L

L

11

LM3670

www.ti.com

SNVS250F – NOVEMBER 2004 – REVISED FEBRUARY 2016

Product Folder Links: LM3670

Submit Documentation Feedback

Copyright © 2004–2016, Texas Instruments Incorporated

7.3 Feature Description

7.3.1 Circuit Operation

The LM3670 operates as follows. During the first portion of each switching cycle, the control block in the LM3670

turns on the internal PFET switch. This allows current to flow from the input through the inductor to the output

filter capacitor and load. The inductor limits the current to a ramp with a slope of:

(1)

by storing energy in a magnetic field. During the second portion of each cycle, the controller turns the PFET

switch off, blocking current flow from the input, and then turns the NFET synchronous rectifier on. The inductor

draws current from ground through the NFET to the output filter capacitor and load, which ramps the inductor

current down with a slope of:

(2)

The output filter stores charge when the inductor current is high, and releases it when low, smoothing the voltage

across the load.

7.3.2 Soft Start

The LM3670 has a soft-start circuit that limits in-rush current during start-up. Typical start-up times with a 10-µF

output capacitor and 350-mA load is 400 µs:

Table 1. Typical Start-Up Times for Soft Start

INRUSH CURRENT (mA)

DURATION (µs)

0

32

70

224

140

256

280

256

620

until soft start ends

7.3.3 LDO - Low Dropout Operation

The LM3670 can operate at 100% duty cycle (no switching, PMOS switch is completely on) for low dropout

support of the output voltage. In this way the output voltage is controlled down to the lowest possible input

voltage.

The minimum input voltage needed to support the output voltage is

where

•

•

•

(3)