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LP3988-Q1

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SLAS928B – MARCH 2013 – REVISED NOVEMBER 2016

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Copyright © 2013–2016, Texas Instruments Incorporated

Feature Description (continued)

8.3.5 Current Limit

The fixed current limit (ISC) of the LP3988-Q1 helps protect the regulator during output fault conditions. The

maximum amount of current the device can source is typically 600 mA and is largely independent of input and

output voltage. For reliable operation, do not operate the device in current limit for extended periods of time.

Depending on power dissipation, thermal resistance, and ambient temperature, operating at the current limit may

activate the thermal shutdown circuitry.

8.3.6 Thermal Shutdown (TSD)

Thermal shutdown (TSD) protection disables the output when the junction temperature rises to approximately

160°C, This shutdown causes the device dissipation to go to zero, which allows the device to cool. When the

device junction temperature cools to approximately 140°C, the output circuitry is automatically enabled.

Depending on power dissipation, thermal resistance, and ambient temperature, the thermal protection circuit may

cycle ON and OFF. This cycling limits the dissipation of the regulator, protecting it from damage as a result of

overheating. The thermal shutdown circuitry of the LP3988-Q1 has been designed to protect against temporary

thermal overload conditions. The TSD circuitry was not intended to replace proper heat sinking. Continuously

running the LP3988-Q1 device in TSD may degrade device reliability and lifetime.

8.3.7 Fast Turnon Time

The LP3988-Q1 utilizes a speed-up circuit to ramp up the internal VREF voltage to its final value to achieve a fast

output turnon time.

8.4 Device Functional Modes

8.4.1 Normal Operation

The device regulates to the nominal output voltage under the following conditions:

•

The input voltage (VIN ) is at least either 2.5 V, or VOUT + 200 mV, whichever is greater.

•

The enable voltage (VEN) is at least 1.2 V (VIH).

•

The output current is no more than the maximum rated current of 150 mA.

•

The device junction temperature (TJ) is no more than the maximum specified operating junction temperature.

8.4.2 Dropout Operation

If the input voltage is lower than the nominal output voltage plus the specified dropout voltage (VDO), but all other

conditions are met for normal operation, the device operates in dropout mode. In this mode of operation, the

output voltage is the same as the input voltage minus the dropout voltage. The transient performance of the

device is significantly degraded because the pass device (PMOS transistor), no longer controls the current

through the LDO. Line or load transients while operating in dropout can result in large output voltage deviations.

8.4.3 On/Off Input Operation

The LP3988-Q1 is turned off by pulling the EN pin low, and turned on by pulling it high. If this feature is not used,

the EN pin must be tied to VIN to keep the regulator output on at all time. To assure proper operation, the signal

source used to drive the EN input must be able to swing above and below the specified turnon/turnoff voltage

thresholds listed in the Electrical Characteristics section under VIL and VIH.

8.4.4 Disabled

The device is disabled under the following conditions:

•

VEN is no more than 0.6 V (VIL).

•

TJ is greater than the thermal shutdown temperature.