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Why no spread spectrum for Class D?

Ken Dillinger
Expert 1220 points
Other Parts Discussed in Thread: TAS5414

I am looking to develop a new project and one of the requirements our friend FCC Part 15B.  I have been looking at Maxim's product line and, of course, they tout slope control, spread spectrum, good PCB layout, etc.  They seem to market the spread spectrum feature fairly aggressively, but there seems to be a difference in philosophy.  I know TI has this technology in clocking, but it has not been adopted in audio.

Should I care? Or should I stick to the old main stays of shielding, filtering, and layout?

 

-Ken

  • 0
    TI__Expert 4320 points

    Ken

    a good question - and the following comments should be noted as my personal comments and not necessarily the official TI recommendations.

    spread spectrum can be good if you have some specific problems with certain peaks - they can be spread out - and then hopefully below the limits - but they don't disappear, so even though you use spread spectrum, you still need to pick the right inductors with low parallel capacitance, you still need to pick decoupling capacitors with low series inductance and low series resistance, you still need to make a good PCB layout that minimizes the antennas etc

    we have also seen that adding spread spectrum to the clock in an class-d amp can lead to increased audio noise - so is that an acceptable compromise?

    I have seen that two different PCB layouts had more than 20dB differences at a given MHz peak due to resonances - here the use of spread spectrum would not make a big differences. One example is the TAS5414 - that use fixed clock, but still passes the strict EMI requirements in automotive - the key here is that the TAS5414 uses slow switching to lower the EMI.

    rgds,

    Kim

  • 0 in reply to Kim Nordtorp Madsen
    Expert 1220 points

    Hello Kim,

     

    It's ok.  You can say that Maxim doth play well with the specmanship.  I won't tell anyone.  :-D

    In all seriousness, your reply helps.  I have been trying to read up on Spread Spectrum in audio applications and from my own cursory research and your opinions here, it does not seem at all like  a magic bullet.  More-so with the fact that one still has to have good layout and component selection.

    I will also look more indepth to the noise performance of both types of audio amplifiers (SS and non-SS) to see what the differences are.  

    Thanks again.

    -Ken