LP2992 datasheet(13/15 Pages) NSC

13 / 15 page

Application Hints (Continued)

It should also be noted that the ESR of a typical Tantalum will

increase about 2:1 as the temperature goes from 25˚C down

to −40˚C, so some guard band must be allowed.

ON/OFF INPUT OPERATION

The LP2992 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

IN to keep the regulator

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

ON/OFF. To prevent

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

ON/OFF (see Electri-

cal Characteristics).

REVERSE INPUT-OUTPUT VOLTAGE

The PNP power transistor used as the pass element in the

LP2992 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

IN (and out the ground pin), which can

damage the part.

In any application where the output may be pulled above the

input, an external Schottky diode must be connected from

IN to VOUT (cathode on VIN, anode on VOUT), to limit the

reverse voltage across the LP2992 to 0.3V (see Absolute

Maximum Ratings).

LLP Mounting

The LLP package requires specific mounting techniques

which are detailed in National Semiconductor Application

Note # 1187. Referring to the section PCB Design Recom-

mendations, it should be noted that the pad style which

should be used with the LLP package is the NSMD

(non-solder mask defined) type.

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