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LM3671MF-1.5 Fiches technique(PDF) 10 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
No de pièce LM3671MF-1.5
Description  2MHz , 600mA Step-Down DC-DC Converter in SOT23-5
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM3671MF-1.5 Fiches technique(HTML) 10 Page - National Semiconductor (TI)

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Operation Description
DEVICE INFORMATION
The LM3671, a high efficiency step down DC-DC switching
buck converter, delivers a constant voltage from either a
single Li-Ion or three cell NiMH/NiCd battery 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 cur-
rent required - PWM, PFM, and shutdown. The device oper-
ates in PWM mode at load currents of approximately 80 mA
or higher, having voltage tolerance of ±4% with 90% effi-
ciency or better. Lighter load currents cause the device to
automatically switch into PFM for reduced current consump-
tion (I
Q = 16 µA typ) and a longer battery life. Shutdown
mode turns off the device, offering the lowest current con-
sumption (I
Q, 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
recommended to keep the part in shutdown until the input
voltage is 2.8V or higher.
CIRCUIT OPERATION
The LM3671 operates as follows. 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–VOUT)/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
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
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 in-
versely proportional to the input voltage is introduced.
While in PWM (Pulse Width Modulation) 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.
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 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.
PFM OPERATION
At very light loads, the converter enters PFM mode and
operates with reduced switching frequency and supply cur-
rent to maintain high efficiency.
The part will automatically transition into PFM mode when
either of two conditions occurs for a duration of 32 or more
clock cycles:
A. The inductor current becomes discontinuous.
B. The peak PMOS switch current drops below the I
MODE
level, (Typically I
MODE < 30mA + VIN/42
Ω ).
20108423
FIGURE 5. Typical PWM Operation
www.national.com
10


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