LM3671MF-1.25 National Semiconductor, LM3671MF-1.25 Datasheet - Page 13

• Manufacturer Part Number: LM3671MF-1.25
• Description: DC/DC CONVERTER, 1.25V, SMD, 3671
• Manufacturer: National Semiconductor

Specifications of LM3671MF-1.25

• Primary Input Voltage: 2.25V
• No. Of Outputs: 1
• Output Voltage: 1.25V
• Output Current: 600mA
• Voltage Regulator Case Style: SOT-23
• No. Of Pins: 5
• Operating Temperature Range: -25°C To +85°C
• Svhc: No
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
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 (Pulse Width Modulation), PFM (Pulse
Frequency 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
typ) and a longer battery life. Shutdown mode turns off the
device,
(I
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 2.7V 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
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
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 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 com-
SHUTDOWN
offering
= 0.01 µA typ).
the
OUT
lowest
/L.
current
IN
–V
OUT
)/L, by storing
consumption
Q
= 16 µA
13
parator 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. This allows the inductor
current more time to decay, thereby preventing runaway.
PFM OPERATION
At very light load, the converter enters PFM mode and
operates with reduced switching frequency and supply cur-
rent to maintain high efficiency.
The part automatically transitions into PFM mode when ei-
ther of two conditions occurs for a duration of 32 or more
clock cycles:
level, (Typically I
A. The NFET current reaches zero.
B. The peak PMOS switch current drops below the I
FIGURE 6. Typical PWM Operation
MODE
<
30mA + V
IN
/42 Ω ).
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MODE