EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: eee on November 12, 2013, 04:51:54 pm
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Hi guys! I'm new here, just joined after seeing some amazing videos of Dave on youtube. :-+
I know quite a bit about DC boost IC chips, and constant current chips you can buy, but what about a combination of the two? Does such a chip exist that does 2 in 1?
If not, will using a DC boost convertor, in series with a constant current chip work fine, or will there be issues such as impedance mismatch?
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Thanks for your reply AcHmed99! :)
The issue is that the supply voltage varies between 9.7V and 11.7, yet the LED requires at least 12.5V to be driven at the desired 1A. If I set this chip to operate at 1A, will it automatically boost the voltage to whatever the LED requires? (the hotter the LED gets, the less it's Vf is...)
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Correct, you won't be able to do the two simultaneously, but nothing will prevent you from using a voltage mode power controller as a current mode controller. You can use most any boost converter IC that is advertised as voltage mode, and simply configure it to regulate output current. This is commonly done for LED circuits. Current is regulated and the output voltage is clamped by the Vfwd of the LEDs.
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And just to make the idea complete in the thread... I drew up a simplified schematic to show how to use a basic buck voltage regulator as a current regulator with a LED string.
As a voltage regulator, you would set a top and bottom feedback resistor to divide the desired output voltage down to the feedback voltage. Then the current limiting resistor RLIM would limit the current through the LED string.
As a current regulator, the feedback voltage is now developed across RSET, so choose RSET = VFB/ISET. The controller will now adjust the voltage output to whatever value it needs to be at to maintain ISET across RSET