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LP3991

SNVS296J – DECEMBER 2006 – REVISED JUNE 2016

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8.2.2.2 Input Capacitor

An input capacitor is required for stability. It is recommended that a 1-µF capacitor be connected between the

LP3991 IN pin and ground (this capacitance value may be increased without limit).

This capacitor must be located a distance of not more than 1 cm from the IN pin and returned to a clean analog

ground. Any good-quality ceramic, tantalum, or film capacitor may be used at the input.

NOTE

Tantalum capacitors can suffer catastrophic failures 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, a surge current rating sufficient for the application must be

ensured by the manufacturer.

There are no requirements for the equivalent series resistance (ESR) on the input capacitor, but tolerance,

temperature, and voltage coefficients must be considered when selecting the capacitor to ensure the capacitance

remains

≊ 1 µF over the entire operating temperature range.

8.2.2.3 Output Capacitor

Correct selection of the output capacitor is critical to ensure stable operation in the intended application.

The output capacitor must meet all the requirements specified in the recommended capacitor table over all

conditions in the application. these conditions include DC bias, frequency and temperature. Unstable operation

results if the capacitance drops below the minimum specified value.

The LP3991 is designed specifically to work with very small ceramic output capacitors. For voltage options of 1.5

V and higher, A 4.7-µF ceramic capacitor (dielectric type X7R or X5R) with an ESR between 5 m

Ω to 500 mΩ, is

suitable in the LP3991 application circuit. However, on lower VOUT options a 2.2-µF may be employed with only a

small increase in load transient.

Other ceramic types such as Y5V and Z5U are less suitable owing to their inferior temperature characteristics.

(See Capacitor Characteristics.)

It is also recommended that the output capacitor is placed within 1 cm of the OUT pin and returned to a clean,

low impedance, ground connection.

It is possible to use tantalum or film capacitors at the device output, VOUT, but these are not as attractive for

reasons of size and cost. (See Capacitor Characteristics.)

8.2.2.4 No-Load Stability

The LP3991 remains stable and in regulation with no external load. This is an important consideration in some

circuits, for example CMOS RAM keep-alive applications.

8.2.2.5 Capacitor Characteristics

The LP3991 is designed to work with ceramic capacitors on the input and output to take advantage of the

benefits they offer. For capacitance values around 4.7 µF, ceramic capacitors give the circuit designer the best

design options in terms of low cost and minimal area.

For both input and output capacitors, careful interpretation of the capacitor specification is required to ensure

correct device operation. The capacitor value can change greatly dependant on the conditions of operation and

capacitor type.

In particular, to ensure stability, the output capacitor selection must take account 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 also show some decrease over time due to

aging. The capacitor parameters are also dependant on the particular case size with smaller sizes giving poorer

performance figures in general.