LP3984

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SNVS160F – OCTOBER 2001 – REVISED OCTOBER 2013

APPLICATION HINTS

External Capacitors

Like any low-dropout regulator, the LP3984 requires external capacitors for regulator stability. The LP3984 is

specifically designed for portable applications requiring minimum board space and smallest components. These

capacitors must be correctly selected for good performance.

Input Capacitors

An input capacitance of

≊ 1 µF is required between the LP3984 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 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 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,

it must be specified by the manufacturer to have a surge current rating sufficient for the application.

There are no requirements for the ESR on the input capacitor, but tolerance and temperature coefficient must be

considered when selecting the capacitor to ensure the capacitance will be

≊ 1 µF over the entire operating

temperature range.

Output Capacitor

The LP3984 is designed specifically to work with tantalum output capacitors. A tantalum capacitor in 1 to 22 µF

range with 2

Ω to 10Ω ESR range is suitable in the LP3984 application circuit.

It may also be possible to use film capacitors at the output, but these are not as attractive for reasons of size and

cost.

The output capacitor must meet the requirement for minimum amount of capacitance and also have an ESR

(Equivalent Series Resistance) value which is within a stable range (2

Ω to 10Ω).

No-Load Stability

The LP3984 will remain stable and in regulation with no external load. This is specially important in CMOS RAM

keep-alive applications.

On/Off Input Operation

Fast On-Time

process, the noise reduction capacitor at the bypass pin is charged with an internal 70 µA current source. The

current source is turned off when the bandgap voltage reaches approximately 95% of its final value. The turn-on

time is determined by the time constant of the bypass capacitor. The smaller the capacitor value, the shorter the

turn-on time, but less noise gets reduced. As a result, turn-on time and noise reduction need to be taken into

design consideration when choosing the value of the bypass capacitor.

Copyright © 2001–2013, Texas Instruments Incorporated

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