LM3407 Datasheet, PDF (8/22 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

LM3407
SNVS553B – JANUARY 2008 – REVISED MAY 2013
SIMPLIFIED FUNCTIONAL BLOCK DIAGRAM
FS
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VIN
VIN
+
3.6V ^-
Clock
Generator
UVLO
6
DIM
DIM
400 k:
Slope Compensation
+
-
PWM
Comparator
3.6V
5 PA
EN
3.6V
SD
-
EA
gm
+
VCC
regulator
SD
VCC
Set
DIM
Q1
SWITCH
CONTROL
Reset
Waveform shaping and
Average
Current Sense
198mV
+
-
VCC
LX
ISNS
GND
FUNCTIONAL DESCRIPTION
OVERVIEW
The LM3407 is a constant current output floating buck switching converter with wide input voltage range and low
feedback current sense reference voltage. These characteristics make the LM3407 an efficient solution to
provide constant current to high power LEDs. The device is ideal for automotive, industrial and general lighting
applications where high power LEDs are used as the lighting source. The LM3407 has an integrated power N-
MOSFET that makes the application solution compact and simple to implement. An external 1% thick-film
resistor allows the converter output voltage to adjust as needed to deliver constant current within 10% accuracy
to a serially connected LED string of varying number and type. Converter switching frequency is adjustable from
300 kHz to 1 MHz. The LM3407 features a dimming input to enable LED brightness control by Pulse Width
Modulation (PWM). Additionally, a separate enable pin allows for low power shutdown. An exposed pad MSOP-8
PowerPAD package provides excellent heat dissipation and thermal performance. Input UVLO and output open-
circuit protection ensure a robust LED driver solution.
FLOATING BUCK SWITCHING CONVERTER
The LM3407 is designed for floating buck configuration. Different from conventional buck converters, a low side
power N-MOSFET is used. The floating buck configuration simplifies the driver stage design and reduces the die
size of the power MOSFET. Additionally, the connections of the power diode, inductor and output capacitor are
switched to ground with a ground referenced power switch, Q1. The extraction of inductor current information can
be easily realized by a simple current sensing resistor. These benefits combine to provide a high efficiency, low
cost, and reliable solution for LED lighting applications.
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