SN65HVD09: RS422 output short circuit protection- If outputs nB+ and nB- short

There is not separate current limiting function; the device just does not have enough drive strength to let more than about 75 mA flow. (This is a typical value; the guaranteed maximum is 260 mA.) The current will flow indefinitely, or until it goes into thermal shutdown. The device will continue to work normally when the temperature has gone down.

Thermal shutdown happens when the temperature reaches the limit. This depends on the number of shorted outputs and the ambient temperature.

Hi Ajith,

So while a typical (room temp - ideal conditions) test of a short is about 75mA as Clemens mentioned - it can be up to +/-260mA worst case condition. The impedance on the driver for A and B make up a combined total of typically ~150 Ohms (from VOD, VOH, and VOL specs in datasheet - figures are not worth as much as table data - pictures are one random device and can't be taken as "this is what will always happen" you have to use the tables for worst case approximations)  - so worst case power short is going to be around 0.26^2 * 150 = appx. 10W - so one short can damage the device if you push against max ratings as you shouldn't violate. As even at temperatures below 25C this device realistically can't handle more than 2.5W. Generally speaking you won't get 260mA - it probably will be lower - but even at 75mA you are looking at  appx. 840mW of power - so typical scenario 1 short is probably safe - more than 1 could damage device. 

The power rating can be increased by cooling the IC with air-flow - by how much is really going to depend on multiple design factors for heat management - and even then it probably isn't going to be great. 

for your additional questions:

1. Its about total power dissipated - so if the power dissipated over the device doesn't increase the junction temperature to a point of failure - as indicated by the temperature junction ratings. Typical applications probably can handle one short indefinitely unless you are violating power rating which is possible - so shorts should be handled quickly and if more than one shorts you most likely will have a problem. 

2. It depends on operating temperature - but 0 to 1 is the safe zone. This part does not actually comply with RS-422 or RS-485 so it has a much higher output impedance and higher short circuit current allowed that + the 9 channels makes short circuit operation not really protected with this device - unless you can keep it cool - do not run this at higher temps if you expect shorts. For RS-422 systems there isn't a bus contention issue (only 1 RS-422 driver on a bus - so no contention)  but RS-485 could have multiple drivers - so your firmware needs to protect device. 

3. The power dissipation assumption is assumption  is not correct - you are looking at around almost 1W of power per channel - typical case; at TA = 70C - you will not experience typical - so one short could damage device.

Ultimately - if you can't protect the part from experiencing A/B shorts the part could break. The work-around is to use multiple modern RS-485 ICs. They will have lower output resistance (many of the THVD devices are around 10 to 20 Ohms - compared to 150 - some modern devices include the THVD1550 (more cost effective) and THVD1450 (higher performance) ) and they will cap at 250mA - so you will see ~1/2 W to 1W  per device. Since there are multiple IC's every channel could short indefinitely without the need for additional protection (usually that would implemented in software and at the cost of reduced throughput) which would make the design simpler. Also if you need RS-485 or RS-422 compliance - this part is definitely not compliant with standard . Also - there is a note that most modern RS-485 transceivers will never output 1.5V or less - we still spec 1.5V as minimum to align with standard - its extremely unlikely you'd hit that on more modern devices when operating in recommended specifications. 

Multi-IC implementations are not ideal when you have one device - and I understand why you may be hesitant to look at a multi-channel system compared to single chip  -but this one device will definitely break if you are worried about multiple channels shorting at higher temps- and any multi channel RS-485 device is going to have the same issues because: 1) Almost all multi-channel RS-485 / RS-422 devices are older and use less sophisticated designs leading to worse performing parts compared to modern devices and 2) even with modern devices - multi-channel RS-485 / RS-422 really can't handle multiple shorts due to the increase of power dissipation over the device - in most multi-channel devices you will not find "all outputs shorted indefinitely protection" - however multi-IC systems using modern devices can. 

Please let me know if you have any other questions. 

Best,

Parker Dodson

Hi Ajith,

When the part goes into thermal shutdown all outputs will go high-z (inputs should already be high-z regardless of shutdown or not) until the device exits thermal shutdown (which is based on device temperature). 

Best,

Parker Dodson

Hi Ajith,

If you hold the device above abs max temperature we do not guarantee it will not be damaged. We have a thermal shutdown function to attempt to prevent damage - but realistically you need to try to avoid even getting to that point  because damage could still occur - so the firmware / control of your system needs to be able to account for that.

Best,

Parker Dodson