LP3878-ADJ_15 Datasheet, PDF (11/25 Pages) Texas Instruments – Micropower 800-mA Low-Noise

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7 Detailed Description
LP3878-ADJ
SNVS311D – MAY 2005 – REVISED FEBRUARY 2015
7.1 Overview
The LP3878-ADJ is an adjustable regulator; the output voltage can be set from 1 V to 5.5 V. The device can
deliver 800-mA continuous load current. Standard regulator features, such as overcurrent and overtemperature
protection, are also included.
The LP3878-ADJ contains other features:
• Low power shutdown current and low ground pin current
• Very low output noise
• 8-lead SO PowerPAD or WSON surface-mount packages to allow for increased power dissipation.
7.2 Functional Block Diagram
ADJ BYPASS IN
6
1
4
OUT
5
1V
V_REF
GROUND 3
Error
Amp
+
-
LP3878-
ADJ
2 N/C
8 SHUTDOWN
7 N/C
7.3 Feature Description
7.3.1 Shutdown Input Operation
The LP3878-ADJ is shut off by pulling the SHUTDOWN input low, and turned on by pulling it high. If this feature
is not to be used, the SHUTDOWN input 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 turnon or turnoff voltage thresholds listed in the Electrical Characteristics under V_ON/OFF.
7.3.2 Reverse Input-Output Voltage
The PNP power transistor used as the pass element in the LP3878-ADJ contains a parasitic diode between the
IN pin and the OUT pin. During normal operation (where the IN pin voltage is higher than the OUT pin voltage)
this parasitic diode is reverse-biased. However, if the OUT pin voltage is pulled above the IN pin voltage this
diode will turn ON, and current will flow into the LP3878-ADJ OUT pin.
In such cases, a parasitic SCR between the IN pin and the GND pin can latch ON which will allow a high current
to flow from the VIN supply, into the IN pin to ground, which can damage the part. In any application where the
OUT pin voltage may be higher than the IN pin voltage, even momentarily, an external Schottky diode must be
connected from the IN pin to the OUT pin (cathode to IN pin, anode to OUT pin), to limit the reverse voltage
across the LP3878-ADJ to 0.3 V (see Absolute Maximum Ratings).
7.3.3 Low Output Noise
With a 10-nF capacitor on the BYPASS pin, the output noise is only 18 µV.
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