LM3674_15 Datasheet, PDF (10/26 Pages) Texas Instruments – LM3674 2-MHz, 600-mA Step-Down DC-DC Converter in SOT-23

LM3674
SNVS405G – DECEMBER 2005 – REVISED APRIL 2015
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7.3 Feature Description
7.3.1 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:
V_IN-V_OUT
L
(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:
-V_OUT
L
(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.
7.3.2 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.
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.
V_SW
2V/DIV
I_L
200 mA/DIV
V_OUT
V_IN = 3.6V
V_OUT = 1.5V
I_OUT = 400 mA
10 mV/DIV
AC Coupled
TIME (200 ns/DIV)
Figure 18. PWM Operation
10
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