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TPS43000: audible noise in PFM mode in ZETA tology configuration

Maurizio Bonelli
Expert 1400 points
Part Number: TPS43000

Hello,

I have developed 15 W power supply powered by a 1-cell LiIon battery. Due the mechanical constraints of the enclosure and to minimize the EMI, the design is compact and all the power components are close each other to minimize path lenght and loop areas. Particularly the 2 inductors are next each other.

The noise is audible only at low loads, when the operation is in PFM discountinuous mode. When it passes to continuous mode, the noise stops.

I have used 2x47 uF MURATA ceramics capacitor as input decoupling network: adding smaller capacitors, i.e. 22 nF + 10 pF reduces the noise but it is still audible to the most sensible ears.

I would like to know if someone has had a similar experience and if there is a root cause that can be used to understand how modify the circuit or the layout.

Thanks for the reply.

  • 0
    TI__Guru 73150 points
    Can you share your schematic, and the switch node waveform?
  • 0 in reply to Youhao Xi
    Expert 1400 points

    Hello, sorry for delay but my Company was closed for Spring Festival holiday.

    The waform are below, the single PFM packet and the repetitive signal.

    About the schematic, I am sorry but currently I cannot share due company policy. In any case it is a standard circuit with 2 independent inductors with same value of 3,3 uH, at switching frequency of 1 MHz. 

  • 0 in reply to Maurizio Bonelli
    TI__Guru 73150 points
    The waveform looks normal, and you should disable the PFM mode to keep the switching frequency from dropping .
  • 0 in reply to Youhao Xi
    Expert 1400 points
    Thanks XiYouhao, I already know that if I disable the PFM mode the noise stops.
    PFM mode is necessary to increase the efficiency because for the most of the time the load is light, less than half the inductor ripple current.
    A solution could be to increase the inductors value, but in that case the respons time due the sudden higher load request becomes too high as well as the overshoots, but I suppose this is academy.
    Then, I am not yet sure if the root cause of the noise is to ascribe to the inductors or to the ceramic capacitors (that have a piezo behaviour when AC voltage is present at their terminals): if the capacitors, I have to suppose the fly ones are the source of noise more than the input ones, don't I?

    Thanks
  • 0 in reply to Maurizio Bonelli
    TI__Guru 73150 points
    The noise may come from both the ceramic capacitors and the inductor. You may also try to play with the inductor, say use a smaller or bigger value and see if the PFM frequency can be moved out of audible range.
  • 0 in reply to Youhao Xi
    Expert 1400 points
    When the load changes, stil remain the PFM range of current, also the frequency changes, then there are no combinations that can reduce this.
    I'm afraid I have to find the solution using mechanical tricks more than electronic ones.
  • 0 in reply to Maurizio Bonelli
    TI__Guru 73150 points
    You are right. It is possible that the PFM at a different load level may alleviate the audible noise. Also I heard some engineers alter the capacitor and inductor orientations on the PCB such that the vibration is not reinforced among these parts. You may search for such design tricks online.