Application Hints

V

To prevent misoperation, ensure that V

before start-up is initiated. This scenario can occur in sys-

tems with a backup battery using reverse-biased "blocking"

diodes which may allow enough leakage current to flow into

the V

the main power is removed. Using low leakage diodes or a

resistive pull down can prevent the voltage at V

above 50mV. Large bulk capacitors connected to V

also cause a start-up problem if they do not discharge fully

before re-start is initiated (but only if V

below 1V). A resistor connected across the capacitor will

allow it to discharge more quickly. It should be noted that the

probability of a "false start" caused by incorrect logic states

is extremely low.

EXTERNAL CAPACITORS

Like any low-dropout regulator, external capacitors are re-

quired to assure stability. These capacitors must be correctly

selected for proper performance.

INPUT CAPACITOR: An input capacitor of at least 10µF is

required. Ceramic, Tantalum, or Electrolytic capacitors may

be used, and capacitance may be increased without limit.

OUTPUT CAPACITOR: An output capacitor is required for

loop stability. It must be located less than 1 cm from the

device and connected directly to the output and ground pins

using traces which have no other currents flowing through

them (see PCB Layout section).

The minimum value of output capacitance that can be used

for stable full-load operation is 10µF, but it may be increased

without limit. The output capacitor must have an ESR value

as shown in the stable region of the curve (below).Tantalum

capacitors are recommended for the output capacitor.

ESR Curve

20063170

SELECTING A CAPACITOR

It is important to note that capacitance tolerance and varia-

tion with temperature must be taken into consideration when

selecting a capacitor so that the minimum required amount

of capacitance is provided over the full operating tempera-

ture range. In general, a good Tantalum capacitor will show

very little capacitance variation with temperature, but a ce-

ramic may not be as good (depending on dielectric type).

Aluminum electrolytics also typically have large temperature

variation of capacitance value.

Equally important to consider is a capacitor’s ESR change

with temperature: this is not an issue with ceramics, as their

ESR is extremely low. However, it is very important in Tan-

talum and aluminum electrolytic capacitors. Both show in-

creasing ESR at colder temperatures, but the increase in

aluminum electrolytic capacitors is so severe they may not

be feasible for some applications (see Capacitor Character-

istics Section).

CAPACITOR CHARACTERISTICS

CERAMIC: For values of capacitance in the 10 to 100 µF

range, ceramics are usually larger and more costly than

tantalums but give superior AC performance for bypassing

high frequency noise because of very low ESR (typically less

than 10 m

Ω). However, some dielectric types do not have

good capacitance characteristics as a function of voltage

and temperature.

Z5U and Y5V dielectric ceramics have capacitance that

drops severely with applied voltage. A typical Z5U or Y5V

capacitor can lose 60% of its rated capacitance with half of

the rated voltage applied to it. The Z5U and Y5V also exhibit

a severe temperature effect, losing more than 50% of nomi-

nal capacitance at high and low limits of the temperature

range.

X7R and X5R dielectric ceramic capacitors are strongly rec-

ommended if ceramics are used, as they typically maintain a

ing ratings of temperature and voltage. Of course, they are

typically larger and more costly than Z5U/Y5U types for a

given voltage and capacitance.

TANTALUM: Solid Tantalum capacitors are recommended

for use on the output because their typical ESR is very close

to the ideal value required for loop compensation. They also

work well as input capacitors if selected to meet the ESR

requirements previously listed.

Tantalums also have good temperature stability: a good

quality Tantalum will typically show a capacitance value that

varies less than 10-15% across the full temperature range of

125˚C to −40˚C. ESR will vary only about 2X going from the

high to low temperature limits.

The increasing ESR at lower temperatures can cause oscil-

lations when marginal quality capacitors are used (if the ESR

of the capacitor is near the upper limit of the stability range at

room temperature).

ALUMINUM: This capacitor type offers the most capaci-

tance for the money. The disadvantages are that they are

larger in physical size, not widely available in surface mount,

and have poor AC performance (especially at higher fre-

quencies) due to higher ESR and ESL.

Compared by size, the ESR of an aluminum electrolytic is

higher than either Tantalum or ceramic, and it also varies

greatly with temperature. A typical aluminum electrolytic can

exhibit an ESR increase of as much as 50X when going from

25˚C down to −40˚C.

It should also be noted that many aluminum electrolytics only

specify impedance at a frequency of 120 Hz, which indicates

they have poor high frequency performance. Only aluminum

electrolytics that have an impedance specified at a higher

frequency (between 20 kHz and 100 kHz) should be used for

the LP387X. Derating must be applied to the manufacturer’s

ESR specification, since it is typically only valid at room

temperature.

www.national.com

9