REVERSE CURRENT PATH

The power transistor used in the LP2980 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

occur under these conditions as long as the current flowing

into the output pin does not exceed 100 mA.

ON/OFF INPUT OPERATION

The LP2980 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

LP2980 input voltage or another logic supply. The high-level

voltage may exceed the LP2980 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.

For best results during assembly, alignment ordinals on the

PC board may be used to facilitate placement of the micro

SMD device.

Micro SMD Light Sensitivity

Exposing the micro SMD device to direct sunlight will cause

misoperation of the device. Light sources such as Halogen

lamps can also affect electrical performance if brought near

to the device.

The wavelenghts which have the most detrimental effect are

reds and infra-reds, which means that the fluorescent light-

ing used inside most buildings has very little effect on perfor-

mance. A micro SMD test board was brought to within 1 cm

of a fluorescent desk lamp and the effect on the regulated

output voltage was negligible, showing a deviation of less

than 0.1% from nominal.

DS012078-50

FIGURE 3. 10 µF ESR Range

DS012078-34

FIGURE 4. LP2980 Reverse Current Path

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