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The spec for the HCPL3700M (AC voltage detection IC) lists an "External Clearance" of ≥7.4mm.

Is this saying that no component or exposed trace should be within 7.4mm on any side of the chip? Or is it just stating the clearance spec that its own footprint is meeting? (I note that the AC pins are 7.62mm from each other and from the low voltage pins on the other side of the IC, which seems suspiciously close to 7.4mm.)

If it's specifying clearance to other components, then as far as I understand this would contradict most recommendations for clearances of high voltage conductors on circuit boards. E.g. 240VAC comes to ~2.5mm, depending on the standard. Creepage specs are naturally a bit higher, but still well under 7.4mm. And after all, the HCPL3700M's own AC input pins have slightly less than 2.54mm clearance from their neighboring (post-internal-rectifier) pins.

To get the 2.5mm figure I used the Saturn calculator for an external, uncoated trace, 340V peak, under 3050m, IPC-2221B. In my current board layout, the nearest metal is ~3mm. Would this be safe?

Relevant section from the PDF:

clearance specification from spec sheet

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It's only a description of the part, not a prescription of how to use it.

When selecting parts for isolation, one needs to know the separation distance across the isolation barrier, in order to meet isolation requirements, including creepage and clearance (as defined in standards such as IEC 60950-1, 62368-1, etc.). So, they provide this description to allow for easy integration.

You don't have to have other parts any particular distance away from it, so long as creepage and clearance are honored.

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It depends on what the max voltage and the IEC\IPC specs the design needs to adhear to. The IPC-2221B is a good starting point. One problem is since this design is connected directly to AC mains it might be subject to arcing requirements above 340V. Since I have not tested a product to such standards I can't comment on the standard, but I do know that regular power supplies have to be able to handle more than that. It also matters if this is an industrial or residential product as there are different standards for each. If you are getting this tested at a lab, get the requirements from them.

The IEC 61000-4-5 standard defines a standardized test method and different levels of protection based on installation class and coupling methods. Dc power supplies are typically concerned with installation classes 3-5, which have test requirements from 1kV to 4kV (Table 1).

enter image description here Source: https://www.cui.com/blog/the-basics-of-power-supply-surge-protection

To get the 2.5mm figure I used the Saturn calculator for an external, uncoated trace, 340V peak, under 3050m, IPC-2221B. In my current board layout, the nearest metal is ~3mm. Would this be safe?

Yes, if that is the max voltage then yes. The HCPL3700M could handle up to 6000V so it needs a very large creepage distance. Also remember the difference, if you can't get the clearance distance you can increase it with barriers or gaps.

This image says it best for creepage vs clearance.

enter image description here Source: https://www.protoexpress.com/blog/importance-pcb-line-spacing-creepage-clearance/

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It refers to the clearance between the 2 sides, the high voltage measuring one and the isolated one, meaning the clearance between pins 1-4 and 5-8, there should be at least 7.4mm non copper space between the traces of each side to have proper insulation at peak voltage

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