LP3981

SNVS159G – OCTOBER 2001 – REVISED MAY 2013

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APPLICATION HINTS

POWER DISSIPATION AND DEVICE OPERATION

The permissible power dissipation for any package is a measure of the capability of the device to pass heat from

the power source, the junctions of the IC, to the ultimate heat sink, the ambient environment. Thus, the power

dissipation is dependant on the ambient temperature and the thermal resistance across the various interfaces

between the die and ambient air.

As stated in the notes for Absolute Maximum Ratings and Operating Ratings, the allowable power dissipation for

the device in a given package can be calculated using the equation:

(1)

With a

temperature of 125°C and an ambient temperature of 25°C. The device in a VSSOP package returns a figure of

0.476W, (

The actual power dissipation across the device can be represented by the following equation:

(2)

This establishes the relationship between the power dissipation allowed due to thermal considerations, the

voltage drop across the device, and the continuous current capability of the device. The device can deliver

300mA but care must be taken when choosing the continuous current output for the device under the operating

load conditions.

EXTERNAL CAPACITORS

Like any low-dropout regulator, the LP3981 requires external capacitors for regulator stability. The LP3981 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

≊ 2.2µF is required between the LP3981 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 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

≊ 2.2µF over the entire operating

temperature range.

OUTPUT CAPACITOR

The LP3981 is designed specifically to work with very small ceramic output capacitors. A ceramic capacitor

(dielectric types Z5U, Y5V or X7R) in 2.2 to 22 µF range with 5m

Ω to 500mΩ ESR range is suitable in the

LP3981 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 CAPACITOR CHARACTERISTICS).

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 (5 m

Ω to 500 mΩ).

NO-LOAD STABILITY

The LP3981 will remain stable and in regulation with no external load. This is specially important in CMOS RAM

keep-alive applications.

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