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TPS62933: Audible noise relative to PFM

zhi gao
Prodigy 10 points
Part Number: TPS62933

I designed a DC-DC circuit with TPS62933, input 18V, output 15V 2.5A, inductor=6.8uH, Fsw=500kHz. Cout>200uF.

when this circuit work with the load which is an Audio PA. the load is light when no music playing, then an audible noise could be heart. 

I measured the figure of pin5 SW as follows, the switch pulses were not consecutive.

I guess the audible noise is relevant to the type of switching pulse. and switching mode goes into PFM which has BURST of pulses.

then I changed the inductor from6.8uH to 3.3uH, Fsw also be changed to 1.1MHz by changing RT value.

I measured the figure of pin5 SW again as follows, the switch pulses were consecutive, and no audible noise.

MY QUESTION:

does the change of inductor from 6.8uH to 3.3uH is reasonable to avoiding audible noise? 

why the switch pulses are not consecutive when inductor is 6.8uH, and why the switch pulses are consecutive when inductor is 3.3uH, how is the principle?

  • 0
    TI__Genius 16632 points

    Hello Gao,

    1. I got that at light load you saw multi-pulse pulses with 6.8uH, while stable single pulses with 3.3uH. How about heavy load? Does device work stable at 2.5A?

    2. Could you please share your full schematic and Cout value? Seems you used a quite large Cout, which may affect loop stability. I can do some simulations based on your design.

  • 0 in reply to Miranda Gu
    Prodigy 10 points

    Hi Miranda,

        thanks for your reply.

        1. it's stable single pulses when heavy load. stable single pulses could be saw when another 60mA load current is added. loading test at heavy load such as 2.5A will be updated later.

        2. schematics as follows. Cout are 200uF E-CAP and 174uF MLCC, thank for your simulation work.

      

  • 0 in reply to zhi gao
    TI__Genius 16632 points

    Hi Gao,

    1. Today I did bench test on TPS62933EVM using your component values: Rfb: 10K/178K (15Vout); L: 6.8uH; RT floating (500kHz); Cout: 2x100uF electrolytic capacitor + 8x22uF MLCC (effective capacitance 4uF per cap with 15V DC bias).

    I could reproduce the multiple-pulse phenomenon with 6.8uH inductor at light load (range: 0A~70mA), attach 18Vin-50mA waveform below. 

    And I checked 3.3uH inductor, could also see multiple-pulse with 3.3uH at light load, but the range is smaller than 6.8uH, loading 0A~50mA.

    2. Then I measured both 3.3uH and 6.8uH bode plots. Unfortunately the results didn't give me much clues. Both phase margin look OK, just see the very narrow loop bandwidth due to large Cout.

    18Vin-15Vout-2.5A-6.8uH-RT floating: --- Bandwidth: 6.06kHz; Phase margin: 57.659deg

    18Vin-15Vout-2.5A-3.3uH-RT pulled to GND:   --- Bandwidth: 4.62kHz; Phase margin: 45.459deg

    3. My bench setup is different with yours (different components P/N, different board layout). As you already found 3.3uH could solve your audible noise problem. I think you could move forward to evaluate 3.3uH solution. 

    Please check all loading range device waveforms, load/line transient performance and other needed validation tests. 

    Keep me updated if you have further test progress.