Application Hints (Continued)
DS012506-41
If the output is forced above the input by more than a VBE
,
this diode will become forward biased and current will flow
from the VOUT terminal to VIN
.
<
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 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).
DS012506-39
FIGURE 2. 5V/10 µF ESR Curves
DS012506-43
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
DS012506-40
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 VIN to keep the
regulator on at all times (the ON/OFF input must not be left
floating).
FIGURE 3. 3V/3.3 µF ESR Curves
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.
DS012506-42
Important: the regulator shutdown function will operate incor-
rectly if a slow-moving signal is applied to the ON/OFF input.
FIGURE 4. 3V/10 µF ESR Curves
Micro SMD Mounting
REVERSE CURRENT PATH
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.
The power transistor used in the LP2981 has an inherent di-
ode connected between the regulator input and output (see
below).
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