LP3985

SNVS087AC – OCTOBER 2000 – REVISED MAY 2013

www.ti.com

APPLICATION HINTS

EXTERNAL CAPACITORS

Like any low-dropout regulator, the LP3985 requires external capacitors for regulator stability. The LP3985 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 capacitance of

≊ 1µF is required between the LP3985 input pin and ground (the amount of the

capacitance 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

analog ground. A ceramic capacitor is recommended although a good quality tantalum or film capacitor may be

used at the input.

Important: 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 verified by the manufacturer to have a surge current rating sufficient for the application.

There are no requirements for the ESR on the input capacitor, but tolerance and temperature coefficient must be

considered when selecting the capacitor to ensure the capacitance will remain within the operational range over

the full range of temperature and operating conditions.

OUTPUT CAPACITOR

Correct selection of the output capacitor is important to ensure stable operation in the intended application.

The output capacitor must meet all the requirements specified in the recommended capacitor table over all

conditions in the application. These conditions include DC-bias, frequency and temperature. Unstable operation

will result if the capacitance drops below the minimum specified value. (See the next section Capacitor

Characteristics).

The LP3985 is designed specifically to work with very small ceramic output capacitors. A 1.0µF ceramic

capacitor (dialectric type X7R) with ESR between 5m

Ω to 500mΩ is suitable in the LP3985 application circuit.

X5R capacitors may be used but have a narrower temperature range. With these and other capacitor types (Y5V,

Z6U) that may be used, selection is dependant on the range of operating conditions and temperature range for

that application. (see section on Capacitor Characteristics).

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

reasons of size and cost (see next section Capacitor Characteristics).

It is also recommended that the output capacitor be placed within 1cm from the output pin and returned to a

clean ground line.

CAPACITOR CHARACTERISTICS

The LP3985 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 1µF to 4.7µF range, ceramic capacitors are the smallest, least

expensive and have the lowest ESR values (which makes them best for eliminating high frequency noise). The

ESR of a typical 1µF ceramic capacitor is in the range of 20m

Ω to 40mΩ, which easily meets the ESR

requirement for stability by the LP3985.

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 dependant on the conditions of operation 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. Capacitance value 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 43 shows a typical graph showing a comparison of

capacitor case sizes in a Capacitance vs. DC Bias plot. As shown in the graph, as a result of the DC Bias

14

Submit Documentation Feedback

Copyright © 2000–2013, Texas Instruments Incorporated

Product Folder Links: LP3985