LP2981-N

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SNOS773K – MARCH 2000 – REVISED APRIL 2013

APPLICATION HINTS

EXTERNAL CAPACITORS

Like any low-dropout regulator, the external capacitors used with the LP2981-N must be carefully selected to

assure regulator loop stability.

Input Capacitor:

An input capacitor whose value is

≥ 1 μF is required with the LP2981-N (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-N 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 values (refer to Figure 34, Figure 35, Figure 36, and Figure 37).

NOTE

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-N and returned to a clean

analog ground.

CAPACITOR CHARACTERISTICS

Tantalum:

Tantalum capacitors are the best choice for use with the LP2981-N. Most good quality tantalums can be used

with the LP2981-N, 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 temperatures and a capacitor that is near the upper limit

for stability 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 below for important

information on ceramic capacitors).

Ceramic:

Ceramic capacitors are not recommended for use at the output of the LP2981-N. This is because the ESR of a

ceramic can be low enough to go below the minimum stable value for the LP2981-N. 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 electrolytics are not typically used with the LP2981-N. They must meet

the same ESR requirements over the operating temperature 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 because the electrolyte can freeze.

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