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TPS23758: TPS23758 Output Ripple Sorcery

eljeffo
Intellectual 560 points
Part Number: TPS23758

I copied the design from the TPS23758-EVM and measured some output ripple.  The result is fine, but I don't understand it.

In school we're taught that a single connected line is the same voltage along the line.  So I'm checking the output ripple along the Vout line and it's different at every point.

All the way from super noisy at D2, less noisy at Q1, and decreasing with each capacitor and more.  That's a decrease of output ripple of 1.54v -> 224mv.

I understand how capacitance can smooth out the ripple, but am I seeing correctly that the ripple truly reduces along the length of the line?  That's crazy!  

What concept is this sorcery? :)

  • 0
    Intellectual 560 points

    Oh and... please tell your friends... thanks for making a chip&circuit that emits fairly low EMI.  It's a refreshing break from some of the other solutions we're looking at.

  • 0 in reply to eljeffo
    Intellectual 560 points

    Also I tried out the dithering function.  I couldn't see much of a difference in output on my TEM cell.  Is there a particular spectrum I should be looking at?  I'm looking between 30mhz and 800mhz.

  • 0
    TI__Mastermind 21320 points

    Hello,

    The measurement on the gate of Q1 is not the same node as the output. The other thing is that the most reliable measurement method for the output ripple is across one of the capacitors using the tip & barrel method. Measuring directly on the FET will introduce noise into the measurement.

    So when we look along the caps the differences are not as significant. However, can you confirm you setup?

    Additionally, did you use the same probe or different probes?

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to eljeffo
    TI__Mastermind 21320 points

    Hello,

    For the dithering function, it lowers EMI at the switching frequency, 250kHz. 

    Instead of switching perfectly at 250kHz each cycle, dithering essentially puts the switching frequency somewhere between  ex. 248kHz and 252kHz (tolerance is set by resistor). Therefore the switching noise is spread out over a larger frequency range, and thus a lower dB at 250kHz + harmonics. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Intellectual 560 points

    Hi Michael,

    I'll do the tip and barrel thing when I get to the office again. :)

    And thanks for the note on the dithering.  That should be easy enough to verify now that I know what to look for.  

    Jeff

  • 0 in reply to eljeffo
    Intellectual 560 points

    Oh and yes, same probe used on all points along the output trace.  Just stuck ground nearby (about 2cm away) and touched all the points.

  • 0 in reply to eljeffo
    TI__Mastermind 21320 points

    Hello,

    Using the ground connector clip that is 2cm long can create a ground loop that is susceptible to noise. Let us know your results. Thanks!

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments