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Limitations to analog dimming of NCL30105?

Discussion in 'General Electronics Discussion' started by eem2am, Nov 6, 2012.

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  1. eem2am

    eem2am

    414
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    Aug 3, 2009
    Hello,

    I wish to use the NCL30105 Current mode controller as a LED driver.

    NCL30105 DATASHEET:
    http://www.onsemi.com/pub_link/Collateral/NCL30105-D.PDF

    However, the peak current set point voltage is too high for me……its 1V maximum.

    I need to make it 260mV maximum.

    ….to make it 260mV maximum, would you put a 39K resistor from the Sstart pin to ground.? (20uA * 39000 / 3) = 0.26V

    ….if you did this, and you wanted to further control the LED current by analog dimming to make the LEDs dimmer, then you would not be able to make the LEDs go any dimmer, because attaching a dimming control voltage to the SSTart pin cannot be done without adding a series diode as in fig 23, page 13……..as you can see, the voltage across this diode is 0.6V, …..so obviously the sstart pin voltage cannot be taken any lower than 600mV……is this true?
     
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