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MAX13451E FAULT signal behavior

Category: Datasheet/Specs
Product Number: MAX13451E

Hello ADI support team,

According to the datasheet of the MAX13451E it should indicate a short circuit condition (on the RS485 bus) with a HIGH FAULT output, but we can't get this feature working.

We see the following with VCC = 5V, VL = 1V8, SRL = INV = TERM100 = nTERM = 0V, DE = nRE = DI = 1V8, 100 kOhm pull-down resistor on the FAULT pin:

  • With no errors on the bus signals we can measure ~60 mV at the FAULT pin
  • With the A+B signals shorted together we can measure ~200 mV at the FAULT pin
  • With the A+B signals connected to GND we can measure ~200 mV at the FAULT pin
  • With either of the A or the B signals connected to 6V (over-voltage condition) we can see a valid HIGH state at the FAULT pin

What can be the issue?
Thanks for any help.

  • Hello,

    Tying the A or B signals to GND will not assert the FAULT pin; it needs to be below GND to trigger. 

    I will need to look more into why the shorted signals did not assert the FAULT pin. What is the driver output current when A and B are shorted together?

  • Hello,

    The short-circuit current between A and B is ~53 mA.

  • Hello,

    I have confirmed that since the FAULT pin is a push-pull driver, an external pull-down resistor is not required. With the resistor connected, what are you measuring at VL? Does the VL supply have enough current capability to source the pull-down resistor?

  • Hello,

    We now tested the MAX13451E off-circuit (on a breadboard) and we see the same behavior: the FAULT only goes up to ~200 mV with a short-circuit on the A+B signals.
    With the VCC=5V we tested with three different VL voltages (1V8, 3V3, 5V), but we basically see no current consumption on the VL pin (with and without the 100 kOhm pull-down resistor on the FAULT pin).
    As I wrote in my first post the FAULT definitely changes if we add the short-circuit, but it only goes from ~60 mV to ~200 mV. It only goes close to the VL voltage if we add an over-voltage condition to the A/B signals.

  • Hello,

    Some questions:

    • Is the current in your supply being limited?
    • Have you tested a different IC to rule out the possibility that the part has been damaged?
    • Were you able to assert the FAULT pin by forcing A or B below GND while DE is high? At this point, is the short-circuit test the only one that is not asserting the FAULT pin?

    Since the FAULT works as expected for an over-voltage condition, it could indicate that something in your application is preventing an overcurrent condition.

  • Hello,

    I'm sorry for my late reply.

    • I can confirm that we have nothing in our test setup that could limit the short-circuit current between the A and the B signals at that low level (~53 mA). The MAX13451E simply doesn't output more.
      I can see in the datasheet that two different short-circuit currents are specified:
      • Driver Short-Circuit Output Current: max. 280 mA
      • Driver Short-Circuit Foldback Output Current: min. 15 mA
        • Can this "Foldback current" explain the lower current we measure?
        • What is the minimum current level that should set the FAULT output HIGH?
    • We tested multiple devices in and off circuit.
    • Yes, I can confirm that the FAULT output goes HIGH if we apply an external voltage to the A or to the B signal that is higher than the VCC or lower than the GND.
  • Hello,

    Apologies for the delay. It is odd that FAULT does not get asserted for the short circuit condition, but does for the under- and overvoltage conditions.

    It does not seem like the current levels rose high enough to trigger the foldback circuitry, but I am unsure what current level would set the FAULT output high. I have reached out to the team more closely involved with this part and will get back to you once I receive a response. For now, I have some other questions:

    • Can you clarify what you meant by testing multiple devices "in and off circuit"? Was the "off circuit" test a standalone IC, or was it still connected to other components?
    • Is there anything connected to your FAULT pin, like an LED or microcontroller input?
  • Hello,

    Thanks for checking the trigger level for the current limit in advance.

    • Yes, during the "off circuit" testing we soldered a brand new IC on a TSSOP-DIP breakout and we did the measurements on a breadboard. We only had a multimeter connected to the FAULT pin in this setup.
    • On our PCB ("in circuit") the FAULT signal is connected to an input pin of a GPIO expander.
  • Hello,

    Unfortunately I was not able to get a specific answer from the team, so I will have to dig around in some older documentation to see if I can find anything.

    I have also ordered the part myself to see if I am able to replicate the issue you are seeing. Please give me some time for the parts to arrive and for the testing. 

    I know you mentioned testing multiple ICs already, so would you be able to send over photos of the top marks? I'd like to verify the ICs that you have.


  • Hello,

    We have 5 identical parts on our PCB and all of them behave the same as described above:

    This is one of the parts that we later tested on a breadboard and also behaves the same:

    Thanks for your help in advance.