REVERSE CURRENT PATH

The power transistor used in the LP2981 has an inherent di-

ode connected between the regulator input and output (see

below).

If the output is forced above the input by more than a V

this diode will become forward biased and current will flow

from the V

to the part.

The internal diode can also be turned on by abruptly step-

ping the input voltage to a value below the output voltage.

To prevent regulator mis-operation, a Schottky diode should

be used in any application where input/output voltage condi-

tions can cause the internal diode to be turned on (see be-

low).

As shown, the Schottky diode is connected in parallel with

the internal parasitic diode and prevents it from being turned

on by limiting the voltage drop across it to about 0.3V.

ON/OFF INPUT OPERATION

The LP2981 is shut off by pulling the ON/OFF input low, and

turned on by driving the input high. If this feature is not to be

used, the ON/OFF input should be tied to V

regulator on at all times (the ON/OFF input must not be left

floating).

To ensure 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 which guar-

antee an ON or OFF state (see Electrical Characteristics).

The ON/OFF signal may come from either a totem-pole out-

put, or an open-collector output with pull-up resistor to the

LP2981 input voltage or another logic supply. The high-level

voltage may exceed the LP2981 input voltage, but must re-

main within the Absolute Maximum Ratings for the ON/OFF

pin.

It is also important that the turn-on/turn-off voltage signals

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

greater than 40 mV/µs.

Important: the regulator shutdown function will operate incor-

rectly if a slow-moving signal is applied to the ON/OFF input.

Micro SMD Mounting

The micro SMD package requires specific mounting tech-

niques which are detailed in National Semiconductor Appli-

cation Note # 1112. Referring to the section

Surface Mount

Technology (SMT) Assembly Considerations, it should be

noted that the pad style which must be used with the 5-pin

package is the NSMD (non-solder mask defined) type.

DS012506-39

FIGURE 2. 5V/10 µF ESR Curves

DS012506-40

FIGURE 3. 3V/3.3 µF ESR Curves

DS012506-42

FIGURE 4. 3V/10 µF ESR Curves

DS012506-41

DS012506-43

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