MC33097 - fail-safe output FS0B connection

[matej_15]

Hi!

we have a question regarding the Functional Safety and external circuitry associated with the fail-safe output FS0B of the MC33907LAE SBC.

The MC33907 Safety Manual states the following on Page 38:

It says that we need to have a 5k1 FS0B series resistor in order for it to be robust against automotive transients. Our project has a custom circuitry connected to fail-safe output FS0B which combines FS0B signal and Safety MCU (MPC5748G) signal making a redundant path [SM_021]. Only the combined fault signal goes externally out of the system. Our schematics is shown below:

Our questions:

1.

Is the above schematic sufficient to say that our circuit is "robust against automotive transients which can appears on the battery line" as stated in the Safety Manual - [SM_018] and if not, why?

2.

How was the recommended 5k1 series resistor value calculated?

3.

On the Page 38 of the Safety Manual it is written "Implementation: a redundant signal coming from the MCU must be connected to the safety switch with a pull-down resistor.".

As shown in the schematics above, we have a pull-up connected to N-MOSFET's gate on our redundant SMCU signal. This pull-up ensures that the signal combining with the FS0B signal (drain pin of Q2203) will be low by default, same as, I believe, is the reasoning for pull-down resistor in the Implantation text from above. Is it safe to say that our circuitry with pull-up resistor delivers the same functionality as the recommended pull-down resistor and therefore the assumption [SM_021] is fulfilled? 

Best regards,

Matej

[r40959]

our answer from William Sanfins: Hello Matej, See my answers below : 1. Correct, your circuitry is robust against automotive transients. I would recommend to displace capacitor C2223 (10nF) the closest to FS0B pin to protect it against ESD. 2. FS0B safety output is load dump proof and robust against ISO 7637-2:2011 pulses with a serial resistor to limit the current during the high transient pulse on the line. The resistance value is calculated based on the ISO pulses parameters, the FS0B pin spec etc. 3. According to your schematic, yes it is. FS0B is an active low signal, therefore in the safety manual schematic we assumed to have also an active low signal coming from the MCU as a redundant path, hence the pull down. Thank you