Power Supply Rejection Ratio (V

20039108

Application Hints

EXTERNAL CAPACITORS

Like any low-dropout regulator, the LP3928 requires external

capacitors for regulator stability. The LP3928 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

≅ 1 µF is required between the

LP3928 V

may be increased without limit).

This capacitor must be located a distance of not more than

1 cm from the V

ground. Any good quality ceramic, tantalum, or film capacitor

may be used at the input.

Important: Tantalum capacitors can suffer catastrophic fail-

ures 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 on the input capaci-

tor, but tolerance and temperature coefficient must be con-

sidered when selecting the capacitor to ensure the capaci-

tance will be

≅ 1 µF over the entire operating temperature

range.

FAST ON-TIME

The LP3928 utilizes a speed up circuitry to ramp up the

internal V

output turn on time.

CAPACITOR CHARACTERISTICS

The LP3928 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 1 µF to 4.7 µF 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 1 µF

ceramic capacitor is in the range of 20 m

Ω to 40 mΩ, which

easily meets the ESR requirement for stability by the

LP3928.

The ceramic capacitor’s capacitance can vary with tempera-

ture.

Most large value ceramic capacitors (

≅ 2.2 µF) are manufac-

tured 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 ceramic capaci-

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

1 µF to 4.7 µF 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.

OUTPUT CAPACITOR

The LP3928 is designed specifically to work with very small

ceramic output capacitors, any ceramic capacitor (dielectric

types Z5U, Y5V or X7R) in 1.5 µF to 22 µF range with 5 m

Ω

to 500 m

Ω ESR range is suitable in the LP3928 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 section Capacitor Characteristics).

The output capacitor must meet the requirement for mini-

mum amount of capacitance and also have an ESR (Equiva-

lent Series Resistance) value which is within a stable range.

The output capacitor should be placed as near to the V

CCB

pin as possible.

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