Application Hints

EXTERNAL CAPACITORS

Like any low-dropout regulator, the external capacitors used

with the LP2982 must be carefully selected to assure regula-

tor loop stability.

INPUT CAPACITOR: An input capacitor whose value is

≥

1 µF is required with the LP2982 (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 LP2982 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

Figure 1, Figure 2).

IMPORTANT: The output capacitor must maintain its ESR in

the stable region over the full operating temperature to as-

sure 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 LP2982 and returned to a clean analog

ground.

LOW-CURRENT OPERATION: In applications where the

to the output capacitor.

Circuitry inside the LP2982 is specially designed to reduce

operating (quiescent) current at light loads down to about

65 µA.

The mode of operation which yields this very low quiescent

current also means that the output capacitor ESR is critical.

For optimum stability and minimum output noise, it is recom-

mended that a 10

Ω resistor be placed in series with the out-

put capacitor in any applications where I

CAPACITOR CHARACTERISTICS

TANTALUM: Tantalum capacitors are the best choice for

use with the LP2982. Most good quality tantalum can be

used with the LP2982, but check the manufacturer’s data

sheet to be sure the ESR is in range.

It is important to remember that ESR increases sharply at

upper limit for stability at room temperature can cause insta-

bility 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 LP2982. This is because the ESR of

a ceramic can be low enough to go below the minimum

stable value for the LP2982. A good 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-

trolytic are not typically used with the LP2982. They must

meet the same ESR requirements over the operating tem-

perature range, which is more difficult because of their large

increase in ESR 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 electrolytic are not operational below −25˚C be-

cause the electrolyte can freeze.

BYPASS CAPACITOR

The 0.01 µF capacitor connected to the bypass pin to reduce

noise must have very low leakage.

The current flowing out of the bypass pin comes from the

bandgap reference, which is used to set the output voltage.

This capacitor leakage current causes the output voltage to

decline by an amount proportional to the current. Typical val-

This data is valid up to a maximum leakage current of about

500 nA, beyond which the bandgap is so severely loaded

that it can not function.

Care must be taken to ensure that the capacitor selected will

not have excessive leakage current over the operating tem-

perature range of the application.

A high quality ceramic capacitor which uses either NPO or

COG type dielectric material will typically have very low leak-

age. Small surface mount polypropylene or polycarbonate

film capacitors also have extremely low leakage, but are

slightly larger than ceramics.

DS012679-43

FIGURE 1.

5V/2.2 µF ESR Curves

DS012679-44

FIGURE 2.

3V/4.7 µF ESR Curves

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