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Original question:

TMS320F280049C: Questions about processor core 1.2V and external 3.3V power up sequence?

TMS320F280037: injection current

Konrad Holzmann
Prodigy 140 points
Part Number: TMS320F280037


Hello Team,

I would like to know the structure of the internal overvoltage protection of the digital an analog inputs. Are there diodes to VDDx and GND? How much current can be injected to the pins.

Reason:

Especially while powering up the system on some pins the voltage is higher than VDDx, but with low current, e.g. for measuring supply voltage on anlog pins. Achiveing Section "6.12.1.4.2 Signal Pins Power Sequence" is not so easy, some external circuit is needed. 

If there could be some higher voltage, e.g. 0.8V simple protection diodes can be used, a series resistor to the input pin can limit the current. 0.3V is hard to achive with diodes.

Regards Konrad

  • 0
    TI__Prodigy 470 points

    Hi,

    The expert is out of the office because of Holiday. Please expect a reply by Tuesday.

    Thanks.

  • 0
    TI__Expert 5730 points

    Konrad,

    There are small internal protection diodes from the pins to the respective VDDA/VDDIO and also VSS.

    It isn't recommended to put any voltage on signal pins before the supplies are available.  However if you limit the current and the duration of current injection is transient then this should be okay.  The Absolute Maximum rating is 20mA but for intentional repeated injection you should keep it <2mA, the lower the better.

    There are IBIS models of the ESD clamps available in the models section of the product folder if you'd like to simulate the combination of series resistor and external diode.  I have seen the circuit you propose used for this purpose.

    Best regards,

    Jason