LP2980-ADJEP datasheet(7/10 Pages) NSC

7 / 10 page

Ripple Rejection

20207021

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

≥1 μF is 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 operation.

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.

20207022

2.2 μF ESR Curves

20207023

4.7 μF ESR Curves

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-AD-

JEP 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 temper-

atures).

Off-brand capacitors should be avoided, as some poor quality

Tantalums are seen with ESR's > 10

Ω, and this usually caus-

es oscillation problems.

One caution about Tantalums if they are used on the input:

the ESR of a Tantalum is low enough that it can be destroyed

by surge current if powered up from a low impedance source

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