LP3990

SNVS251I – MAY 2004 – REVISED MAY 2013

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APPLICATION HINTS

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.

INPUT CAPACITOR

An input capacitor is required for stability. It is recommended that a 1.0µF capacitor be connected between the

LP3990 input pin and ground (this capacitance value may be increased without limit).

This capacitor must be located a distance of not more than 1cm 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 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 guaranteed 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 1.0µF over the entire operating temperature range.

OUTPUT CAPACITOR

The LP3990 is designed specifically to work with very small ceramic output capacitors. A 1.0µF ceramic

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

Ω to 500mΩ, is suitable in the LP3990

application circuit.

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 5m

Ω to 500mΩ for stability.

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.

CAPACITOR CHARACTERISTICS

The LP3990 is designed to work with ceramic capacitors on the output to take advantage of the benefits they

offer. For capacitance values in the range of 0.47µF to 4.7µF, ceramic capacitors are the smallest, least

expensive and have the lowest ESR values, thus making them best for eliminating high frequency noise. The

ESR of a typical 1.0µF ceramic capacitor is in the range of 20m

Ω to 40mΩ, which easily meets the ESR

requirement for stability for the LP3990.

For both input and output capacitors, careful interpretation of the capacitor specification is required to ensure

correct device operation. The capacitor value can change greatly, depending on the operating conditions and

capacitor type.

In particular, the output capacitor selection should take account of all the capacitor parameters, to ensure that the

specification is met within the application. The capacitance can vary with DC bias conditions as well as

temperature and frequency of operation. Capacitor values will also show some decrease over time due to aging.

The capacitor parameters are also dependant on the particular case size, with smaller sizes giving poorer

performance figures in general. As an example, Figure 1 shows a typical graph comparing different capacitor

case sizes in a Capacitance vs. DC Bias plot. As shown in the graph, increasing the DC Bias condition can result

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