LM3670MFX-2.5 National Semiconductor, LM3670MFX-2.5 Datasheet - Page 10

• Manufacturer Part Number: LM3670MFX-2.5
• Description: Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits
• Manufacturer: National Semiconductor

www.national.com
Typical Performance Characteristics
Operation Description
DEVICE INFORMATION
The LM3670, 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
LM3670 has the ability to deliver up to 350 mA depending on
the input voltage and output voltage (voltage head room),
and the inductor chosen (maximum current capability).
There are three modes of operation depending on the cur-
rent required - PWM, PFM, and shutdown. PWM mode
handles current loads of approximately 70 mA or higher.
Lighter output current loads cause the device to automati-
cally switch into PFM for reduced current consumption (I
15 µA typ) and a longer battery life. Shutdown mode turns off
the device, offering the lowest current consumption
(I
Q, SHUTDOWN
V
= 0.1 µA typ).
V
SW
IN
, V
, V
OUT
OUT
(I
LOAD
PFM Mode
, I
, I
Soft Start
INDUCTOR
INDUCTOR
= 350mA)
vs. Time
vs. Time
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Q
=
(unless otherwise stated: V
10
The LM3670 can operate up to a 100% duty cycle (PMOS
switch always on) for low drop out control of the output
voltage. In this way the output voltage will be controlled
down to the lowest possible input voltage.
Additional features include soft-start, under voltage lock out,
current overload protection, and thermal overload protection.
As shown in Figure 1, only three external power components
are required for implementation.
CIRCUIT OPERATION
The LM3670 operates as follows. During the first portion of
each switching cycle, the control block in the LM3670 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
V
SW
, V
OUT
IN
PWM Mode
, I
=3.6V, V
INDUCTOR
OUT
vs. Time
=1.8V) (Continued)
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