ISO1412: safety limting values - IEC 60747-17:2020 + COR1:2021 (VDE 0884-17) - protection of interface pins for TIA/EIA RS-485 and RS-422, PROFIBUS

Hello Jonathan, 

(+) [FAQ] Are there any suggestions from TI on how to comply with the "Safety Limiting Values"? or (+) [FAQ] [H] Quality, Reliability, and Certifications

Typically, a series resistor in line with the VCC pin is used to protect the device in the event of a VCC to GND short. In this case the device will draw an excessive current which could lead to further problems if maximum junction temperature is exceeded. 

There is a level of fault protection built into the device 

"9.3.2 Failsafe Receiver

The differential receiver of the ISO14xx devices has failsafe protection from invalid bus states caused by:
• Open bus conditions such as a broken cable or a disconnected connector
• Shorted bus conditions such as insulation breakdown of a cable that shorts the twisted-pair
• Idle bus conditions that occur when no driver on the bus is actively driving"

Further protection can be provided using a TVS diode on the bus pins.

Best,
Andrew

Hello Andrew,

thank you for your reply!

"Typically, a series resistor in line with the VCC pin is used to protect the device in the event of a VCC to GND short. In this case the device will draw an excessive current which could lead to further problems if maximum junction temperature is exceeded. " - This is 100% correct, but in my opinion not enough, even if it is the most important protection on each side.

From my point of view a protection like in "Figure 5. The Digital Isolator Serves as an Interface in This Example, Providing Isolation between an ADC or AFE and an MCU." of your "Technical Article Understand and Apply Safety-limiting Values for Digital Isolators" ( https://www.ti.com/lit/ta/sszt202/sszt202.pdf ) is required on each pin on each side except each side the ground ones.

Could you please provide more detailed information about the internal fault protection built into the device to protect the isolation barrier, such as internal series resistors that have inherent physical properties, so that you can argue with this to justify the isolation of the device as inhernet physical property?

In the end, it is about to obtain a justification under which conditions the isolation of ISO14xx is an inherent physical property.


Best Regards,
Jonathan

Hello Jonathan, 

The internal fault protection is the DC bus fault protection (+/-18V) that is on the A/B lines. If further protection is needed from transients or overvoltage's under normal operation, then a TVS diode or surge protector needs to be used. 

However, I understand that you are more concerned about the safety limiting values. Please understand that safety limiting values are only relevant when the device has already failed from a transient and a DC fault is present on the pin of a failing device.

The safety limiting values are concerned about protecting the isolation barrier after the device has already failed. For most systems the VCC1/2 pins are connected to an LDO or some other power device.

For an example, if an LDO fails, it is common that the LDO fails short.  This would mean that the VIN (for example 12V) is now continuously applied to the VCC1 this would damage side 1 of the isolator causing a VCC to GND short. If the 12V is not limited with a resistor, then this will cause the device to heat up and it could cause damage to the other side of the isolator and the side two isolation barrier. 

Side 1's other pins are the transceiver's logic pins. These will be connected to an MCU or other logic device that is not capable of supplying enough current to cause secondary damage in the event of a failure. Therefore, only VCC1 needs a safety limiting resistor

Similarly, VCC2 would need to be protected. However, the A and B RS-485 pins would only need protection if a DC fault is present. 

Best,
Andrew