Application Hints

EXTERNAL CAPACITORS

Like any low-dropout regulator, the external capacitors used

with the LP2981 must be carefully selected to assure regula-

tor loop stability.

INPUT CAPACITOR: An input capacitor whose value is

≥ 1 µF is required with the LP2981 (amount of capacitance

can be increased without limit).

This capacitor must be located a distance of not more than

0.5" from the input pin of the LP2981 and returned to a clean

analog ground. Any good quality ceramic or tantalum can be

used for this capacitor.

OUTPUT CAPACITOR: The output capacitor must meet

both the requirement for minimum amount of capacitance

and E.S.R. (equivalent series resistance) value. Curves are

provided which show the allowable ESR range as a function

of load current for various output voltages and capacitor val-

ues (refer to

Figures 1, 2, 3, 4).

IMPORTANT: The output capacitor must maintain its ESR in

the stable region over the full operating temperature range to

assure stability. Also, capacitor tolerance and variation with

temperature must be considered to assure the minimum

amount of capacitance is provided at all times.

This capacitor should be located not more than 0.5" from the

output pin of the LP2981 and returned to a clean analog

ground.

CAPACITOR CHARACTERISTICS

TANTALUM: Tantalum capacitors are the best choice for

use with the LP2981. Most good quality tantalums can be

used with the LP2981, but check the manufacturer’s data

sheet to be sure the ESR is in range.

It is important to remember that ESR increases at lower tem-

peratures and a capacitor that is near the upper limit for sta-

bility at room temperature can cause instability when it gets

cold.

In applications which must operate at very low temperatures,

it may be necessary to parallel the output tantalum capacitor

with a ceramic capacitor to prevent the ESR from going up

too high (see next section for important information on ce-

ramic capacitors).

CERAMIC: Ceramic capacitors are not recommended for

use at the output of the LP2981. This is because the ESR of

a ceramic can be low enough to go below the minimum

stable value for the LP2981. A 2.2 µF ceramic was measured

and found to have an ESR of about 15 m

Ω, which is low

enough to cause oscillations.

If a ceramic capacitor is used on the output, a 1

Ω resistor

should be placed in series with the capacitor.

ALUMINUM: Because of large physical size, aluminum elec-

trolytics are not typically used with the LP2981. They must

meet the same ESR requirements over the operating tem-

perature range, more difficult because of their steep increase

at cold temperature.

An aluminum electrolytic can exhibit an ESR increase of as

much as 50X when going from 20˚C to −40˚C. Also, some

aluminum electrolytics are not operational below −25˚C be-

cause the electrolyte can freeze.

ON/OFF Pin Current vs

V

ON/OFF

DS012506-36

ON/OFF Threshold

vs Temperature

DS012506-37

DS012506-38

FIGURE 1. 5V/3.3 µF ESR Curves

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