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AM26C31: output short circuit

Hironori Maeda
Prodigy 10 points
Part Number: AM26C31
Other Parts Discussed in Thread: AM26LV31E

Hello.

I would be grateful if you could answer the following questions.

question

If the negative side of the driver output is shorted to GND, will the device fail due to overcurrent, etc.?

Connect the output Y to 74LV14 through a 1kΩ pull-up resistor and Z to GND as shown in 1.

A is a purchased motor driver, and B is an interface board that we designed ourselves.

The reason for this connection is that A was previously an open collector output as shown in 2.

　　　

Originally, ASG/BSG/TIM shown in 1 and 2 were output to B, but they were not used.

It was changed from 2 to 1 and 2 out of 21 units broke down.

One of them is shown in 3. I think you can see that the IC is burnt out.

The motor driver manufacturer explained that the negative side of the differential output was shorted to GND and the signal was added to the input, which led to damage to the device.

However, 19 devices were not damaged, which led to this question.

that's all, thank you very much.

JEOL Ltd.  Maeda

  • 0
    TI__Guru 51932 points

    Hi Hironori,

    So yes - a short circuit current can indeed break the device - but its a bit more complicated than just shorting the device. I will say a direct connection of driver output to ground will most likely fry the device. 

    This device is rated to -170mA for short circuit current. So it can be shorted but usually not for very long. While this device doesn't state it directly many devices in this family have a limit that any short circuit current is to last for no more than 1s and no more than 1 output is shorted at a time. The issue isn't the current itself necessarily but the heating of the IC that the current causes. If the junction heats up too much the device can be destroyed - if the junction is cooled without hitting its max junction temperature (it doesn't say - but it should be ~150C) - in other similar parts you can look at multiple watts of energy lost by shorting multiple driver pins to ground - even 1 can cause a large temperature increase.

    So I do think the issue is the driver to ground. If unused I'd put a resistor of equivalent value as the impedance to ground seen by A. Since this seems to be point to point - using a 12K ohm to ground on the unused driver pins would be a good start - if there are multiple transceivers that are not shown it is advisable size the resistor with the following formula:  12K/( total # of transceivers - 1) - so for 2 devices (as shown above) 12K - for 3 devices 6K, 4 devices 4k.... etc.

    So in conclusion:

    1. Shorting an output all the time will raise the risk of raising the junction temperature to a failure point. While brief shorts are possible - extended ones will have much higher chances of frying the junction.

    2. The reason not all failed is because not all the junctions heated up enough - however high failures would be more or less expected in the current setup.

    3. Instead of grounding unused driver outputs use a resistor to ground so that the driver isn't shorted - 12K is recommended for point to point (which is what I think your application looks like) but it can really be anything that isn't too low of an impedance (I'd say 375 Ohms -but for power consumption reasons I would go with bigger resistor - so that minimal current is coming out of Z) 

    Please let me know if you have any other questions! 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 10 points

    Hello Dodson-san,

    Thank you for your prompt reply.

    I roughly understand the mechanism that leads to the failure of the device.

    Lastly, please let me know the following.

    The above sentence, "While this device doesn't state it directly many devices in this family have a limit that any short circuit current is to last for no more than 1s and no more than 1 output is shortened at a time." Please tell me where is it?

    Best regards,

    Hironori Maeda

  • +1 in reply to Hironori Maeda
    TI__Guru 51932 points

    Hi Hironori,

    Its not listed in this datasheet - but it is in datasheets of similar parts in the same technology family (for example see AM26LV31E Datasheet, Page 5, footnote #2) . It is just in general good advise to follow regardless of what part you are using for these types of applications, especially when extra heat dissipation measures are unavailable as extended short circuits can  destroy pretty much any part. Essentially the short circuit protection on the part gives some time for a short to be resolved before it is destroyed as it is current limited. Sorry if that wasn't clear in my first response. The fix for the problem should be just to use a resistor to ground on the unused driver outputs.

    Please let me know if you have any further questions!

    Best,

    Parker Dodson