Output Impedance (V

Output Impedance (V

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10136466

Application Hints

EXTERNAL CAPACITORS

Like any low-dropout regulator, the LP3985 requires external

capacitors for regulator stability. The LP3985 is specifically

designed for portable applications requiring minimum board

space and smallest components. These capacitors must be

correctly selected for good performance.

INPUT CAPACITOR

An input capacitance of ) 1µF is required between the

LP3985 input pin and ground (the amount of the capacitance

may be increased without limit).

This capacitor must be located a distance of not more than

1cm 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-

ures due to surge current when connected to a low-

impedance source of power (like a battery or a 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 the ESR on the input capaci-

tor, but tolerance and temperature coefficient must be con-

sidered when selecting the capacitor to ensure the capaci-

tance will be ) 1µF over the entire operating temperature

range.

OUTPUT CAPACITOR

The LP3985 is designed specifically to work with very small

ceramic output capacitors. A ceramic capacitor (temperature

characteristics X7R, X5R, Z5U, or Y5V) in 1 to 22 µF range

with 5m

Ω to 500mΩ ESR range is suitable in the LP3985

application circuit.

It may 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 a stable range

(5 m

Ω to 500 mΩ).

NO-LOAD STABILITY

The LP3985 will remain stable and in regulation with no

external load. This is specially important in CMOS RAM

keep-alive applications.

CAPACITOR CHARACTERISTICS

The LP3985 is designed to work with ceramic capacitors on

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

capacitance values in the range of 1µF to 4.7µF range,

ceramic capacitors are the smallest, least expensive and

have the lowest ESR values (which makes them best for

eliminating high frequency noise). The ESR of a typical 1µF

ceramic capacitor is in the range of 20 m

Ω to 40 mΩ, which

easily meets the ESR requirement for stability by the

LP3985.

The ceramic capacitor’s capacitance can vary with tempera-

ture. The capacitor type X7R, which operates over a tem-

perature range of -55˚C to +125˚C, will only vary the capaci-

() 2.2µF) are manufactured with Z5U or Y5V temperature

characteristics. Their capacitance can drop by more than

50% as the temperature goes from 25˚C to 85˚C. Therefore,

X7R is recommended over Z5U and Y5V in applications

where the ambient temperature will change significantly

above or below 25˚C.

Tantalum capacitors are less desirable than ceramic for use

as output capacitors because they are more expensive when

comparing equivalent capacitance and voltage ratings in the

1µF to 4.7µF range.

Another important consideration is that tantalum capacitors

have higher ESR values than equivalent size ceramics. This

means that while it may be possible to find a tantalum

capacitor with an ESR value within the stable range, it would

have to be larger in capacitance (which means bigger and

more costly ) than a ceramic capacitor with the same ESR

value. It should also be noted that the ESR of a typical

tantalum will increase about 2:1 as the temperature goes

from 25˚C down to −40˚C, so some guard band must be

allowed.

NOISE BYPASS CAPACITOR

Connecting a 0.01µF capacitor between the C

and ground significantly reduces noise on the regulator out-

put. This cap is connected directly to a high impedance node

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