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LED driver compensation

treez
Intellectual 970 points
Other Parts Discussed in Thread: TPS61165, TPS61175, TPS61170

hello,

i was reading the excellent  ti application note by Jeff Falin concerning LED driver compensation.

 

please could you tell how to adjust the transfer function given there for when the LEDs have an inductor in series with them to smooth out the current?

  • 0
    TI__Mastermind 36110 points

    This would require a lot of math and testing to come up with an accurate answer.  The better solution might be to add more capacitance to the output to minimize the ouput voltage ripple.  This will minimize the current ripple through the LEDs. Some people place a capacitor across the LEDs as well.  Unless the current ripple is excessive, it should not be a problem.  The ripple is at the converter switching frequency, so it is not visible. 

    Can you comment on your application?

  • 0 in reply to Matthew Day
    Intellectual 970 points

    Thankyou Mr Day,

    As you know, the great article to which i refer is:

    http://www.techonline.com/article/pdf/showPDFinIE.jhtml?id=2184007921

    My application is Car Headlights, DRL's ChimSL's etc

    Referring to Equation 3 in the article, i am wondering why there is no inclusion of the compensation capacitor which goes with the TPS61165 ? (220nF recomended)

    The compensation capacitor as you know, is part of the feedback loop.

     

     

  • 0 in reply to treez
    TI__Guru*** 143880 points

    My original intent on the that article was for it to apply to any current mode controlled LED driver, not only the TPS61165.  So, I left out the compensation network.  You can include the transconductance amplifier transfer function by simply including the pole created by the compensation capacitor.  The TPS61170 is the closest part to the TPS61165 and its datasheet gives the complete transfer function.  The TPS61175 datasheet, EVM and design app note explains it better.