LP2989 Datasheet, PDF (15/34 Pages) National Semiconductor (TI) – Micropower/Low Noise, 500 mA Ultra Low-Dropout Regulator For Use with Ceramic Output Capacitors

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LP2989
SNVS083O – FEBRUARY 2005 – REVISED MARCH 2015
Feature Description (continued)
7.3.4 Short Circuit Protection (Current Limit)
The internal current limit circuit is used to protect the LDO against high-load current faults or shorting events. The
LDO is not designed to operate in a steady-state current limit. During a current-limit event, the LDO sources
constant current. Therefore, the output voltage falls when load impedance decreases. Note also that if a current
limit occurs and the resulting output voltage is low, excessive power may be dissipated across the LDO, resulting
in a thermal shutdown of the output. A foldback feature limits the short-circuit current to protect the regulator from
damage under all load conditions. If OUT is forced below 0 V before EN goes high and the load current required
exceeds the foldback current limit, the device may not start correctly.
7.3.5 Thermal Protection
The device contains a thermal shutdown protection circuit to turn off the output current when excessive heat is
dissipated in the LDO. The thermal time-constant of the semiconductor die is fairly short, and thus the output
cycles on and off at a high rate when thermal shutdown is reached until the power dissipation is reduced. The
internal protection circuitry of the device is designed to protect against thermal overload conditions. The circuitry
is not intended to replace proper heat sinking. Continuously running the device into thermal shutdown degrades
its reliability.
7.4 Device Functional Modes
7.4.1 Operation With 16 V ≥ V_IN > V_OUT(TARGET) + 1 V
The device operates if the input voltage is equal to, or exceeds V_OUT(TARGET) + 1 V. At input voltages below the
minimum V_IN requirement, the devices does not operate correctly, and output voltage may not reach target value.
7.4.2 Operation with Shutdown Control
If the voltage on the SHUTDOWN pin is less than 0.18 V, the output is ensured to be OFF. When the voltage on
the SHUTDOWN pin is more than 1.6 V the output is ensured to be ON. Operating with the SHUTDOWN pin
voltage between 0.18 V and 1.6 V is strongly discouraged as the status of the output is not ensured.
7.4.3 Shutdown Input Operation
The LP2989 is shut off by driving the SHUTDOWN pin low, and turned on by pulling it high. If this feature is not
to be used, the SHUTDOWN should be tied to V_IN to keep the regulator output on at all times.
To assure proper operation, the signal source used to drive the Shutdown input must be able to swing above and
below the specified turn-on/turn-off voltage thresholds listed in the Electrical Characteristics section under V_SD.
To prevent mis-operation, the turn-on (and turn-off) voltage signals applied to the Shutdown input must have a
slew rate which is ≥ 40 mV/µs.
CAUTION
The regulator output voltage cannot be ensured if a slow-moving AC (or DC) signal is
applied that is in the range between the specified turn-on and turn-off voltages listed
under the electrical specification V_SD (see the Electrical Characteristics table).
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