PFM Mode

V

PWM Mode

V

20075822

20075823

Soft Start

V

(I

20075824

Operation Description

DEVICE INFORMATION

The LM3670, a high efficiency step down DC-DC switching

buck converter, delivers a constant voltage from either a

single Li-Ion or three cell NiMH/NiCd battery to portable

devices such as cell phones and PDAs. Using a voltage

mode

architecture

with

synchronous

rectification,

the

LM3670 has the ability to deliver up to 350 mA depending on

the input voltage and output voltage (voltage head room),

and the inductor chosen (maximum current capability).

There are three modes of operation depending on the cur-

rent required - PWM, PFM, and shutdown. PWM mode

handles current loads of approximately 70 mA or higher.

Lighter output current loads cause the device to automati-

cally switch into PFM for reduced current consumption (I

15 µA typ) and a longer battery life. Shutdown mode turns off

the

device,

offering

the

lowest

current

consumption

(I

The LM3670 can operate up to a 100% duty cycle (PMOS

switch always on) for low drop out control of the output

voltage. In this way the output voltage will be controlled

down to the lowest possible input voltage.

Additional features include soft-start, under voltage lock out,

current overload protection, and thermal overload protection.

As shown in Figure 1, only three external power components

are required for implementation.

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

by storing energy in a magnetic field. During the second

portion of each cycle, the controller turns the PFET switch

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