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LM3678 Datasheet(PDF) 9 Page - Texas Instruments

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No. de pieza LM3678
Descripción Electrónicos  High-Performance Miniature 1.5-A Step-Down DC-DC Converter for Handheld Applications
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Fabricante Electrónico  TI1 [Texas Instruments]
Página de inicio  http://www.ti.com
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LM3678 Datasheet(HTML) 9 Page - Texas Instruments

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LM3678
www.ti.com
SNVS464B – APRIL 2008 – REVISED JANUARY 2009
OPERATION DESCRIPTION
DEVICE INFORMATION
The LM3678, a high-efficiency step-down DC-DC switching buck converter, delivers a constant voltage from a
single Li-Ion battery and input voltage rails from 2.5 V to 5.5 V to portable devices such as cell phones and
PDAs. Using a voltage mode architecture with synchronous rectification, the LM3678 has the ability to deliver up
to 1.5 A, depending on the input voltage, output voltage, ambient temperature and the inductor chosen.
Additional features include soft start, undervoltage 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 0.5 V. It is recommended to keep the part in shutdown until the
input voltage is 2.5 V or higher.
CIRCUIT OPERATION
During the first portion of each switching cycle, the control block in the LM3678 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 (VIN–VOUT)/L, 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 - VOUT/L.
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 device 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.
The output voltage is regulated by switching at a constant frequency and then modulating the energy per cycle to
control power to the load. At the beginning of each clock cycle the PFET switch is turned on and the inductor
current ramps up until the comparator trips and the control logic turns off the switch. The current limit comparator
can also turn off the switch in case the current limit of the PFET is exceeded. Then the NFET switch is turned on
and the inductor current ramps down. The next cycle is initiated by the clock turning off the NFET and turning on
the PFET.
Figure 25. Typical PWM Operation
Copyright © 2008–2009, Texas Instruments Incorporated
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