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LIN Bus protection circuit

What is the proper value for FB1 in relation to the surrounding capacitors in this diagram?

The FB1 resistance is measured to 0.76ohm and package size is 0603. All components are AEC-Q200 rated.

The diagram is made from an existing application (Bosch WDA motor.) The LIN bus line in question is intercommunication between two motors (synced dual motor setup.)

Update (additional info):

A similar design can be found on the NCV7425EVB EV-board from ONSemi, see schematic on page 4.

NXP also mentions ESD protection using Ferrite Bead for the TJA1020 LIN transceiver, see pages 30-31.

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  • \$\begingroup\$ Is pin 1 of D1 connected to anything? There is a wire shown with one end attached to D1 but on the schematic no indication of if the wire is connected to anything else. \$\endgroup\$ Dec 9, 2023 at 13:03
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    \$\begingroup\$ It's just connected to the BSI on another separate LIN Bus, which btw has kind of a similar protection with two caps and a ferrite bead in-between, just different value caps (2 x 100pF) on that side, prob. because BSI is LIN master, motor 1 is slave, but seemingly also master to motor 2 on a separate LIN line. Anyway, D1 is just a dual bus line protector and shouldn't be important what's on the other line. \$\endgroup\$
    – ajdalshov
    Dec 10, 2023 at 11:43

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The function of this ferrite bead is EMC protection, i.e. it protects from high frequency interference (HF) in and out of the circuit. Typically, these ferrites are specified with how much resistance they provide at 100 Mhz frequency, like e.g. 1k ohms @100 MHz. The measured DC resistance should be low, so that it does not interfere with other LIN bus requirements. 0.76 ohms should not be a problem in this case.

Overall, it is therefore not possible to tell you the proper (HF) value of this ferrite without knowing about the overall EMC requirements and performance of the total system.

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FB1, C1 and C2 are a third-order low-pass filter with a low Q factor. In EMC, a high Q factor (if you remplace FB1 by a self) can be problematic, as it can lead to oscillations.

You need a simulator to choose FB1.

The cutoff frequency should be large enough not to round off the LIN signal, and small enough to comply with EMC protection requirements.

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For a prototype or one-off design that needs no EMC certification, it's not crazy to simply choose a bead of similar resistance in the same package. If EMC is a problem, you'll have something to test. Fixing EMC problems is generally much more complicated than just choosing the right bead.

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