Application Hints

OPERATION DESCRIPTION

The LP5996 is a low quiescent current, power management

IC, designed specifically for portable applications requiring

minimum board space and smallest components. The

LP5996 contains two independently selectable LDOs. The

first is capable of sourcing 150mA at outputs between 0.8V

and 3.3V. The second can source 300mA at an output volt-

age of 0.8V to 3.3V.

INPUT CAPACITOR

An input capacitor is required for stability. It is recommended

that a 1.0µF capacitor be connected between the LP5996

input pin and ground (this capacitance value may be in-

creased 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: 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 (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 1.0µF over the entire operating temperature range.

OUTPUT CAPACITOR

The LP5996 is designed specifically to work with very small

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

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

Ω to

500m

Ω, is suitable in the LP5996 application circuit.

For this device the output capacitor should be connected

between the V

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

device output, C

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 5m

Ω to 500mΩ for stability.

NO-LOAD STABILITY

The LP5996 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 LP5996 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 LP5996.

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 1 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 (0.7µF in this case). Note that the graph

shows the capacitance out of spec for the 0402 case size

capacitor at higher bias voltages. It is therefore recom-

mended 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 capacitance value of ceramic capacitors varies with

temperature. The capacitor type X7R, which operates over a

temperature range of -55˚C to +125˚C, will only vary the

similar tolerance over a reduced temperature range of -55˚C

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

1µF are manufactured with Z5U or Y5V temperature charac-

teristics. 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

0.47µF 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

20171540

FIGURE 1. Graph Showing a Typical Variation in

Capacitance vs DC Bias

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