LM3671_08 Datasheet, PDF (15/24 Pages) National Semiconductor (TI) – 2MHz, 600mA Step-Down DC-DC Converter

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 volt-
age 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 Fre-
quency 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 protec-
tion, current overload protection, and thermal shutdown 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 rec-
ommended 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 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_IN–V_OUT)/L, by storing en-
ergy 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.
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 rect-
angular 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 av-
erage 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 propor-
tional 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 in-
ductor 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.
20108423
FIGURE 7. Typical PWM Operation
Internal Synchronous Rectification
While in PWM mode, the LM3671 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 LM3671 to protect itself and
external components during overload conditions. PWM mode
implements current limiting using an internal comparator 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. This allows the inductor current more
time to decay, thereby preventing runaway.
PFM OPERATION
At very light load, the converter enters PFM mode and oper-
ates with reduced switching frequency and supply current to
maintain high efficiency.
The part automatically transitions into PFM mode when either
of two conditions occurs for a duration of 32 or more clock
cycles:
A. The NFET current reaches zero.
B. The peak PMOS switch current drops below the I_MODE
level, (Typically I_MODE < 30mA + V_IN/42 Ω ).
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