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TMS320F28027: tolerance of COMP input pin vs very short negative spikes

Felix Elsaesser
Prodigy 40 points
Part Number: TMS320F28027

Hello,

I am using this microcontroller to digital contol a flyback DC-DC converter. To improve efficiency I want to monitor a voltage which can become negative. I use the following circuit to fit the signal inside the maximum ratings of the input pins of the microcontroller:

Unfortunatily I still get a negative voltage spikes for about 20-40 ns (with a Frequency of 150 kHz). The negative voltage spikes range from -0.5 V to -3.5 V.

I want to compare this signal with the internal COMP. Can its input whitstand such short peaks? A fast response time is very important so I want to avoid additional OPAMS.

  • 0 in reply to Felix Elsaesser
    TI__Guru 62975 points

    Felix,

    I have to officially advise against this because it violates the Absolute Maximum Ratings of no more than 300mV below VSSA.  I personally would not expect lasting damage from such short pulses, but I also have no evidence that they can be sustained for an extended period of time.

    However, even if lasting damage is avoided, I would not expect the comparator to operate within datasheet specifications when the stricter Recommend Operating Conditions are violated.  This would likely inject enough disturbance into the device to make the comparator performance unreliable.

    Would you be able to use an extended resistor divider network to precondition the signal similar to what is recommended for the PGA on F28004x?  You would probably need to disable hysteresis on the comparator if you try this.

    -Tommy

  • 0 in reply to tlee
    Prodigy 40 points

    Hello Tommy,

    thank you for your reply. I am not quite sure how I should extend my resistor divider. In the document you showed they use a DAC to offset the voltage. But as far as I unterstood the internal DAC of TMS320F28027 is only for the internal COMP and cant be accessed.

    Is it possible to use an existing voltage level (like my 3.3 VCC) instead of an DAC?

  • 0 in reply to Felix Elsaesser
    TI__Guru 62975 points

    Correct, F28027 does not have a buffered-DAC, but you can provide an external reference voltage to provide an offset.  As you have pointed out, you might be able to use a voltage divider to create a reference voltage from your 3.3V supply.

    Are you sure that an op-amp wouldn't work here?

  • 0 in reply to tlee
    Prodigy 40 points

    I think an op-amp with the right specs could work. Unfortunatly I have not found one yet where I can be sure it will work properly. 

    The specs are:

    Vsupply 3 - 15 V

    single supply

    propagation delay <50 ns 

    able to withstand the mentioned negative voltage spikes ( -3.5 V)

    I found now several op-amps which stated in the datasheet they could withstand voltages beyond their maximum ratings for a limited time. Unfortunatly they seemed all to slow for my purpose. The ones which should be fast enough did not include such statements. But even with those statements I am still breaking the maximum ratings.

    Can you recommend an op-amp which I could use?

  • 0 in reply to Felix Elsaesser
    TI__Guru 62975 points

    Felix,

    I recommend asking for amplifier selection advice in the Amplifiers Forum.  Generally speaking, I would not expect components to operate correctly when the input signal exceeds the range of the power supplies.

    You can still use a resistor network to precondition the signal into your single supply range.  I recommend using an op-amp to buffer the resistor network output because the embedded analog modules on F2802x expect low-impedance input signals.

    -Tommy