TMS320F280049C: A2 pin impedance on PowerOff DSP

Francesc Paradell

Part Number: TMS320F280049C

We are working on a 2 phases DCDC converter using 2 TMS320F280049C DSPs.

We have connected the output current sensing of phase1 to A0 Analog input on DSP1 to A2 Analog input on DSP2.

While booth DSPs are Powered On booth are reading a correct value, but when we power Off DSP2, DSP1 is reading a wrong value, and we read near to 40% of the real value.

Looks like DSP2 is somehow adding an unexpected impedance that is affecting the voltage connected to A0 on DSP1.

Can you explain the behavior of A2 pin during a Power Off sequence?

over 2 years ago

0 Francois Charlot over 2 years ago

TI__Mastermind 29107 points

Hello Francesc,

As you expect, you've got some current flowing into DSP2. This is because its input stage is powered off, so your analog signal is powering some internal paths of DSP2.

We cannot give you any input impedance for this input when the device is powered off because this behavior is forbidden. You need to add an analog switch to your design so as to isolate DSP2 from the analog signal when DSP2 is powered off.

Best regards,
François.

0 Francesc Paradell over 2 years ago in reply to Francois Charlot

Prodigy 60 points

And is it possible to reconfigure somehow the pin before power off to avoid this issue?

0 Francois Charlot over 2 years ago in reply to Francesc Paradell

TI__Mastermind 29107 points

Hi Francesc,

Unfortunately not. Because some level of I/O configuration may be performed by software, but it is lost once you power off the device.

Again, this configuration of feeding a non-zero voltage to a device that's powered off is forbidden, because in your case this voltage is not only going to the ADC input but it is also powering a part of the input circuitry, which is drawing a significant current compared to that of a simple analog input. Hence the large voltae drop that you measure.

Best regards,
François.

0 NADA SOURI over 2 years ago in reply to Francois Charlot

Prodigy 60 points

Hello Francois,

Thanks for your feedback.

Also we have seen that when we turn off the DSP, the 3v3 doesn't go to 0v, it goes to 1,8v.

Do you think it is normal with what Francesc mentioned before?

Thanks.

Nada

0 Francois Charlot over 2 years ago in reply to NADA SOURI

TI__Mastermind 29107 points

Hello Nada,

Without having seen the schematic (please do not post full schematics on this public forum), I would say that if you want to power off DSP2 while keeping DSP1 active, you need to fully isolate DSP2 so as to avoid powering or partially DSP2 through its I/O pins.

Best regards,
François.

0 Francesc Paradell over 2 years ago in reply to Francois Charlot

Prodigy 60 points

Thank you François,

So we are thinking in to change our strategy from supervisor and instead of PowerOff the faulty phase we can keep it in reset using the XRSn line.

Which will be the behavior of the ADC input if we force the XRSn?

0 Francois Charlot over 2 years ago in reply to Francesc Paradell

TI__Mastermind 29107 points

Hello Francesc,

That looked to be a good idea. However this is also an illegal behavior as pet our datasheet. Please see this section of the document. You truly need to isolate the device pins if you intend to keep it powered off or in the reset state. Sorry for that.

Best regards,
François.

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TMS320F280049C: A2 pin impedance on PowerOff DSP