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LMD18200: Help in determining safety of applying 10mA to PWM inputs of this part when it is unpowered

Edward Riess
Prodigy 120 points
Part Number: LMD18200


Would you be able to confirm that it would be safe to apply +10mA to unpowered LMD18200 H-Bridge switches at each of the two PWM inputs (pins 5 and 17 in our packages) with respect to the SGND and PGND terminals (at pins 8, 18, 19, and 20) for some amount of time, or preferably an infinite amount of time? A “Yes” answer, preferably for the infinite-time case, would be very helpful for a test in unrelated circuitry that would cause this condition to occur.

 

Please let me know if there are other stipulations regarding other inputs/outputs that would affect your determination.


Thank you.

Ed Riess

 

Edward A. Riess

Common Control Unit (CCU) Lead Engineer

INTEGRATED MISSION SYSTEMS / L3HARRIS TECHNOLOGIES

t +1 513 573 6220 / m + 513 312 3742 

L3Harris.com / Edward.Riess@L3Harris.com

7500 Innovation Way / Mason, OH 45040-9699 / USA

 

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  • 0
    TI__Guru** 114905 points

    Hi Ed,

    Can you clarify what you are attempting to do?

    By +10mA, do you mean current flowing into the inputs?

    Will you be running this test at temperature?

    We can assess your query in more detail after you reply.

  • 0 in reply to Rick Duncan
    Prodigy 120 points

    Thank you, Rick.  I realized that I should have mentioned that the inputs would be driven by +15V, with respect to the SGND and PGND pin potentials (at circuit ground), connected through about 1.5kΩ resistors, thinking that if protection diodes or other voltage-limiting mechanisms were in-place, the maximum current into the inputs would be 10mA.  The tests would be at ambient (room) temperature.

     

    We are trying to test a particular component in a completely assembled (top-level) avionics unit without taking it apart. To perform the test, one section of each of several circuit boards, one-at-a-time, would be powered, and the result of that is the 15V appearing on a rail that would apply the mentioned potentials to the PWM inputs.

     

    Thank you very much for responding.

    Ed Riess

  • 0 in reply to Edward Riess
    TI__Guru** 114905 points

    Hi Ed,

    Performing this test in the manner you proposed is not recommended.

    Unfortunately, there is no current flowing at +15V on the inputs.

    This means the inputs are exposed to the full voltage (3V above the absolute maximum). This could impact the input integrity.

    Is there any way to limit the voltage at the inputs when performing these tests?

  • 0 in reply to Rick Duncan
    Prodigy 120 points

    Rick,

    Thank you for your pursuit of an answer to this question.

    In this circuit, there is no means to either lower the voltage to a level that TI might declare safe, or to probe and connect-to-ground all affected inputs at the same time.  Therefore, unless TI can provide assurance that the proposed test is safe due to input currents being absolutely limited to 1mA (which we guarantee), we will abandon this approach and consider this inquiry closed.  So, it seems worthwhile to ask this final question:  would the 1mA current-limit of this stimulus be low enough to unequivocally ensure that no stress would result to circuitry at these inputs?

    Your answer will be our guide.

    Thank you.

  • 0 in reply to Edward Riess
    TI__Guru** 114905 points

    Hi Ed,

    Please abandon this approach.

    Voltage is the concern in this case, not current. As the voltage is above the absolute maximum of 12V, there could be stress to the inputs in this condition. 

  • 0 in reply to Rick Duncan
    Prodigy 120 points

    OK, Rick.  Thank you for your investigation into this option.  We will abandon this approach to resolve the issue and very much appreciate your help.

    Ed Riess