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SN74LV1T00: SN74LV1T00DBVR safety analysis

Alex Khramov23
Intellectual 290 points
Part Number: SN74LV1T00

For the schematic safety analysis I want to discuss situation when 5V supplied SN74LV1T00DBVR output connected to the ground over 22 Ohm resistor.

In this situation output current exceed 25 mA  Absolute Maximum output current . Should I expect any safety related critical problems like smoke or ignition?

The same question for the situation when two  5V supplied SN74LV1T00DBVR buffers outputs are shorted ( one has the logical H and second the logical L)

Do you have any official document about?

  • 0
    TI__Mastermind 20100 points

    Hi Alex,

    You might experience smoke, probably an overcurrent event leading to that pin being shorted to VCC or GND on the device since the FET sees too much current. I would suggest increasing the resistor value and/or adding a couple of diodes to prevent the devices from being shorted. The datasheet is the document in this case.

    Best,

    Malcolm

  • 0 in reply to Malcolm Lyn
    Intellectual 290 points

    Hi Malcolm,

    Could you please to show the diodes proposed connection.

    BR

    Alex

  • 0 in reply to Alex Khramov23
    TI__Mastermind 20100 points

    Hi Alex,

    Can I ask why you shorted the output of two channels together such that one can drive high while the other drives low? Maybe there is a better solution to the problem you are trying to solve.

    The best solution is to not short the outputs in that manner. The second best solution would be to add larger resistors at both outputs to limit the amount of current. The diode solution as shown below functionally turns the device into an open drain device and would require a pulldown resistor.

    Best,

    Malcolm

  • 0 in reply to Malcolm Lyn
    Intellectual 290 points

    Hi Malcolm,

    The device I analyze according to the Safety Standard has the external connector.

    This connector has 3 pins.

    First SN74LV1T00 buffer output connected to the pin 1 over 22 Ohm resistor.

    Second SN74LV1T00 buffer output connected to the pin 2 over 22 Ohm resistor.

    Pin 3 connected to the Ground.

    During normal operation they are not shorted of course, but for the Safety Standard requirements I should 

    evaluate the possible risk smoke or ignition in the next cases

    Pin 1 shorted to the Pin 2. 

    Pin 1 shorted to the Pin 3.

    Pin 2 shorted to the Pin 3.

    For the situation when Pin 1 connected to the Pin 2 ( buffers output are shorted ) , this risk situation I guess

    when buffers outputs have the opposite values, isn't?

    At this moment I think the best solution is to increase the serial resistor , but it will limit the minimal pulse width

    transferred over the buffers because resistor together with connectors ( male-female) capacitance and input buffer/ layout 

    capacitance ( receiver ) will create the RC low pass filter. 

    Maybe you can recommend the similar buffer in the same package but with higher output current ( this or another logic

    family )  in order to keep the serial resistor as minimum as possible but still not overcome the Absolute Max. Rating buffer

    output  current. 

    The situation when the buffers outputs will be broken after the shorts I described before but didn't create smoke is

    acceptable. This was my point in the first mail.

     

    BR

    Alex

  • +1 in reply to Alex Khramov23
    TI__Mastermind 20100 points

    Hi Alex,

    For highest drive strength I would use the below device (not a translating device):

    SN74LVC1G00 data sheet, product information and support | TI.com

    There are some devices with TTL compatible inputs that could work but none of them share the same package.

    Best,

    Malcolm