Datasheet

-V
OUT
L
V
IN
-V
OUT
L
LM3674
SNVS405F –DECEMBER 2005–REVISED MAY 2013 www.ti.com
OPERATION DESCRIPTION
DEVICE INFORMATION
The LM3674, 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 LM3674 has the ability to deliver up to 600
mA depending on the input voltage, output voltage, ambient temperature and the inductor chosen.
There are two modes of operation depending on the current required - Pulse Width Modulation (PWM), and
shutdown. The device operates in PWM throughout the I
OUT
range. 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 overload
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 LM3674 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 the 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 Pulse Width Modulation (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.
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.
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