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LP3981

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SNVS159H – OCTOBER 2001 – REVISED JULY 2015

With an RθJA = 56.5°C/W, the device in the WSON package returns a value of 1.77 W with a maximum junction

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

0.565 W (R θJA = 177°C/W).

The actual power dissipation across the device can be represented by Equation 2:

(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 300

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

conditions.

The R

θJA value is not a characteristic of the package by itself but of the package, the printed circuit board

(PCB), and other environmental factors. Equation 2 is only valid when the application configuration matches the

EIA/JEDEC JESD51-7 (High-K) configuration in which RθJA was either measured or modeled. Few, if any, user

applications conform to the PCB configuration defined by the EIA/JEDEC standards. As a result, the RθJA values

are useful only when comparing assorted packages that have been measured or modeled to the EIA/JEDEC

standards, but are of little use to estimate real world junction temperatures.

The EIA/JEDEC standard JESD51-2 provides methodologies to estimate the junction temperature from external

measurements (

ψJB references the temperature at the PCB, and ψJT references the temperature at the top

surface of the package) when operating under steady-state power dissipation conditions. These methodologies

have been determined to be relatively independent of the PCB attached to the package when compared to the

more typical RθJA. Refer to Semiconductor and IC Package Thermal Metrics application report, SPRA953, for

specifics.

9.2.2.2 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.

9.2.2.3 Input Capacitor

An input capacitance of

≊ 2.2 µF 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 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.

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, 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 is

≊ 2.2 µF over the entire operating

temperature range.

9.2.2.4 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 the 2.2-µF to 22-µF range with 5-m

Ω to 500-mΩ ESR is suitable in the

LP3981 application circuit.

It is also 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

equivalent series resistance (ESR) value which is within a stable range (5 m

Ω to 500 mΩ).

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