LP2987 datasheet(19/24 Pages) NSC

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Application Hints (Continued)

IMPORTANT: The output capacitor must maintain its ESR in

the stable region

over the full operating temperature range of

the application to assure stability.

The minimum required amount of output capacitance is

4.7 µF. Output capacitor size can be increased without limit.

It is important to remember that capacitor tolerance and

variation with temperature must be taken into consideration

when selecting an output capacitor so that the minimum

required amount of output capacitance is provided over the

full operating temperature range. A good Tantalum capacitor

will show very little variation with temperature, but a ceramic

may not be as good (see next section).

The output capacitor should be located not more than 0.5”

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

CAPACITOR CHARACTERISTICS

TANTALUM: A solid tantalum capacitor is the best choice for

the output capacitor on the LM2987/8. Available from many

sources, their typical ESR is very close to the ideal value

required on the output of many LDO regulators.

Tantalums also have good temperature stability: a 4.7 µF

was tested and showed only a 10% decline in capacitance

as the temperature was decreased from +125˚C to −40˚C.

The ESR increased only about 2:1 over the same range of

temperature.

However, it should be noted that the increasing ESR at lower

temperatures present in all tantalums can cause oscillations

when marginal quality capacitors are used (where the ESR

of the capacitor is near the upper limit of the stability range at

room temperature).

CERAMIC: The ESR of ceramic capacitor can be low

enough to cause an LDO regulator to oscillate: a 2.2 µF

ceramic was measured and found to have an ESR of 15 m

Ω.

If a ceramic capacitor is to be used on the LP2987/8 output,

Ω resistor should be placed in series with the capacitor to

provide a minimum ESR for the regulator.

A disadvantage of ceramic capacitors is that their capaci-

tance varies a lot with temperature: Large ceramic capaci-

tors are typically manufactured with the Z5U temperature

characteristic, which results in the capacitance dropping by

50% as the temperature goes from 25˚C to 80˚C.

This means you have to buy a capacitor with twice the

minimum C

OUT to assure stable operation up to 80˚C.

ALUMINUM: The large physical size of aluminum electrolyt-

ics makes them unsuitable for most applications. Their ESR

characteristics are also not well suited to the requirements of

LDO regulators. The ESR of a typical aluminum electrolytic

is higher than a tantalum, and it also varies greatly with

temperature.

A typical aluminum electrolytic can exhibit an ESR increase

of 50X when going from 20˚C to −40˚C. Also, some alumi-

num electrolytics can not be used below −25˚C because the

electrolyte will freeze.

POWER-ON RESET DELAY

A power-on reset function can be easily implemented using

the LP2987/8 by adding a single external capacitor to the

Delay pin. The Error output provides the power-on reset

signal when input power is applied to the regulator.

The reset signal stays low for a pre-set time period after

power is applied to the regulator, and then goes high (see

Timing Diagram below).

10001709

Timing Diagram for Power-Up

The external capacitor c

DLY sets the delay time (TDELAY).

The value of capacitor required for a given time delay may

be calculated using the formula:

DLY =TDELAY/(5.59 X 10

To simplify design, a plot is provided below which shows

values of C

DLY versus delay time.

10001711

Plot of C

DLY vs TDELAY

LP2987 Datasheet(PDF) 19 Page - National Semiconductor (TI)

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