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 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 amount of output capacitance that can be

used for stable operation is 10µF. For general usage across

all load currents and operating conditions, the part was

characterized using a 10µF Tantalum input capacitor. The

minimum and maximum stable ESR range for the output

capacitor was then measured which kept the device stable,

assuming any output capacitor whose value is greater than

10µF (see Figure 1 below).

It should be noted that it is possible to operate the part with

an output capacitor whose ESR is below these limits, as-

suming that sufficient ceramic input capacitance is provided.

This will allow stable operation using ceramic output capaci-

tors (see next section).

OPERATION WITH CERAMIC OUTPUT CAPACITORS

LP385X voltage regulators can operate with ceramic output

capacitors if the values of the input and output capacitors are

selected appropriately. The total ceramic output capacitance

must be equal to or less than a specified maximum value in

order for the regulator to remain stable over all operating

conditions. This maximum amount of ceramic output capaci-

tance is dependent upon the amount of ceramic input ca-

pacitance used as well as the load current of the application.

This relationship is shown in Figure 2, which graphs the

maximum stable value of ceramic output capacitance as a

function of ceramic input capacitance for load currents of

1.5A.

If the maximum load current is 1.5A and a 10µF ceramic

input capacitor is used, the regulator will be stable with

ceramic output capacitor values from 10µF up to about

150µF. When calculating the total ceramic output capaci-

tance present in an application, it is necessary to include any

ceramic bypass capacitors connected to the regulator out-

put.

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).

20031070

FIGURE 1. ESR Curve for C

Input Capacitor)

20031095

FIGURE 2. Maximum Ceramic Output Capacitance vs

Ceramic Input Capacitance

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