LM3670_15 Datasheet, PDF (11/25 Pages) Texas Instruments – LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits

LM3670
www.ti.com
SNVS250E – NOVEMBER 2004 – REVISED FEBRUARY 2013
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 current 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_Q = 15 µA typ) and a longer battery life. Shutdown mode turns off the device, offering the lowest
current consumption (I_SHUTDOWN = 0.1 µA typ).
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
V_IN-V_OUT
L
(1)
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
(2)
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 inversely 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).
Copyright © 2004–2013, Texas Instruments Incorporated
Product Folder Links: LM3670
Submit Documentation Feedback
11