LP2989 datasheet(15/19 Pages) TI

15 / 19 page

Application Hints

LLP PACKAGE DEVICES

The LP2989 is offered in the 8 lead LLP surface mount

package to allow for increased power dissipation compared

to the SO-8 and Mini SO-8. For details on thermal perfor-

mance as well as mounting and soldering specifications,

refer to Application Note AN-1187.

For output voltages < 2V, see LP2989LV datasheet.

EXTERNAL CAPACITORS

Like any low-dropout regulator, the LP2989 requires external

capacitors for regulator stability. These capacitors must be

correctly selected for good performance.

Input Capacitor

An input capacitor whose size is at least 2.2 µF is required

between the LP2989 input and ground (the amount of ca-

pacitance may be increased without limit).

Characterization testing performed on the LP2989 has

shown that if the amount of actual input capacitance drops

below about 1.5 µF, an unstable operating condition may

result. Therefore, the next larger standard size (2.2 µF) is

specified as the minimum required input capacitance. Ca-

pacitor tolerance and temperature variation must be consid-

ered when selecting a capacitor (see Capacitor Character-

istics section) to assure the minimum requirement of 1.5 µF

is met over all operating conditions.

The input capacitor must be located at a distance of not

more than 0.5" from the input pin and returned to a clean

analog ground. Any good quality ceramic or tantalum may be

used for this capacitor, assuming the minimum capacitance

requirement is met.

Output Capacitor

The LP2989 requires a ceramic output capacitor whose size

is at least 4.7µF. The actual amount of capacitance on the

output must never drop below about 3.5µF or unstable op-

eration may result. For this reason, capacitance tolerance

and temperature characteristics must be considered when

selecting an output capacitor.

The LP2989 is designed specifically to work with ceramic

output capacitors, utilizing circuitry which allows the regula-

tor to be stable across the entire range of output current with

an output capacitor whose ESR is as low as 4 m

Ω.Itmay

also be possible to use Tantalum or film capacitors at the

output, but these are not as attractive for reasons of size and

cost (see next section Capacitor Characteristics).

The output capacitor must meet the requirement for mini-

mum amount of capacitance and also have an ESR (equiva-

lent series resistance) value which is within the stable range.

Curves are provided which show the stable ESR range as a

function of load current (see ESR graph below).

10133938

Stable Region For output Capacitor ESR

Important: The output capacitor must maintain its ESR

within the stable region over the full operating temperature

range of the application to assure stability.

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. (See Capacitor Character-

istics section).

The output capacitor must be located not more than 0.5"

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

Noise Bypass Capacitor

Connecting a 10 nF capacitor to the Bypass pin significantly

reduces noise on the regulator output. However, the capaci-

tor is connected directly to a high-impedance circuit in the

bandgap reference.

Because this circuit has only a few microamperes flowing in

it, any significant loading on this node will cause a change in

the regulated output voltage. For this reason, DC leakage

current through the noise bypass capacitor must never ex-

ceed 100 nA, and should be kept as low as possible for best

output voltage accuracy.

The types of capacitors best suited for the noise bypass

capacitor are ceramic and film. High-quality ceramic capaci-

tors with either NPO or COG dielectric typically have very

low leakage. 10 nF polypropolene and polycarbonate film

capacitors are available in small surface-mount packages

and typically have extremely low leakage current.

CAPACITOR CHARACTERISTICS

Ceramic

The LP2989 was designed to work with ceramic capacitors

on the output to take advantage of the benefits they offer: for

capacitance values in the 4.7 µF range, ceramics are the

least expensive and also have the lowest ESR values (which

makes them best for eliminating high-frequency noise). The

ESR of a typical 4.7 µF ceramic capacitor is in the range of

10 m

Ω to 15 mΩ, which easily meets the ESR limits required

for stability by the LP2989.

One disadvantage of ceramic capacitors is that their capaci-

tance can vary with temperature. Many large value ceramic

capacitors (

≥ 2.2 µF) are manufactured with the Z5U or Y5V

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