LP2982 Datasheet, PDF (14/17 Pages) National Semiconductor (TI) – Micropower 50 mA Ultra Low-Dropout Regulator

Application Hints (Continued)
REVERSE CURRENT PATH
The power transistor used in the LP2982 has an inherent di-
ode connected between the regulator input and output (see
below).
DS012679-39
If the output is forced above the input by more than a V_BE,
this diode will become forward biased and current will flow
from the V_OUT terminal to V_IN. This current must be limited to
< 100 mA to prevent damage 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 misoperation, 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).
DS012679-40
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 LP2982 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_IN to keep the
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
LP2982 input voltage or another logic supply. The high-level
voltage may exceed the LP2982 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.
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