IN

EN

GND

OUT

ON

OFF

1.2 M

LP3991

9

LP3991

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SNVS296J – DECEMBER 2006 – REVISED JUNE 2016

Product Folder Links: LP3991

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7 Detailed Description

7.1 Overview

The LP3991 device is a monolithic integrated low-dropout voltage regulator with a low input operating voltage

tolerance. Key protection circuits, including output current limitation and thermal shutdown, are integrated in the

device. Using the EN pin, the device may be controlled to provide a shutdown state, in which negligible supply

current is drawn. The LP3991 is designed to be stable with space-saving ceramic capacitors as small as 0402

case size.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Post-Buck Regulator

Linear post-regulation can be an effective way to reduce ripple and switching noise from DC-DC convertors while

still maintaining a reasonably high overall efficiency.

The LP3991 is particularly suitable for this role due to its low input voltage requirements. In addition, there is

often no need for a separate input capacitor for the LP3991 as it can share the output cap of the DC-DC

convertor.

Care of PCB layouts involving switching regulators is paramount. In particular, the ground paths for the LDO

must be routed separately from the switcher ground and star connected close to the battery. Routing of the

switch pin of the DC-DC convertor must be kept short to minimize radiated EMI. A low pass filter such as a ferrite

bead or common mode choke on the battery input leads can further reduce radiated EMI.

Figure 19 shows a typical example using an LM3673, 350-mA DC-DC buck regulator with a nominal output of

1.8 V and a 1.5-V LP3991 device. The overall efficiency is greater than 70% over the full Li-Ion battery voltage

The LP3991-1.5 remains in regulation down to an input voltage of 1.65 V; thus, in this case, a 1.8-V buck with

5% tolerance is adequate for all conditions of temperature and load.