LP3987

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SNVS164G – OCTOBER 2001 – REVISED MAY 2013

APPLICATION HINTS

EXTERNAL CAPACITORS

Like any low-dropout regulator, the LP3987 requires external capacitors for regulator stability. The LP3987 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 LP3987 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. Any good quality ceramic, 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 specified 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 be

≊ 1µF over the entire operating

temperature range.

OUTPUT CAPACITOR

The LP3987 is designed specifically to work with very small ceramic output capacitors. A ceramic capacitor

(dielectric types Z5U, Y5V or X7R) in 1 to 4.7 µF range with 5m

Ω to 500mΩ ESR range is suitable in the LP3987

application circuit.

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).

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

(Equivalent Series Resistance) value which is within a stable range (5 m

Ω to 500 mΩ).

NO-LOAD STABILITY

The LP3987 will remain stable and in regulation with no external load. This is specially important in CMOS RAM

keep-alive applications.

CAPACITOR CHARACTERISTICS

The LP3987 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 20 m

Ω to 40 mΩ, which easily meets the ESR

requirement for stability by the LP3987. The ceramic capacitor's capacitance can vary with temperature. Most

large value ceramic capacitors (

≊ 2.2µF) are manufactured with Z5U or Y5V temperature characteristics, which

results in the capacitance dropping by more than 50% as the temperature goes from 25°C to 85°C.

A better choice for temperature coefficient in a ceramic capacitor is X7R, which holds the capacitance within

±15%. Tantalum capacitors are less desirable than ceramic for use as output capacitors because they are more

expensive when comparing equivalent capacitance and voltage ratings in the 1µF to 4.7µF range.

Another important consideration is that tantalum capacitors have higher ESR values than equivalent size

ceramics. This means that while it may be possible to find a tantalum capacitor with an ESR value within the

stable range, it would have to be larger in capacitance (which means bigger and more costly ) than a ceramic

capacitor with the same ESR value. It should also be noted that the ESR of a typical tantalum will increase about

2:1 as the temperature goes from 25°C down to

−40°C, so some guard band must be allowed.

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