Application Hints

EXTERNAL CAPACITORS

In common with most regulators, the LP3992 requires exter-

nal capacitors for regulator stability. The LP3992 is specifi-

cally designed for portable applications requiring minimum

board space and smallest components. These capacitors

must be correctly selected for good performance.

INPUT CAPACITOR

An input capacitor is required for stability. It is recommended

that a 1.0µF capacitor be connected between the LP3992

input pin and ground (this capacitance value may be in-

creased 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

The LP3992 is designed specifically to work with very small

ceramic output capacitors. A 1.0µF ceramic capacitor (di-

electric types Z5U, Y5V or X7R) with ESR between 5m

Ω to

500m

Ω, is suitable in the LP3992 application circuit.

For this device the output capacitor should be connected

between the C

connect the output capacitor directly to the V

case C

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).

The output capacitor must meet the requirement for the

minimum value of capacitance and also have an ESR value

that is within the range 5m

Ω to 500mΩ for stability.

NO-LOAD STABILITY

The LP3992 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 LP3992 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, ceramic

capacitors are the smallest, least expensive and 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 LP3992.

The temperature performance of ceramic capacitors varies

by type. Most large value ceramic capacitors (

≥ 2.2µF) are

manufactured with Z5U or Y5V temperature characteristics,

which results in the capacitance dropping by more than 50%

as the temperature goes from 25˚C to 85˚C.

A better choice for temperature coefficient in a ceramic

capacitor is X7R. This type of capacitor is the most stable

ture range.

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.

SHUTDOWN AND ENABLE

The LP3992 features an active low shutdown pin, V

which turns the device off when pulled low. The device

output is enabled when the shutdown pin is pulled high. In

the shutdown mode the regulator output is off and the device

typically consumes 3nA.

If the application does not require the shutdown feature, the

V

permanently on.

To ensure proper operation, the signal source used to drive

the V

specified turn-on/off voltage thresholds listed in the Electrical

Characteristics section under V

FAST TURN ON AND OFF

The controlled shutdown feature of the device provides a

fast turn off by discharging the output capacitor via an inter-

nal FET device. This discharge is current limited by the

RDS

circuitry within the reference block allowing a very fast ramp

of the output voltage to reach the target voltage.

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