Hello: I am forwarding this inquriy from my customer:
"We are having issues in production with our RS422 interfaces that use these components. Haven't figured out the root cause yet.Can the DS26LV31T handle a short circuit on its outputs?If so, for how long? The datasheet is not clear on how long the short can last.What would be some failure modes of a short that remained too long?Also, is the short circuit current limit controlled by the device or does it have to be limited externally?Can TI/National comment on whether they had any lot issues in the past 2 years? "
Please address this issue for the customer. Thanks. George Biner, Arrow LA FAE
Greetings -
The RS-422 driver output is specified at 150mA MAX for a short to ground. If one shorts the output, up to that magnitude of current can flow, and worse case fault power is 3.6(150mA) or 540mW per fault. Thermal damage could occur depending upon total power dissipation (VCC x ICC) and power of other outputs all added up, and then doing a standard junction temperature calculation Tj = Ta + PD (Theta JA), if the fault current exceeded Tj of 150, then a thermal damage is possible due to the violation of abs max for Tj. If the Ta = -40C there is more room for surviving this fault. If it is 85C, then a larger issue as there is less power dissipation package capacity there.
In general RS-422 parts have only a current limit feature. If a more robust output is needed RS-485 is a better choice as it also includes thermal shutdown protection and senses the high die temp and turns off to protect itself.
More details would need to be know to determine what happened for example: short to what voltage? How many outputs? Supply voltage, Ta, other channel status etc.
Some applications add a series resistor in the legs of the output to add additional current limited if needed. I prefer the RS485 protection myself. This is a high volume device, I am not aware of other reported issues or of any fab changes in the past two years.
A damaged unit could be returned for a PQA to see what is visible (ESD, EOS, blow metal etc). That might help to determine when / why the damage is occuring.
John Goldie
Video Interface - DPS / SVA