LP2985AIBP-1.5 datasheet(11/17 Pages) TI

Part #	LP2985AIBP-1.5

Description	Micropower 150 mA Low-Noise Low-Dropout Regulator

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Manufacturer	TI [Texas Instruments]

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EXTERNAL CAPACITORS

Like any low-dropout regulator, the LP2985LV requires ex-

ternal capacitors for regulator stability. These capacitors

must be correctly selected for good performance.

Input Capacitor

An input capacitor whose capacitance is

≥ 1 µF is required

between the LP2985LV input and ground (the amount of

capacitance may be increased without limit).

This capacitor must be located a distance of not more than 1

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

Any good quality ceramic, tantalum, or film capacitor may be

used at the input.

Important: Tantalum capacitors can suffer catastrophic fail-

ure due to surge current when connected to a low-

impedance source of power (like a battery or very large

capacitor). If a Tantalum capacitor is used at the input, it

must be guaranteed by the manufacturer to have a surge

current rating sufficient for the application.

There are no requirements for ESR on the input capacitor,

but tolerance and temperature coefficient must be consid-

ered when selecting the capacitor to ensure the capacitance

will be

≥ 1 µF over the entire operating temperature range.

Output Capacitor

The LP2985LV 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 5 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 sectionCapacitor 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 graphs Figure 1 and

Figure 2).

Important: The output capacitor must maintain its ESR

within the stable region over the full operating temperature

range of the application to assure stability.

The LP2985LV requires a minimum of 2.2 µF on the output

(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. It should be noted that

ceramic capacitors can exhibit large changes in capacitance

with temperature (see next section, Capacitor Characteris-

tics).

The output capacitor must be located not more than 1 cm

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. It should be noted that

the capacitor is connected directly to a high-impedance cir-

cuit 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

The LP2985LV was designed to work with ceramic capaci-

tors on the output to take advantage of the benefits they

offer: for capacitance values in the 2.2 µF to 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 2.2 µF ceramic

capacitor is in the range of 10 m

Ω to 20 mΩ, which easily

meets the ESR limits required for stability by the LP2985LV.

10129519

FIGURE 1. LP2985LV 2.2µF Stable ESR Range

10129520

FIGURE 2. LP2985LV 4.7µF Stable ESR Range

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