 10 µF aluminum

electrolytic or 1 µF solid tantalum. The output capacitor’s

ESR must be less than 10

Ω, or the zero added to the regu-

lator frequency response by the ESR could reduce the

phase margin, creating oscillations. The shaded area in the

Output Capacitor ESR graph indicates the recommended

ESR range. An input capacitor, of at least 1 µF solid tantalum

or 10 µF aluminum electrolytic, is also needed if the regulator

is situated more than 6 inches from the input power supply

filter.

MINIMUM LOAD

A minimum load current of 500 µA is required for proper op-

eration. The external resistor divider can provide the mini-

mum load, with the resistor from the adjust pin to ground set

to 2.4 k

Ω.

SETTING THE OUTPUT VOLTAGE

The output voltage of the LM2991 is set externally by a resis-

tor divider using the following equation:

V

where V

REF

= −1.21V. The output voltage can be pro-

grammed within the range of −3V to −24V, typically an even

Adjust Pin Current

DS011260-25

Low Voltage Behavior

DS011260-26

Maximum Power

Dissipation (TO-220)

DS011260-27

Maximum Power Dissipation

(TO-263) (See Note 3)

DS011260-28

Output Capacitor ESR Range

DS011260-5

www.national.com

5