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AM26LS31: Configurations for safety operation

Jejomar Ildefonso
Genius 9510 points
Part Number: AM26LS31

Hi Team,

The AM26LS31 datasheet states that it has a positive- and negative-current limiting feature for protection from line fault conditions.

Does the device automatically enables this feature? Would you be able to give us a figure on how much the current is minimized?

Is there also an available external circuit that you can share for added protection?



Your inputs will be highly appreciated. Thank you so much in advance.

Kind Regards,

Jejomar

  • 0
    TI__Guru 62920 points

    Hello Jejomar,

    The current limit is always enabled and will prevent the output current from exceeding the value given in the "IOS" (short-circuit output current) specification in the datasheet.  If further current limiting is needed you could evaluate the use of series resistances (especially ones with a positive temperature coefficient), although note that this will have the drawback of introducing some attenuation to the signal.

    Regards,
    Max

  • 0 in reply to Miguel Robertson
    TI__Genius 9510 points

    Hello Max,

    Thanks for the helpful response.

    Our customer would like to use an external schottky diode at the input with a knee set at 5.5V, provided that the impedance and parasitic capacitance does not load down the input and slow down the rising edge. Would this work?


    Kind Regards,

    Jejomar

  • 0 in reply to Jejomar Ildefonso
    TI__Guru 62920 points

    Jejomar,

    Would this diode connect from each Y/Z output to the ground of the transmitting AM26LS31?  If so I don't see this introducing issues.  If the goal is to prevent excessive output current (beyond the specifies IOS limit), though, it may not be effective against all fault conditions.  For example, if a high-level output were shorted to ground it would still result in high current flow from the transceiver (since this would be within the working voltage range of the diode circuit).

    Regards,
    Max

  • 0 in reply to Miguel Robertson
    TI__Genius 9510 points

    Hello Max,

    The diode will be connected on the input lines. Would this limit the input to 5V and reduce current draw? 

    If not, what happens when the input voltage exceeds 5V?


    Regards,

    Jejomar

  • 0 in reply to Jejomar Ildefonso
    TI__Guru 62920 points

    Hi Jejomar,

    The short-circuit current limiting function is for the Y/Z output pins.  Were you concerned about limiting the input current to an input pin (G, /G, 1A, 2A, 3A, or 4A)?  If that is the case then the diode solution would be effective.  It would limit the input voltage to within the maximum supported range, and within this range the input current is relatively low (see II, IIH, adn IIL specifications in Table 6.5 of the datasheet for details).

    Regards,
    Max

  • 0 in reply to Miguel Robertson
    TI__Genius 9510 points

    Hello Max,

    We have shared your answers with the customer and they are very grateful.

    Thanks a lot for your help.


    Kind Regards,

    Jejomar