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TMS570LS3137-EP: Forward Voltage of ADC Clamp Diodes

Shihab
Expert 1225 points

Part Number: TMS570LS3137-EP

Hi,

We have provided clamping diodes (whose Vf at 25C is 380mV and at 125C is 0.2V at IF=100mA) externally to protect the ADC inputs . The external clamp diode is provided because the clamping current can be as high as 100mA during a fault condition.

So I was wondering whether the external clamping diodes with the above stated Vf rating will guarantee the protection of ADC inputs? Can you please assess and reply?

I have measured the VF of diodes provided inside TMS570LS3137-EP for ADC channel protection. The VF is measured as ~0.7V.

As per datasheet ADC input voltage should be (VAI < VSSAD – 0.3 or VAI > VCCAD + 0.3).

I am hoping that the value 0.3 is the VF of the protection diode. Can you please tell me whether 0.3 is provided in the datasheet by considering an operation upto 125C temperature?.

Can I assume that the nominal VF of the protection diode is 0.7V at 25C and at 125C the VF will be reduced to 0.3V?. Can I consider that, the statement (VAI < VSSAD – 0.3 or VAI > VCCAD + 0.3) is valid at higher temperature where as if I operate the DSP at 25C, then the ADC input will be protected upto (VAI < VSSAD – 0.7 or VAI > VCCAD + 0.7)?

Thanks,

Shihab.

  • 0
    TI__Guru 62930 points
    Shihab,
    The diodes in the device are not clamp diodes, but ESD protection diodes.
    The specifications you reference are recommended operating conditions. Exceeding these will not damage device, but device may no longer meet specifications. You must stay within this range to insure you are meeting required input conditions and insure specified performance.
    From a reliability standpoint, these pins are specified in the absolute max ratings table as -0.3 to 5.5V independent of supply voltage.

    If this answers your question, please click "Verify it as the answer"
    Regards,
    Wade
  • 0 in reply to Wade Vonbergen
    Expert 1225 points

    Hi Wade,


    Thank you for the reply.
    I would like to get a little bit more information on this.

    In our application the ADC input voltage exceedance will happen only during a fault condition which is considered as abnormal condition and occurs rarely.

    The ADC input voltage goes to -0.5V during this fault condition and once the fault is cleared the ADC input voltage will come back to within the recommended input voltage range.

    Whether the ADC channel will meet its specifications if the input voltage comes back within recommended range after the fault is cleared? We are not worried about that particular channel of ADC meeting its datasheet specifications during fault condition.

    Thanks,
    Shihab.

  • 0 in reply to Shihab
    TI__Guru 62930 points

    Technically an excursion to -0.5V will violate the absolute maximum ratings and violate the warranty.

    However, these inputs also have an IIK specification.

    If the current is limited to within the IIK spec, then you can safely violate the input voltage spec.

    The IIK spec is +/-10mA.   So, if your current is less than 10mA during the transient fault, then you are not violating the absolute max ratings.

    If this answers your question, please click "Verify it as the answer"
    Regards,
    Wade

  • 0 in reply to Wade Vonbergen
    Expert 1225 points
    Hi Wade,

    Can I interpret IIK (Input Clamp Current) as the current flowing through the ESD protection diodes?
    If yes, then at normal lab ambient temperature I have measured the VF of this diode using multimeter and found to be ~ +0.7V.
    In my design, at 25C the minimum ADC input during fault condition is < |-500mV|. This voltage is less than VF of the ESD diode at 25C and the diode will not be forward biased. Under this condition the current through the diode should be negligible (it should not be in the range of mA).

    As per datasheet, the absolute max ratings are specified over operating free-air temperature of the DSP (-55C to 125C). According to that, if the operating temperature is 125C then IIK can be upto +/-10mA. Also in general, as the temperature increases the VF of diode reduces. So from the datasheet, my understanding is the -0.3V is a limit to be applied at the maximum operating temperature of DSP (125C) and at temperatures like 25C a limit < -0.3V can be considered and at -55C an even more negative voltage limit can be applied. Please let me know if there is a mistake in my thinking.


    Thanks,
    Shihab.
  • 0 in reply to Shihab
    TI__Guru 62930 points
    We do not specify the diode characteristics this way.
    We only specify the two absolute max conditions.
    If you can confirm that the current is less than +/-10mA, then you will not be violating the absolute max specification.
    Regards,
    Wade