6 Detailed Description

6.1 Overview

The LP2991 is a fixed-output, low-noise, high PSRR, low-dropout regulator that offers exceptional, cost-effective

performance for both portable and nonportable applications. The LP2991 has an output tolerance of 1% across

line, load, and temperature variation (for the new chip) and is capable of delivering 250 mA of continuous load

current.

This device features integrated overcurrent protection, thermal shutdown, output enable, and internal output

pulldown and has a built-in soft-start mechanism for controlled inrush current. This device delivers excellent line

and load transient performance. The operating ambient temperature range of the device is –40°C to 125°C.

6.2 Functional Block Diagram

Bandgap

Reference

VREF = 1.2 V

IN

OUT

GND

Internal

Controller

UVLO

Current

Limit

Thermal

Shutdown

ON/OFF

GND

Output

Pull-down

GND

BYPASS

GND

6.3 Feature Description

6.3.1 Output Enable

The ON/OFF pin for the device is an active-high pin. The output voltage is enabled when the voltage of the

ON/OFF pin is greater than the high-level input voltage of the ON/OFF pin and disabled with the ON/OFF pin

voltage is less than the low-level input voltage of the ON/OFF pin. If independent control of the output voltage is

not needed, connect the ON/OFF pin to the input of the device.

The device has an internal pulldown circuit that activates when the device is disabled by pulling the ON/OFF pin

voltage lower than the low-level input voltage of the ON/OFF pin, to actively discharge the output voltage.

6.3.2 Dropout Voltage

Dropout voltage (VDO) is defined as the input voltage minus the output voltage (VIN – VOUT) at the rated output

current (IRATED), where the pass transistor is fully on. IRATED is the maximum IOUT listed in the Recommended

Operating Conditions table. The pass transistor is in the ohmic or triode region of operation, and acts as a

switch. The dropout voltage indirectly specifies a minimum input voltage greater than the nominal programmed

output voltage at which the output voltage is expected to stay in regulation. If the input voltage falls to less than

the nominal output regulation, then the output voltage falls as well.

LP2992

SNVS171L – NOVEMBER 2001 – REVISED FEBRUARY 2025

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