Datasheet

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LM3370
SNVS406N –NOVEMBER 2005–REVISED MAY 2013 www.ti.com
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
automatically switch into PFM mode to maintain high efficiency with low supply current (I
Q
= 20µA typ.) per
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.
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