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SN65LBC184: Supply pin to GND pin very low Resistance Failure

Joe Saggio
Prodigy 40 points
Part Number: SN65LBC184


I'm troubleshooting a design around this part and have had 2 PCBs drawing excessive current off the 5V bus that feeds the SN65LBC184. I did not design this circuit, but 2 of these chips have failed in such a way that between the 5V supply Pin and GND pin, there is only 1.85 Ohms...I found this by eliminating devices on the PCB while continually measuring 1.85 Ohms between 5V and GND...might as well be a dead short. This may also be causing the 5V LDO to fail in an instance or two. Since I did not design this chip into the product, and also since the chip (and 485 coms) is not being used at this time, this is a new finding.

Long story short, removing R13...which is shorting pin6 and 7 too GND greatly reduces the PCBs excessive current inrush..not sure why these terminating resistors would have been installed like this, but I suspect it could be the reason for the 5V to GND failure of the IC...

I'm looking for wisdom from anyone at TI or anywhere that has designed with this chip and can confirm my suspicions...since there is no firmware tested and validated to drive this chip,  could it be coming "up" upon power on to the PCB in an undetermined state and driving outputs high,.,...thus sinking current to GND. Note, after the power up "event" and current limiting on a 24V bench supply that supplies the PCB/5V regulator,  the PCB draws normal current assuming U3 has not failed as described....There are MANY PCBs that have not failed like this and 3 that have. 

I have also attached a scope capture of how the 5V supply comes up on the regulators 5V output....24V is fed into the regulator with 100mA current limit set on 24V bench supply...for purposes of testing and debugging I bumped that 24V current limit up to 500mA and it still went into limiting....by removing R13, I am able to push down the 24V supply limit to 200mA - voltage shelving and current limiting is greatly reduced.

5V startup without R13 (and also basically same without U3)

5V startup with R13 & U3 installed on 5V Rail

  • 0
    TI__Guru 62920 points
    Hi Joe,

    How is the DE pin being handled? If it is not held low, then the output driver of the LBC184 will be active and try to drive a signal onto the bus. Since both outputs are shorted to ground, though, this will cause large currents to flow from VCC and out either the A or B pin (depending on the state of the D pin) into the termination resistances. The LBC184 will source up to 250 mA into a short circuit, and under this condition the IC will be dissipating a large amount of power (250 mA * 5 V = 1.25 W). This would raise the internal temperature, and if this scenario persisted for some time I would expect the device lifetime to degrade and the failure rate to increase.

    I'd recommend removing those 0-Ohm termination resistors. I suspect they were included in the design as placeholders in case termination needed to be added, and that populating them with 0 Ohms was not intended (there is no case where it would make sense to do this). Making sure DE is low would also solve the issue of overloading the driver by making sure it is disabled (high impedance).

    If you've already done both of these things and are still seeing failures please let me know. What I mentioned above seems like the most likely root cause based on the application circuit, but there could be other factors to consider as well.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 40 points

    Hi Max, 

    Thank you for the speedy response! Your explanation is clear and makes complete sense - very appreciated as I believe we have dialed down to the root cause. 

    I have removed the "placeholder" terminating resistors. I'll certainly go check code and see how the DE pin is being handled - and hold it low.

    I removed the resistors on 6 PCBs yesterday after writing in this forum and all (as shown on scope grabs) performed in *much* friendlier manner on power-on...

    Thanks Again - look forward to future questions/answers as they arise :-)