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TPS65981: Errant VBUS Voltage

Ben Schroeder1
Prodigy 70 points
Part Number: TPS65981
Other Parts Discussed in Thread: PCM2705C

TPS65981_RCLAMP0582_CONNECTIONS.pdf

Hello,

We have a board design that uses both a TPS65981 and PCM2705C to provide 5V, 3A charging and USB audio from an Ipad.  All prototype/breadboard testing worked perfectly and is identical to our final board except the final board design uses some Semtech RCLAMP0582N.TCT ESD diode arrays protecting the USB Data and CC lines.

The problem is that our final board doesn't recognize or start charging the Ipad. 

Here is some more context about the design:

  • The final design/project has the Ipad fixed in place with the Ipad permanently plugged into our board.  In other words, our designed board won't be powered on without the Ipad plugged in and we won't be reversing the USB type C connection.
  • The PCM2705C is self powered using the same 3.3V supply that provides power to the TPS65981 3.3V input.
  • The PCM2705C USBD+ signal has the pull up 1.5Kohm resistor to this 3.3V supply.
  • We are not making use of the TPS65981 USB Top/Bottom side multiplexer because of board routing/outline constraints.  Instead we have the USB Type C Connector A6 tied to B6 (USBD+) and A7 tied to B7(USBD-). These are then tied directly to the PCM2705C USB data inputs.

Here is some of the troubleshooting I have gone through:

  • I measured all of the TPS65981 voltage regulator outputs and 3.3V input and they are all within the ranges specified in the datasheet.
  • I then probed the VBUS output of the TPS65981 and am measuring around 2-3V regardless of if the Ipad is plugged in or not.  This is an issue  as the VBUS should be ~0V when no device is plugged in.

This errant/leakage VBUS voltage is what is causing the Ipad to not be recognized/charged as there should be no VBUS voltage when plugged in.

My theory is that the RCLAMP0582N.TCT is what is causing the problem.

Since the TPS65981 tried to keep the VBUS voltage ~0V when nothing is connected, I think that the PCM2705C USB D+ 1.5Kohm pullup to 3.3V is back driving some of the RCLAMP0582N.TCT diodes onto the TPS65981 VBUS output. See attached block diagram. 

I was hoping a TI Design Engineer could provide thoughts/comments about my theory or provide other ideas about where this errant VBUS voltage is coming from?  

Thanks,

Ben Schroeder