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AM26LV31: Blown outputs (Help needed)

Craig Beiferman
Intellectual 270 points
Part Number: AM26LV31
Other Parts Discussed in Thread: AM26LV32

On my mother board, I am using the AM26LV31 to send step and direction bits to a remote servo controller. (over about 6 feet of wire)

This wire is terminated with a 4.7 ohms in series, and a 120 Ohm termination resistor with TVS diodes on both sides,

and the receiving chip is a AM26LV32 (This is on the servo controller)

The problem is now on at least 3 machines, the outputs of the AM26LV31 have blown out.
If I measure the output to ground, its 4 ohms. Some chips its 450K (I think this is a good chip),
other chips have measured 40K, even down to 4.5k 

What could the explanation be as to why this chip blew out?

and what can I do to protect it?

Admittedly, I have no protection on the mother board side, the chip outputs go directly to a connector.

Thanks for any advice!

  • 0
    TI__Guru 52172 points

    Hi Craig,

    So based on the description you have I don't think the loading is the issue (assuming that the TVS diodes aren't shunting a lot of current during normal operation) 

    A few things:

    1. Minimum RS422 (this device is rated with RS422) input impedance is 4K - in general our devices are usually a bit higher than that but we don't directly spec this so "4K" is the minimum to go by in general, but it could be higher. It definitely sounds like the output is fried in the low impedance devices.

    2. RS422 is a bit tricky when it comes to protection - for driver only devices (such as this one) the output voltage rating isn't -7V to 7V as shown on product page (that is the RX requirement - TX has no such requirement - but I think our automated systems see RS-422 compliant and put that as the common mode range) and the outputs are only rated between -0.5V and 6V abs max ratings. So based on my first pass of the information it seems there could be a possible transient signal on the bus connected to driver pins that could be out of spec - with no protection on the driver side node this could lead to potential failure points. I would assume based on the description you would look at TVS diodes for protection - but if the issue is a surge more robust diodes beyond TVS may be needed. 

    So in conclusion my guess is that there is a potential signal that is frying the driver pins (most likely transient as the driver won't output a voltage that is out of spec as the driver output voltage is derived from VCC voltage) . Using protection diodes/TVS diodes on each output line will help protect the driver side pins from becoming damaged. I'd imagine you are using the multiple channel feature of the part - if this is not needed please let me know as our RS-485 devices are much more robust and backwards compatible with RS-422 systems (same architecture, rs-485 is just a more robust standard), but I understand the multi-channel is most likely why this part was chosen, so if you can't switch, protection diodes that will keep voltages on outputs bounded by abs max conditions should solve the issue, if it is a transient signal on the driver - which based on problem description that seems like the most likely explanation as the loading shouldn't be a problem at all in your setup. 

    Please let me know if you have any other questions and I will see what I can do - and please let me know if you are able to add protection and if that solve the issue.

    Best,

    Parker Dodson