LP5990 datasheet(9/18 Pages) TI1

9 / 18 page

PD =

(TJMAX - TA)

LP5990

www.ti.com

SNVS438B – APRIL 2007 – REVISED DECEMBER 2007

Typical Performance Characteristics (continued).

Unless otherwise specified,CIN = COUT = 1.0µF, VIN = VOUT(NOM) + 1.0V, VEN = 1.0V, IOUT = 1mA , T A = 25°C.

Power Supply Rejection ratio

Output Noise Density

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 (1) of the electrical characteristics, the allowable power dissipation

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

(1)

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

PD = (VIN – VOUT) x IOUT

(2)

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.

(1)

In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may

have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP =

125°C), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the

part/package in the application (

θJA), as given by the following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX). See applications section.

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