TMS320F280039C: The status of GPIO when VDDIO / VDD is not powered.

Wilson Liang2

Hi Sir,

When the customer was verifying the PMBUS application, they found that when multiple F280039C PMBUS devices are connected, some SLAVE devices are not powered at VDD. However, when SDA, SCL, and GND are still connected to the BUS lines, the SDA and SCL lines are pulled low (from 3.3V to 1.05V). They also verified that other GPIOs exhibit a similar voltage pull-down phenomenon when VDD is not powered. Is there a way to correct this? It is difficult to avoid having unpowered devices on the PMBUS (I2C) bus line.

thanks.

9 months ago

0 Masoud Farhadi 9 months ago

TI__Genius 9010 points

Hi Wilson,

The bus high level can back-power the unpowered device through the input protection clamp diodes, which in turn drags the lines down. The absolute-maximum for any digital input voltage must remain between VSS – 0.3 V and VDDIO + 0.3 V. With VDDIO = 0 V, a 3.3 V pull-up violates that limit and can cause leakage/back-power or even damage if sustained. That's why we don’t suggest to leave unpowered slaves on a live bus or another saying power up VDD and VDDIO together. This reinforces that the device is not intended to have its GPIOs driven when its supplies are off.

You may consider adding a PMBus buffer with powered-off high-impedance between the main bus and any node that might be unpowered.

I'm also assigning this thread to our PM BUS expert for more information.

Best Regards,

Masoud

0 Wilson Liang2 9 months ago

Intellectual 540 points

Hi Masoud

Thank you for your explanation.

Do you have any suggestions for PMBus buffer solutions? IC or schematic,

thank you.

+1 Jason Whiles 8 months ago in reply to Wilson Liang2

TI__Expert 6100 points

Wilson,

I don't have a specific suggestion, but there are several options that can be found here depending on your application needs.

I2C & I3C level shifters, buffers & hubs | TI.com

Also, there is another C2000 device family F28P55 which has 'failsafe' IOs for PMBUS and I2C. This means there is not a diode from the pin to supply so there will be minimal current load on the PMBUS bus when the device is powered off.

TMS320F28P550SJ data sheet, product information and support | TI.com

Best regards,

Jason

0 Wilson Liang2 8 months ago in reply to Jason Whiles

Intellectual 540 points

Hi Jason

Noted and thanks.

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TMS320F280039C: The status of GPIO when VDDIO / VDD is not powered.