D

IN

OUT

OUT

P = (V

V

)

I

-

´

A

D

JA

T

T

P

-

=

q

LP5904

SNVS637G – MARCH 2011 – REVISED APRIL 2013

www.ti.com

APPLICATION INFORMATION

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

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:

(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 LP5904 requires external capacitors for regulator stability. The LP5904 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 for good load transient performance. At least a 1.0 µF capacitor has to be connected between

the LP5904 input pin and ground for stable operation over full load current range. Basically, it is ok to have more

output capacitance than input, as long as the input is at least 1.0 µF.

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.

NOTE

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 LP5904, then it is recommended to increase the input capacitor to at

least 10 µF. 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 specified 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 1.0 µF ±30% over the entire operating temperature

range.

OUTPUT CAPACITOR

The LP5904 is designed specifically to work with a very small ceramic output capacitor, typically 1.0 µF. A

ceramic capacitor (dielectric types X5R or X7R) in the 0.5 µF to 10 µF range, and with ESR between 5m

Ω to 500

m

Ω, is suitable in the LP5904 application circuit. For this device the output capacitor should be connected

(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

APPLICATION INFORMATION.

12

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