LM3673

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SNVS434J – JULY 2006 – REVISED NOVEMBER 2006

Typical Performance Characteristics (continued)

Start Up into PWM Mode

Start Up into PFM Mode

Operation Description

DEVICE INFORMATION

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 2.7V to 5.5V 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 - PWM (Pulse Width Modulation), PFM

(Pulse Frequency Modulation), 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

offering the lowest current consumption.

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

protection. As shown in Figure 1, only three external power components are required for implementation.

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

input voltage is 2.7V or higher.

CIRCUIT OPERATION

During the first portion of each switching cycle, the control block in the LM3673 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

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

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

smoothing the voltage across the load.

The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the

load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and

synchronous rectifier at the SW pin to a low-pass filter formed by the inductor and output filter capacitor. The

output voltage is equal to the average voltage at the SW pin.

PWM OPERATION

During PWM operation the converter operates as a voltage-mode controller with input voltage feed forward. This

allows the converter to achieve good load and line regulation. The DC gain of the power stage is proportional to

the input voltage. To eliminate this dependence, feed forward inversely proportional to the input voltage is

introduced.

Copyright © 2006, Texas Instruments Incorporated

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