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LM3370

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SNVS406M – NOVEMBER 2005 – REVISED AUGUST 2011

OPERATION DESCRIPTION

DEVICE INFORMATION

The LM3370, a dual high efficiency step-down DC-DC converter, delivers regulated voltages from input rails

between 2.7V to 5.5V. Using voltage mode architecture with synchronous rectification, the LM3370 has the ability

to deliver up to 600 mA per channel. The performance is optimized for systems where efficiency and space are

critical.

There are three modes of operation depending on the current required: PWM, PFM, and shutdown. PWM mode

handles loads of approximately 70 mA or higher with 90% efficiency or better. Lighter loads cause the device to

channel.

The LM3370 can operate up to a 100% duty cycle (PFET 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.

CIRCUIT OPERATION

During the first portion of each switching cycle, the control block in the LM3370 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.

PWM OPERATION

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

allows the converter to achieve excellent 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.

INTERNAL SYNCHRONOUS RECTIFICATION

While in PWM mode, the LM3370 uses an internal NFET as a synchronous rectifier to reduce rectifier forward

voltage drop and associated power loss. Synchronous rectification provides a significant improvement in

efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier

diode.

CURRENT LIMITING

A current limit feature allows the LM3370 to protect itself and external components during overload conditions.

PWM mode implements cycle-by-cycle current limiting using an internal comparator that trips at 1200 mA (typ.).

If the outputs are shorted to ground the device enters a timed current limit mode where the NFET is turned on for

a longer duration until the inductor current falls below a low threshold, ensuring inductor has more time to decay,

thereby preventing runaway.

Copyright © 2005–2011, Texas Instruments Incorporated

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Product Folder Links: LM3370