Hi Mahmood,

in order to minimize the wasted power and heat dissipation of constant current driver many modern LED drivers work like a boost converter, with the LEDs in the feedback. That's what also Oliver mentioned.

Look at this scheme, for instance:




After power-on the output voltage of this boost converter rises up to the moment, when the voltage drop at R2 equals the internal bandgap reference voltage of 1.24V. So, finally, this converter provides a constant current through R2 and by this through R1. Simply remove R1, put some LEDs in series instead and heavily decrease R2 and you have a constant current source LED driver. This thing actually works, because you can think of a LED working like a resistance, dependent on flowing current. The switcher does not see the LED, but only a "resitance", which varies with flowing current.

A concret example could consist of 7 x 1W LEDs (3V @ 350mA). A current limiting resistor of R2 = 3.6Ohm would then be needed. Input voltage should be raised to 12V.

In the scheme above you see the cap C2, which is elementarily needed to make the boost converter work. This darn thing can make the NUD4700 kill further LEDs, when one fails! During a LED failure the switcher is out of regulation and will begin to raise its output voltage. And after the NUD4700 has turned-on, a too high voltage can drop across the remaining LEDs for a few milliseconds. Such a circuit must be designed very carefully, so that the LED current must never exceed the maximum peak current.


Kai