LM3407_09 Datasheet, PDF (7/16 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

Simplified Functional Block Diagram
Functional Description
OVERVIEW
The LM3407 is a constant current output floating buck switch-
ing 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, in-
dustrial 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 so-
lution 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). Addi-
tionally, a separate enable pin allows for low power shutdown.
An exposed pad eMSOP-8 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. Dif-
ferent 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
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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 in-
ductor 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 light-
ing applications.
The operation of the LM3407 constant current output floating
buck converter is explained below. With the internal switch
Q1 turned ON, current flows through the inductor L1 and the
LED array. Energy is also stored in the magnetic field of the
inductor during the ON cycle. The current flowing through
R_ISNS during the ON cycle is monitored by the Average Cur-
rent Sensing block. The switch will remain ON until the aver-
age inductor current equals 198mV / R_ISNS. When the switch
is turned OFF, the magnetic field starts to collapse and the
polarity of the inductor voltage reverses. At the same time, the
diode is forward biased and current flows through the LED,
releasing the energy stored in the inductor to the output. True
average output current is achieved as the switching cycle
continuously repeats and the Average Current Sensing block
controls the ON duty cycle. A constant current output floating
buck converter only works in Continuous Conduction Mode
(CCM); if the converter enters Discontinuous Conduction
Mode (DCM) operation, the current regulation will deteriorate
and the accuracy of LED current cannot be maintained. The
operating waveforms for the typical application circuit are
shown in Figure 1.
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