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DM355 POWER-UP USB LEAKAGE

JPM
Intellectual 565 points

I wanted to see if anyone had insight into DM355 power-up, in particular with respect to USB.

My power design uses all LDOs. My schematic is the same as LeopardBoard and the EVM. (Both Leopard and EVM tie VDDA13_USB and VDDD13_USB together and feed them by a ferrite bead / capacitor filter.)

My power sequence is as follows: 1V3 ON, wait 10 ms, 1V8 and 3V3 ON.

What I am seeing is when I turn on the 1V3 rail, the 3V3 rail appears to rise and settle at about 550 mV. It seems to resemble some type of internal leakage from the 1V3 rail to the 3V3 rail. When I remove the ferrite bead which feeds VDDA13_USB and VDDD13_USB, all leakage goes away.

So, I conclude there is something internal to the USB peripheral or system interconnect which results in VDDA13_USB or VDDD13_USB or both dumping into the 3V3 rail.

Has anyone else observed this????

My concern is that there may be an internal diode in the USB peripheral from the 1V3 rail to the 3V3 rail which would normally be reverse biased, but during power up is forward biased. I am concerned about the current rating of this internal diode and whether it will fail over time due to stress.

JPM

 

  • 0
    TI__Genius 9510 points

     

    This is probably due to the internal LDO in the USB. When the USB VDDA13_USB voltage is ramped up, you get current to the USB 3.3V supply. This should be OK. Current only occurs during supply ramp. I assume customer does not force the 3.3V supply to 0V, which would cause more current.