Datasheet
V
LX
I
LX
I
D1
V
ISNS
I
LED
, I
L1
I
LED
x R
ISNS
= 198 mV
time
time
time
time
time
I
LED
= 198 mV / R
ISNS
t
OFF
t
ON
T
T = t
ON
+ t
OFF
LM3407
www.ti.com
SNVS553B –JANUARY 2008–REVISED MAY 2013
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 Current Sensing block. The switch will remain ON until the average 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 19.
Figure 19. Operating Waveforms of a Floating Buck Converter
PULSE LEVEL MODULATION (PLM)
The LM3407 incorporates the innovative Pulse Level Modulation technique. With an external 1% thick film
resistor connected to the ISNS pin, the converter output voltage can adjust automatically as needed to deliver
constant current within 10% accuracy to a serially connected LED string of different number and type. Pulse
Level Modulation is a novel method to provide precise constant current control with high efficiency. It allows the
use of low side current sensing and facilitates true average output current regulation regardless of the input
voltage and inductor value. Pulse Level Modulation can be treated as a process that transforms a trapezoidal
pulse chain into a square pulse chain with an amplitude equal to the center of inductor current ramp. Figure 20
shows the waveform of the converter in steady state. In the figure, I
L1
is the inductor current and I
LX
is the switch
current into the LX pin. V
ISNS
is the voltage drop across the current sensing resistor R
ISNS
. V
MSL
is the center of
the inductor current ramp and is a reference pulse that is synchronized and has an identical pulse width to V
ISNS
.
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