TMS320C6748: USB0_VBUS overvoltage & esd protection

Yordan,

Thanks for your quick reply but I have to explain my issue more precisely:

In our design the USB_VBUS power path (from the USB connector) is connected to both a TPS65070 and a C6748 (the former for power supply management and the latter for control)

Only device USB mode is handled (i.e: we don't provide 5V USB power).

We already have tvs and shottky diodes for VBUS protection against over and undervoltage.

The thing is:

The TPS65070 can handle VBUS overvoltage up to 17V (20V max) whereas the USB0_VBUS input on the C6748 is only given for a maximum of 5.5V (up to a maximum of 24hours).

Therefore the USB0_VBUS pin is the weekest point as far as overvoltage is concerned (furthermore 5.5V is not that high indeed...)

Our idea is to insert a serial resistor on the USB0_VBUS path for current limitation in case of overvoltage.

If this idea seems correct, than we have to specifiy its resistance value to ensure that the current drawn, in regards with this input spec, is low enough in case of overvoltage (say 9V) but high enough for

proper operation (i.e higher than input leakage current).

Our choice is something like 15K or 20KOhms...

What is your opinion about all this?

Best regards,

 Bruno

Hi Bruno,

Sorry for the late reply.

I think this is feasible. However, let me consult the design team. They will post their feedback directly here.

Best Regards,
Yordan

Hi,

Is it possible to share the relevant parts of the schematic so we can check it and provide further details if needed?

Best Regards,
Yordan

Hi Yordan,

Here is a hand drawn schematic of the VBUS path.

Explanations:

- BAT54: undervoltage protection

- uClamp0571P :overvoltage protection - Reverse Breakdown Voltage: 7V typ. - 9V max.

- RLimit: current limitation for USB0_VBUS input protection. This actually the subject of this post.

More info:

  The hardware is battery powered (Li-Ion battery)

  This device will used in harsh environment

Regards,

 Bruno

Hi,

I think this should work, but let's wait for the confirmation from the design team to be absolutely sure.

Best Regards,
Yordan

Hi,

A quick update, it seems that 20K resistance value in series with the VBUS pin might be causing problems. We're still investigating this.

Best Regards,
Yordan

Hi Yordan,

Any news?

Hi,
I'm still looking for info on this,
Best Regards,
Yordan

Yordan,

Another question that could resolve our issue:

can you tell me what are the low et high voltage ranges for this input (USB0_VBUS)?

The idea is to see if we can drive this input with a 3.3V driver/buffer/switch etc.

Or, said in other words: is a 3.3V level  high enough for this pin to read it's input as a logical "1" ?

Best Regards,

Bruno