# ISO7142CC: Current Limiting Spec

Part Number: ISO7142CC

Hi Team,

I'm using the ISO7142CC to send signals between two ICs and my application is in a safety critical, therefore I need to account for incorrect connections. I don't see any current limiting spec in the abs max datasheet, only max output current.

What would happen if two outputs of the ISO7142CC were connected together and 'driving' into each other.

Thanks,

Hayden

• Hi Hayden,

The safety limiting values of ISO7142 is mentioned in section 8.3.3 of the data sheet. Limiting currents as mentioned in the table will prevent potential damage to the isolation barrier upon failure of input or output circuitry.

Two outputs of the Isolators should not be connected together. For instance, if one channel is driving high and the other channel is driving low there is a short between supply and ground that will cause excessive current and may damage the device.

Regards

Tejas

Hi Tejas,

Thanks for the info, I must of missed that in the datasheet.

Do you have an idea of what the assumed source impedance of the outputs is? If two outputs were connected directly to each other with no current limiting resistor between, the source impedance would be the only current limiting factor, correct?

Regards,
Hayden

Hi Hayden,

The source impedance should be around 40 ohm. So if two outputs are connected together the total resistance in path should be around 80 ohm.

Can you please explain more about the system, what is condition where two outputs get shorted in your system?

Regards

Tejas

Tejas,

That’s better than I expected!

So, if I have two shorted to each other and worst case one is high and one is low, that would result in:
5.5V/80 = 69mA.

Per the Safety curve below, the case can be over 100C and be able to handle that much current. So, that seems like not a huge issue, though we want to correct in next board rev to avoid unnecessary current draw.

At present, we haven’t been using the outputs that are tied together. We are just developing the software functionality now. So, they have typically been driving into each other with about the same voltage, which would be even less current draw than the above calculation.

System wise - this is for an aerospace electrical engine control module. It's a system that interfaces between the cockpit controls, main flight engine (previously only mechanically controlled) and other actuators around the airplane.

Regards,
Hayden