Texas Instruments LM3671 Evaluation Board LM3671LC-1.8EV/NOPB LM3671LC-1.8EV/NOPB Datenbogen
Produktcode
LM3671LC-1.8EV/NOPB
SNVS294Q – NOVEMBER 2004 – REVISED NOVEMBER 2013
OPERATION DESCRIPTION
Device Information
The LM3671, 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 2.7V to 5.5V to portable devices such as cell phones and PDAs.
Using a voltage mode architecture with synchronous rectification, the LM3671 has the ability to deliver up to 600
mA depending on the input voltage, output voltage, ambient temperature and the inductor chosen.
single Li-Ion battery and input voltage rails from 2.7V to 5.5V to portable devices such as cell phones and PDAs.
Using a voltage mode architecture with synchronous rectification, the LM3671 has the ability to deliver up to 600
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
reduced current consumption (I
(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
reduced current consumption (I
Q
= 16 µA typ) and a longer battery life. Shutdown mode turns off the device,
offering the lowest current consumption (I
SHUTDOWN
= 0.01 µA typ).
Additional features include soft-start, under-voltage protection, current overload protection, and thermal shutdown
protection. As shown in
protection. As shown in
, 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.
input voltage is 2.7V or higher.
Circuit Operation
During the first portion of each switching cycle, the control block in the LM3671 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 (V
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 (V
IN
–V
OUT
)/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 - V
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 - V
OUT
/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.
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.
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.
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.
While in PWM mode, 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.
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.
Copyright © 2004–2013, Texas Instruments Incorporated
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