LP3990

SNVS251J – MAY 2004 – REVISED SEPTEMBER 2014

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The actual power being dissipated in the device can be represented by Equation 2:

(2)

These two equations establish the relationship between the maximum 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.

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

as given by Equation 3:

(3)

combination of the two.

8.2.2.2 External Capacitors

In common with most regulators, the LP3990 requires external capacitors for regulator stability. The LP3990 is

specifically designed for portable applications requiring minimum board space and smallest components. These

capacitors must be correctly selected for good performance.

8.2.2.3 Input Capacitor

An input capacitor is required for stability. It is recommended that a 1-µF capacitor be connected between the

LP3990 IN pin and GND pin (this capacitance value may be increased without limit).

This capacitor must be located a distance of not more than 1 cm from the IN 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 design practices are employed to minimize

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

connect the battery or other power source to the LP3990, then it is recommended that the input capacitor is

increased. 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

approximately 1 µF over the entire operating temperature range.

8.2.2.4 Output Capacitor

The LP3990 is designed specifically to work with very small ceramic output capacitors. A 1-µF ceramic capacitor

(temperature types Z5U, Y5V or X7R/X5R) with ESR between 5 m

Ω to 500 mΩ, is suitable in the LP3990

application circuit.

For this device the output capacitor should be connected between the OUT pin and GND pin.

It is also possible to use tantalum or film capacitors at the device output, but these are not as attractive for

reasons of size and cost (see Capacitor Characteristics).

The output capacitor must meet the requirement for the minimum value of capacitance and also have an ESR

value that is within the range 5 m

Ω to 500 mΩ for stability.

8.2.2.5 No-Load Stability

The LP3990 will remain stable and in regulation with no external load. This is an important consideration in some

circuits, for example CMOS RAM keep-alive applications.

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