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.

SETTING THE OUTPUT VOLTAGE

The output voltage is set using the resistors R1 and R2 (see

Typical Application Circuit). The output is also dependent on

the reference voltage (typically 1.216V) which is measured

at the ADJ pin. The output voltage is given by the equation:

V

This equation does not include errors due to the bias current

flowing in the ADJ pin which is typically about 10 nA. This

100k

Ω , a small error may be introduced by the ADJ bias

current.

The tolerance of the external resistors used contributes a

significant error to the output voltage accuracy, with 1%

resistors typically adding a total error of approximately 1.4%

to the output voltage (this error is in addition to the tolerance

of the reference voltage at V

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 1µF is

required. Ceramic or Tantalum may be used, and capaci-

tance 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 the output capacitance that can be

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

increased without limit. The output capacitor must have an

ESR value as shown in the stable region of the curve (be-

low).

ESR Curve

20074470

C

The capacitor C

improve loop compensation. The correct amount of capaci-

tance depends on the value selected for R1 (see Typical

Application Circuit). The capacitor should be selected such

that the zero frequency as given by the equation shown

below is approximately 45 kHz:

Fz = 45,000=1/(2x

π xR1xC

A good quality ceramic with X5R or X7R dielectric should be

used for this capacitor.

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

www.national.com

9