LM3670MF-1.2EV National Semiconductor, LM3670MF-1.2EV Datasheet - Page 10

• Manufacturer Part Number: LM3670MF-1.2EV
• Description: BOARD EVALUATION LM3670MF-1.2
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3670MF-1.2EV

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.2V
• Current - Output: 350mA
• Voltage - Input: 2.5 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 1MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM3670
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -
• Lead Free Status / Rohs Status: Not Compliant

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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 (Pulse Width Modulation), PFM (Pulse
Frequency Modulation), and shutdown. PWM mode handles
current loads of approximately 70 mA or higher. Lighter
output current loads cause the device to automatically switch
into PFM for reduced current consumption (I
and a longer battery life. Shutdown mode turns off the de-
vice,
(I
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
SHUTDOWN
offering
= 0.1 µA typ).
V
IN
, V
the
OUT
(I
LOAD
, I
Soft Start
lowest
INDUCTOR
= 350mA)
current
vs. Time
Q
20075824
consumption
= 15 µA typ)
(unless otherwise stated: V
10
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 low, smoothing the voltage across
the load.
PWM OPERATION
During PWM operation the converter operates as a voltage-
mode controller with input voltage feed forward. This allows
the converter to achieve excellent 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.
Internal Synchronous Rectification
While in PWM mode, the LM3670 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 LM3670 to protect itself and
external components during overload conditions PWM mode
implements cycle-by-cycle current limiting using an internal
comparator that trips at 620 mA (typ).
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.
IN
= 3.6V, V
OUT
= 1.8V) (Continued)