LP3971 datasheet(39/42 Pages) NSC

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Application Hints

LDO CONSIDERATIONS

External Capacitors

The LP3971’s regulators require external capacitors for

regulator stability. These are specifically designed for por-

table applications requiring minimum board space and small-

est components. These capacitors must be correctly se-

lected for good performance.

Input Capacitor

An input capacitor is required for stability. It is recommended

that a 1.0 µF capacitor be connected between the LDO input

pin and ground (this capacitance value may be increased

without limit).

This capacitor must be located a distance of not more than 1

cm 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: Tantalum capacitors can suffer catastrophic fail-

ures due to surge current when connected to a low imped-

ance source of power (like a battery or a very large capaci-

tor). 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 tem-

perature coefficient must be considered when selecting the

capacitor to ensure the capacitance will remain approxi-

mately 1.0 µF over the entire operating temperature range.

Output Capacitor

The LDO’s are designed specifically to work with very small

ceramic output capacitors. A 1.0 µF ceramic capacitor (tem-

perature types Z5U, Y5V or X7R) with ESR between 5 m

Ω to

500 m

Ω, are suitable in the application circuit.

For this device the output capacitor should be connected

between the V

OUT pin and ground.

It is also possible to use tantalum or film capacitors at the

device output, C

OUT (or VOUT), but these are not as attrac-

tive for reasons of size and cost (see the section 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.

No-Load Stability

The LDO’s will remain stable and in regulation with no ex-

ternal load. This is an important consideration in some cir-

cuits, for example CMOS RAM keep-alive applications.

Capacitor Characteristics

The LDO’s are designed to work with ceramic capacitors on

the output to take advantage of the benefits they offer. For

capacitance values in the range of 0.47 µF to 4.7 µF, 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 1.0 µF ceramic

capacitor is in the range of 20 m

Ω to 40 mΩ, which easily

meets the ESR requirement for stability for the LDO’s.

For both input and output capacitors, careful interpretation of

the capacitor specification is required to ensure correct de-

vice operation. The capacitor value can change greatly, de-

pending on the operating conditions and capacitor type.

In particular, the output capacitor selection should take ac-

count 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 pa-

rameters are also dependant on the particular case size,

with smaller sizes giving poorer performance figures in gen-

eral. As an example, Figure 6 shows a typical graph com-

paring different capacitor case sizes in a Capacitance vs. DC

Bias plot. As shown in the graph, increasing the DC Bias

condition can result in the capacitance value falling below

the minimum value given in the recommended capacitor

specifications table. Note that the graph shows the capaci-

tance out of spec for the 0402 case size capacitor at higher

bias voltages. It is therefore recommended that the capacitor

manufacturers’ specifications for the nominal value capacitor

are consulted for all conditions, as some capacitor sizes

(e.g. 0402) may not be suitable in the actual application.

The ceramic capacitor’s capacitance can vary with tempera-

ture. The capacitor type X7R, which operates over a tem-

perature range of −55˚C to +125˚C, will only vary the capaci-

tance to within ±15%. The capacitor type X5R has a similar

tolerance over a reduced temperature range of −55˚C to

+85˚C. Many large value ceramic capacitors, larger than

1 µF are manufactured with Z5U or Y5V temperature char-

acteristics. Their capacitance can drop by more than 50% as

the temperature varies from 25˚C to 85˚C. Therefore X7R is

recommended over Z5U and Y5V in applications where the

ambient temperature will change significantly above or be-

low 25˚C.

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

0.47 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

20180723

FIGURE 6. Graph Showing a Typical Variation in

Capacitance vs. DC Bias

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