Datasheet

-t
=
C
DELAY
-
V
O
V
REF
R
P
ln
1
+
-
SHDN
GND
IN
OUT
LP3982
FAULT
100k
V
IN
CC
LMC7225
MICROPROCESSOR
RESET
C
DELAY
R
P
V
O
= 3V
0.1PF
T
JA
(160 - T
A
)
(0.300 x V
D
)
LP3982
www.ti.com SNVS185D –FEBRUARY 2002–REVISED APRIL 2013
(6)
Fault Detection
The LP3982 provides a FAULT pin that goes low during out of regulation conditions like current limit and thermal
shutdown, or when it approaches dropout. The latter monitors the input-to-output voltage differential and
compares it against a threshold that is slightly above the dropout voltage. This threshold also tracks the dropout
voltage as it varies with load current. Refer to Figure 4 in the typical characteristics section.
The FAULT pin requires a pull-up resistor since it is an open-drain output. This resistor should be large in value
to reduce energy drain. A 100 kΩ pull-up resistor works well for most applications.
Figure 20 shows the LP3985 with delay added to the FAULT pin for the reset pin of a microprocessor. The
output of the comparator stays low for a preset amount of time after the regulator comes out of a fault condition.
Figure 20. Power on Delayed Reset Application
The delay time for the application of Figure 20 is set as follows:
(7)
The application is set for a reset delay time of 8.8 ms. Note that the comparator should have high impedance
inputs so as to not load down the V
REF
at the CC pin of the LP3982.
Shutdown
The LP3982 goes into sleep mode when the SHDN pin is in a logic low condition. During this condition, the pass
transistor, error amplifier, and bandgap are turned off, reducing the supply current to 1 nA typical. The maximum
voltage for a logic low at the SHDN pin is 0.4V. A minimum voltage of 2V at the SHDN pin will turn the LP3982
back on. The SHDN pin may be directly tied to V
IN
to keep the part on. The SHDN pin may exceed V
IN
but not
the ABS MAX of 6.5V.
Figure 21 shows an application that uses the SHDN pin. It detects when the battery is too low and disconnects
the load by turning off the regulator. A micropower comparator (LMC7215) and reference (LM385) are combined
with resistors to set the minimum battery voltage. At the minimum battery voltage, the comparator output goes
low and tuns off the LP3982 and corresponding load. Hysteresis is added to the minimum battery threshold to
prevent the battery's recovery voltage from falsely indicating an above minimum condition. When the load is
disconnected from the battery, it automatically increases in terminal voltage because of the reduced IR drop
across its internal resistance. The Minimum battery detector of Figure 21 has a low detection threshold (V
LT
) of
3.6V that corresponds to the minimum battery voltage. The upper threshold (V
UT
) is set for 4.6V in order to
exceed the recovery voltage of the battery.
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