LM3407_09 Datasheet, PDF (8/16 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

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FIGURE 1. Operating Waveforms of a Floating Buck Converter
PULSE LEVEL MODULATION (PLM)
The LM3407 incorporates the innovative Pulse Level Modu-
lation technique. With an external 1% thick film resistor con-
nected 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 dif-
ferent 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 Modula-
tion 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 2 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 re-
sistor 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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