-t

=

-

+

-

SHDN

GND

IN

OUT

LP3982

FAULT

100k

CC

LMC7225

RESET

0.12F



LP3982

www.ti.com

SNVS185D – FEBRUARY 2002 – REVISED APRIL 2013

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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 k

Ω 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:

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The application is set for a reset delay time of 8.8 ms. Note that the comparator should have high impedance

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

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

exceed the recovery voltage of the battery.

Copyright © 2002–2013, Texas Instruments Incorporated

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