LP38869MHESLASHNOPB datasheet(17/23 Pages) TI

17 / 23 page

LP38869

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SNVS877B – AUGUST 2012 – REVISED APRIL 2013

BOARD TYPE

THERMAL VIAS

θJA

JEDEC 2-Layer

None

141°C/W

JESD 51-3

131°C/W

78°C/W

65°C/W

JEDEC 4-Layer

59°C/W

JESD 51-7

56°C/W

53°C/W

50°C/W

Noise, PSRR, and Transient Response

The LP38869 is designed to achieve low dropout voltage and low quiescent current in battery-powered systems

while still maintaining good noise, transient response, and AC rejection (see PSRR vs. Frequency in the Typical

Operating Characteristics). When operating from very noisy sources, supply noise rejection and transient

response can be improved by increasing the input and output capacitor values and employing passive post

filtering. The LP38869 output noise is typically 50 µVRMS.

Reverse Input-Output Voltage

A reverse voltage condition will exist when the voltage at the output pin is higher than the voltage at the input pin.

Typically this will happen when VIN is abruptly taken low and COUT continues to hold a sufficient charge such that

the input to output voltage becomes reversed. Alternately, this could also happen if a secondary supply is

connected to the LP38869 output.

While VIN is greater than the UVLO threshold, and the SHDN pin is above the VSHDN(ON) threshold, the control

loop circuitry will attempt to regulate the output voltage. Since the input voltage is less than the output voltage the

control circuit will drive the gate of the pass element to the full ON condition when the output voltage begins to

fall. In this condition, reverse current will flow from the output pin to the input pin, limited only by the RDS(ON) of

the pass element and the output to input voltage differential. This will condition will continue until VIN falls below

the UVLO threshold, or the SHDN pin voltage falls below the VSHDN(OFF) threshold.

The internal PFET pass element in the LP38869 has an inherent parasitic (body) diode. During normal operation,

the input voltage is higher than the output voltage and the body diode is reverse biased. However, if the output is

turned OFF, either by VIN < UVLO or VSHDN < VSHDN(OFF), and the output voltage is more than 500 mV (typical)

above the input voltage the body diode becomes forward biased and current flows from the output pin to the

input pin through the body diode.

The reverse current in the body diode should be limited to less than 1A continuous and less than 5A peak.

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