PD =

(TJMAX - TA)

T

JA

LP5990

SNVS438B – APRIL 2007 – REVISED DECEMBER 2007

www.ti.com

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 dependent on the ambient temperature and the thermal resistance across the various interfaces

between the die and ambient air. As stated in Note 3 of the Operating Ratings table, the allowable power

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

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

PD = (VIN – VOUT) x IOUT

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

drop across the device, and the continuous current capability of the device. These two equations should be used

to determine the optimum operating conditions for the device in the application.

EXTERNAL CAPACITORS

Like any low-dropout regulator, the LP5990 requires external capacitors for regulator stability. The LP5990 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 capacitor is required for stability. The input capacitor should be at least equal to or greater than the

output capacitor. It is recommended that a 1.0 µF capacitor be connected between the LP5990 input pin and

ground.

This capacitor must be located a distance of not more than 1 cm 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: To ensure stable operation it is essential that good PCB practices are employed to minimize ground

impedance and keep input inductance low. If these conditions cannot be met, or if long leads are to be used to

connect the battery or other power source to the LP5990, then it is recommended to increase the input capacitor

to at least 2.2µF. Also, 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 ensured 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

temperature coefficient must be considered when selecting the capacitor to ensure the capacitance will remain

0.3

μF over the entire operating temperature range.

OUTPUT CAPACITOR

The LP5990 is designed specifically to work with very small ceramic output capacitors. A ceramic capacitor

(dielectric types X5R or X7R) 1.0

μF, and with ESR between 5 mΩ to 500 mΩ, is suitable in the LP5990

application circuit.

Other ceramic capacitors such as Y5V and Z5U are less suitable owing to their inferior temperature

characteristics. (See section in Capacitor Characteristics).

For this device the output capacitor should be connected between the VOUT pin and a good ground connection

and should be mounted within 1 cm of the device.

It may also be 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 the section Capacitor Characteristics).

The output capacitor must meet the requirement for the minimum value of capacitance (0.3

μF) and have an ESR

value that is within the range 5 m

Ω to 500 mΩ for stability.

8

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