LP2980-ADJ datasheet(7/9 Pages) NSC

7 / 9 page

Typical Performance Characteristics Unless otherwise specified: T

A = 25˚C, VIN =VO(NOM) + 1V,

IL = 1 mA, ON/OFF pin tied to VIN,RADJ = 86.6k, and test circuit is as shown in Basic Application Circuit. (Continued)

Application Hints

EXTERNAL CAPACITORS

Like any low-dropout regulator, the external capacitors must

be selected carefully to assure regulator loop stability.

INPUT CAPACITOR: An input capacitor whose value is

≥1µFis required (the amount of capacitance may be in-

creased without limit).

Any good quality Tantalum or Ceramic capacitor may be

used here. The capacitor must be located not more than 0.5"

from the input pin and returned to a clean analog ground.

OUTPUT CAPACITOR: The output capacitor must meet

both the requirement for minimum amount of capacitance

and E.S.R. (Equivalent Series Resistance) for stable opera-

tion.

Curves are provided below which show the allowable ESR of

the output capacitor as a function of load current for both

2.2 µF and 4.7 µF. A solid Tantalum capacitor is the best

choice for the output.

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.

Note that this capacitor must be located not more than 0.5”

from the output pin and returned to a clean analog ground.

FEED-FORWARD CAPACITOR: A 7 pF feed-forward ca-

pacitor is required (see Basic Application Circuit). The func-

tion of this capacitor is to provide the lead compensation

necessary for loop stability.

A temperature-stable ceramic capacitor (type NPO or COG)

should be used here.

CAPACITOR CHARACTERISTICS

TANTALUM: The best capacitor choice for the LP2980-ADJ

output is solid Tantalum. The ESR of a good quality Tantalum

is almost perfectly centered in the middle of the “stable”

range of the ESR curve (about 0.5

Ω–1Ω).

The temperature stability of Tantalums is typically very good,

with a total variation of only about 2:1 over the temperature

range of −40˚C to +125˚C (ESR increases at colder tem-

peratures).

Ripple Rejection

DS100001-21

DS100001-22

2.2 µF ESR Curves

DS100001-23

4.7 µF ESR Curves

LP2980-ADJ Datasheet(PDF) 7 Page - National Semiconductor (TI)

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