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TCA9617A: TCA9617A no input signal, output low toggle signal

Thomas Chang15933
Expert 4770 points
Part Number: TCA9617A

Dear Team:

   Our customer used  2pcs TCA9617A in server application, and circuit is as attached.

Customer got strange phenomenon: A side no signal input that is only pull high, but B side got toggle waveform. (data and clock pin got same phenomenon.)

Could you reply customer questions:1. A side voltage should  under B side?

2.Two pcs TCA9617 B side could not series connect it together, right?

3.What is the toggle output root cause?

1st waveform is B side clk. 2nd waveform is B side clk zoom in.

 

  • 0
    TI__Guru 62910 points
    Hi Thomas,

    Yes, per the "Power Supply Recommendations" section of the datasheet, the VCCA voltage should always be less than the VCCB voltage even while the device is disabled and during power ramp. It looks from the schematic like you are using 3.3 V on VCCA and 2.5 V on VCCB on one circuit - is that correct?

    If you are able to operate the device with VCCA < VCCB and still observe this toggling, can you please monitor the corresponding "A" side signal on the oscilloscope at the same time as the "B" side signal? This could help us narrow down a root cause.

    You are right that you cannot connect the "B" side of one device to another. This is because the "B" side introduces a 0.5-V offset to its low-level output so that its steady-state output voltage is held above the low-level input voltage threshold. This is needed so that data flows properly from master to slave and vice-versa.

    Regards,
    Max
  • 0 in reply to Max Robertson
    TI__Expert 4770 points

    Dear Max:

     Thanks for your reply.