Application Hints

EXTERNAL CAPACITORS

In common with most regulators, the LP3994 requires exter-

nal capacitors for regulator stability. The LP3994 is specifi-

cally designed for portable applications requiring minimum

board space and smallest components. These capacitors

must be correctly selected for good performance and to

ensure that their value remains within specification over the

full operating range.

INPUT CAPACITOR

An input capacitor is required for optimum operation and to

ensure stability within the range of specified transient condi-

tions. It is recommended that a 1.0µF capacitor be con-

nected between the LP3994 input pin and ground (this ca-

pacitance value 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 analogue

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 (Equivalent Series

Resistance) on the input capacitor, but tolerance and tem-

perature coefficient must be considered when selecting the

capacitor to ensure the capacitance will remain ) 1.0µF over

the entire operating temperature range.

OUTPUT CAPACITOR

Correct selection of the output capacitor is essential to en-

sure stable operation in the intended application.

The output capacitor must meet all the requirements speci-

fied in the recommended capacitor table over all conditions

in the application. These conditions include DC-Bias, fre-

quency and temperature. Unstable operation may result if

the capacitance drops below the minimum specified value.

(See the Capacitor Characteristics section).

The LP3994 is designed specifically to work with very small

ceramic output capacitors. A 1.0µF ceramic capacitor (type

X7R) with ESR between 5m

Ω to 500mΩ, is suitable in the

LP3994 application circuit. X5R type capacitors may be used

but have a narrower temperature range. With these capaci-

tors and others types (Y5V, Z6U) that may be used, selection

of the capacitor for any application is dependant on the

range of operating conditions and temperature range for that

application. (see section on Capacitor Characteristics).

It may also be possible to use tantalum or film capacitors at

the device output, C

attractive for reasons of size and cost (see the section

Capacitor Characteristics).

It is also recommended that the output capacitor be placed

within 1cm from the output pin and returned to a clean

ground line.

NO-LOAD STABILITY

The LP3994 will remain stable and in regulation with no

external load. This is an important consideration in some

circuits, for example CMOS RAM keep-alive applications.

CAPACITOR CHARACTERISTICS

The LP3994 is designed to work with ceramic capacitors on

the input and output to take advantage of the benefits they

offer. For capacitance values around the 1µF value, ceramic

capacitors give the circuit designer the best design options in

terms of low cost and minimal area. Ceramic capacitors

have the lowest ESR values, thus making them best for

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

ceramic capacitor is in the range of 20m

Ω to 40mΩ, which

easily meets the ESR requirement for stability for the

LP3994.

For both input and output capacitors careful interpretation of

the capacitor specification is required to ensure correct de-

vice operation. The capacitor value can change greatly de-

pendant on the conditions of operation and capacitor type.

In particular the output capacitor selection should take ac-

count of all the capacitor parameters to ensure that the

specification is met within the application. Capacitance value

can vary with DC bias conditions as well as temperature and

frequency of operation. Capacitor values will also show

some decrease over time due to aging. The capacitor pa-

rameters are also dependant on the particular case size with

smaller sizes giving poorer performance figures in general. A

study of manufacturers data on 0402 case size capacitors

shows that these devices may drop below the minimum

specified capacitance due to DC-Bias conditions in conjunc-

tion with other parameters such as temperature and are thus

not recommended for use.

The temperature performance of ceramic capacitors varies

by type. Capacitor type X7R is specified with a tolerance of

X5R has a similar tolerance over the reduced temperature

tors (

≥ 2.2µF) are manufactured with Z5U or Y5V tempera-

ture characteristics, which results in the capacitance drop-

ping by more than 50% as the temperature goes from 25˚C

to 85˚C. Therefore X7R is recommended over these other

capacitor types in applications where the temperature will

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.

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