2-MHz

Oscillator

Soft

Start

Ramp

Generator

Thermal

Shutdown

Undervoltage

Lockout

Frequency

Compensation

+

-

Error

Amp

Control Logic

Driver

Current Limit

Comparator

Ref1

PFM Current

Comparator

Ref2

SW

Zero Crossing

Comparator

FB

EN

VIN

PWM Comparator

pfm_low

pfm_hi

GND

Bandgap

+

-

1 V

Fixed Version

ADJ Version

+

-

0.5 V

+

-

+

-

LM3673

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SNVS434L – JULY 2006 – REVISED OCTOBER 2015

8 Detailed Description

8.1 Overview

The LM3673, a high-efficiency step-down DC-DC switching buck converter, delivers a constant voltage from a

single Li-Ion battery and input voltage ranging from 2.7 V to 5.5 V to portable devices such as cell phones and

PDAs. Using a voltage mode architecture with synchronous rectification, the LM3673 has the ability to deliver up

to 350 mA depending on the input voltage, output voltage, ambient temperature and the inductor chosen.

There are three modes of operation depending on the current required: pulse width modulation (PWM), pulse

frequency modulation (PFM), and shutdown. The device operates in PWM mode at load current of approximately

80 mA or higher. Lighter load current cause the device to automatically switch into PFM for reduced current

Additional features include soft-start, undervoltage protection, current overload protection, and thermal shutdown

protection. As shown in Figure 17, only three external power components are required for implementation. The

part uses an internal reference voltage of 0.5 V. It is recommended to keep the part in shutdown until the input

voltage is 2.7 V or higher.

8.2 Functional Block Diagram

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