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Original question:

SN65LBC172A Temperature Range

SN55LBC172: Short circuit protection on outputs, device behavior

Jesse Muszynski
Prodigy 80 points
Part Number: SN55LBC172

For the SN55LBC172 the datasheet indicates there is positive and negative current limiting and thermal shutdown for protection against line fault conditions; little additional detail is provided on device behavior during these fault conditions.

If 1 of the 4 outputs has Y and Z outputs shorted together are the other outputs affected?

Is there internal impedance on the outputs that limit output current to a safe level? 

Will a single shorted output result in the device entering thermal shutdown?

If the device enters thermal shutdown what state do the outputs go to (for example all outputs go high-Z)?

  • +1
    TI__Guru 52352 points

    Hi Jesse,

    1. So a short circuit doesn't necessarily cause the other outputs issues unless the short causes the device to go into thermal shutdown (then all outputs are high-z) - which is pretty likely for most quad channel RS-485/RS-422. 

    2. There is a current limit to stop the device from sourcing/sinking more than 250mA during short circuit (for defined test condition) 

    3. Potentially - there is a decently high chance that one short could put device into thermal shutdown- please see power dissipation table:

    Exceeding these power ratings may cause the device to go into thermal shutdown - as the power dissipation ratings is the power required to bring junction temperature to max rated temperature (after which thermal shutdown could potentially start) 

    Please note the derating is for TA > 25C not 125C (power rating at 25C is 100 * derating factor higher than power at 125C - so there is a typo).

    So for example if you are using the "W" package at 100C and during a short you exceed 400mW (P_MAX(100C) = (1W - (100C - 25C)) * 8mW/C) then the part may go into thermal shutdown (thermal shutdown may happen at junction temperatures higher than 150C)

    The type of short will also matter. If it is just a Y to Z short - with no external voltages applied the power it would consume would be approximately 17.6 * 0.25^2 or 1.1W (1100mW) if current reached max value (most likely it will be less). The 17.6 comes from a very quick approximation of (VCC - 2*Diode_drop)/(2*RON) = IOS where the 17.6 is 2*RON with the assumption that the diode_drop is 0.3V and IOS reaches its max value of 250mA.  So by this quick approximation short circuits will have a decently high chance of pushing device into thermal shutdown. This is going to be an issue on pretty much any quad channel RS-485 or RS-422 device as they can burn a lot of power during shorts. You can increase the power rating by adding heat sinks to the device - but we don't really specify much information that would help in sizing a heat sink. 

    It would be best if using this part to assume 1 short can throw the device into thermal shutdown (depending on actual current, package, and ambient temperature) so designer should include that consideration into design as it is a potential application scenario that would drive the device into thermal shutdown so there should be  some contingency plan for when/if that happens. 

    4. Outputs go hi-z during thermal shutdown. 

    Please let me know if you have any other questions!

    Best,

    Parker Dodson