Application Hints

EXTERNAL CAPACITORS

Like any low-dropout regulator, the LP2983 requires external

capacitors for regulator stability. These capacitors must be

correctly selected for good performance.

Input Capacitor

An input capacitor whose capacitance is

≥ 1 µF is required

between the LP2983 input and ground (the amount of ca-

pacitance 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 analog ground.

Any good quality ceramic, tantalum, or film capacitor may be

used at the input.

Important: Tantalum capacitors can suffer catastrophic fail-

ure

due

to

surge

current

when

connected

to

a

low-impedance source of power (like a battery or 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 ESR on the input capacitor,

but tolerance and temperature coefficient must be consid-

ered when selecting the capacitor to ensure the capacitance

will be

≥ 1 µF over the entire operating temperature range.

Output Capacitor

The LP2983 is designed specifically to work with ceramic

output capacitors, utilizing circuitry which allows the regula-

tor to be stable across the entire range of output current with

an output capacitor whose ESR is as low as zero ohms. See

Figure 1 below.

The LP2983 requires a minimum of 2.2 µF on the output

(output capacitor size can be increased without limit).

It is important to remember that capacitor tolerance and

variation with temperature must be taken into consideration

when selecting an output capacitor so that the minimum

required amount of output capacitance is provided over the

full operating temperature range. It should be noted that

ceramic capacitors can exhibit large changes in capacitance

with temperature (see next section,

Capacitor Characteris-

tics).

The output capacitor must be located not more than 1 cm

from the output pin and returned to a clean analog ground.

CAPACITOR CHARACTERISTICS

The LP2983 was designed to work with ceramic capacitors

on the output to take advantage of the benefits they offer: for

capacitance values in the 2.2 µF to 4.7 µF range, ceramics

are the least expensive and also have the lowest ESR

values

(which

makes

them

best

for

eliminating

high-frequency noise).

One disadvantage of ceramic capacitors is that their capaci-

tance can vary with temperature. Most large value ceramic

capacitors (

≥ 2.2 µF) are manufactured with the Z5U or Y5V

temperature characteristic, which results in the capacitance

dropping by more than 50% as the temperature goes from

25˚C to 85˚C.

This could cause problems if a 2.2 µF capacitor were used

on the output since it will drop down to approximately 1 µF at

high ambient temperatures (which could cause the LP2983

to oscillate). If Z5U or Y5V capacitors are used on the output,

a minimum capacitance value of 4.7 µF must be observed.

A better choice for temperature coefficient in ceramic capaci-

fortunately, the larger values of capacitance are not offered

by all manufacturers in the X7R dielectric.

ON/OFF INPUT OPERATION

The LP2983 is shut off by driving the ON/OFF input low, and

turned on by pulling it high. If this feature is not to be used,

the ON/OFF input should be tied to V

output on at all times.

To assure proper operation, the signal source used to drive

the ON/OFF input must be able to swing above and below

the specified turn-on/turn-off voltage thresholds listed in the

Electrical Characteristics section under V

mis-operation, the turn-on (and turn-off) voltage signals ap-

plied to the ON/OFF input must have a slew rate which is

≥ 40 mV/µs.

Caution: the regulator output voltage can not be guaranteed

if a slow-moving AC (or DC) signal is applied that is in the

range between the specified turn-on and turn-off voltages

listed under the electrical specification V

cal Characteristics).

REVERSE INPUT-OUTPUT VOLTAGE

The PNP power transistor used as the pass element in the

LP2983 has an inherent diode connected between the regu-

lator output and input. During normal operation (where the

input voltage is higher than the output) this diode is

reverse-biased.

However, if the output is pulled above the input, this diode

will turn ON and current will flow into the regulator output. In

such cases, a parasitic SCR can latch which will allow a high

current to flow into V

damage the part.

In any application where the output may be pulled above the

input, an external Schottky diode must be connected from

V

reverse voltage across the LP2983 to 0.3V (see Absolute

Maximum Ratings).

C

Ceramic to ESR Pin

20029120

FIGURE 1.

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