LM3674MFX-1.2 National Semiconductor, LM3674MFX-1.2 Datasheet - Page 10

• Manufacturer Part Number: LM3674MFX-1.2
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,TSOP,5PIN,PLASTIC
• Manufacturer: National Semiconductor

www.national.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 - PWM (Pulse Width Modulation), and shutdown.
The device operates in PWM throughout the I
Shutdown mode turns off the device, offering the lowest
current consumption (I
Additional features include soft-start, under voltage protec-
tion, current overload protection, and thermal overload pro-
tection. 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
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
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 PWM ( Pulse Width Modulation) operation the con-
verter operates as a voltage-mode controller with input volt-
age feed forward. This allows the converter to achieve ex-
cellent load and line regulation. The DC gain of the power
SHUTDOWN
= 0.01 µA typ).
OUT
range.
10
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 begin-
ning 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.
Internal Synchronous Rectification
While in PWM mode, the LM3674 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 LM3674 to protect itself and
external components during overload conditions. PWM
mode implements current limiting using an internal compara-
tor that trips at 1020 mA (typ). If the output is 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 current
has more time to decay, thereby preventing runaway.
SOFT-START
The LM3674 has a soft-start circuit that limits in-rush current
during start-up. During start-up the switch current limit is
increased in steps. Soft start is activated only if EN goes
from logic low to logic high after Vin reaches 2.7V. Soft start
is implemented by increasing switch current limit in steps of
70mA, 140mA, 280mA, and 1020mA (typ. switch current
limit). The start-up time thereby depends on the output ca-
pacitor and load current demanded at start-up. Typical
start-up times with 10µF output capacitor and 300mA load
current is 350µs and with 10mA load current is 240µs.
LDO - LOW DROP OUT OPERATION
The LM3674-ADJ can operate at 100% duty cycle (no
switching, PMOS switch completely on) for low drop out
support of the output voltage. In this way the output voltage
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