Author Topic: How on EARTH do you lay out a part like this?  (Read 1235 times)

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Offline alexwhittemore

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How on EARTH do you lay out a part like this?
« on: January 20, 2021, 02:24:57 am »
A new client came to me today with a debugging problem they're frantically trying to solve. Long story short, I'm pretty sure the whole issue is how they've laid out USB 3.0 pairs with big 1" stubs running off to a TVS diode, times six for the connector.

Which leads me to the question, because I've seen this a LOT: The Wurth 82401646 comes in an MSOP-8 package, just WILDLY ill suited to coherently routing differential pairs near/past it so as to keep stub length minimized and avoid vias. Like, I can't imagine there's a way to do it for USB3.0 without stubs/vias and while maintaining any SEMBLANCE of impedance control. And yet, that datasheet specifically calls out "high speed differential signals like USB3.0".

Why do these kinds of parts exist? Who uses them, besides people who quickly learn not to? Why does this part tell me it's for USB3.0 when it's virtually guaranteed to tank a USB3 design?

Like, what am I missing here?
 

Online fourfathom

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Re: How on EARTH do you lay out a part like this?
« Reply #1 on: January 20, 2021, 03:22:38 am »
Like this?  There's some coupling between 1/2 and 3/4, but perhaps not too bad.  Or don't I get it?  (it happens)
« Last Edit: January 20, 2021, 03:35:12 am by fourfathom »
 
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Offline alexwhittemore

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Re: How on EARTH do you lay out a part like this?
« Reply #2 on: January 20, 2021, 03:29:00 am »
No - I think you're right. That's the solution a friend just came up with as well. Two of those pairs DO get perilously close. And it's a tight squeeze unless you're working with roughly 3mil prepreg by my estimate (I assumed you wouldn't be able to fit 4 traces underneath at controlled impedance sizing, but maybe). But that might actually be a practical way to do it.

That said, I think I'll probably design in purpose-built fly-by parts still :)
 

Offline Doctorandus_P

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Re: How on EARTH do you lay out a part like this?
« Reply #3 on: May 02, 2021, 06:56:17 pm »
I would also use two via's for GND and Vcc each. One via north, and the other south of pads 7 & 8, and I would add a decoupling capacitor. This capacitor will (partly) short circuit transients, instead of shunting them into your power planes.
 

Offline T3sl4co1l

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Re: How on EARTH do you lay out a part like this?
« Reply #4 on: May 02, 2021, 07:15:28 pm »
Weird, they show a diode from VCC into the TVS.  Well, if that's the case, it's just for bias, and clamping itself, not for bypass.  So, uh, that saves vias, then, huh?...

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Online nctnico

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Re: How on EARTH do you lay out a part like this?
« Reply #5 on: May 02, 2021, 08:16:07 pm »
I would also use two via's for GND and Vcc each. One via north, and the other south of pads 7 & 8, and I would add a decoupling capacitor. This capacitor will (partly) short circuit transients, instead of shunting them into your power planes.
A better way is to put a series resistor between power and VDD. Don't feed transients into your power rail!
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline free_electron

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Re: How on EARTH do you lay out a part like this?
« Reply #6 on: May 03, 2021, 06:49:37 pm »
I would also use two via's for GND and Vcc each. One via north, and the other south of pads 7 & 8, and I would add a decoupling capacitor. This capacitor will (partly) short circuit transients, instead of shunting them into your power planes.
A better way is to put a series resistor between power and VDD. Don't feed transients into your power rail!
correct, and do not forget to put a small capacitor across the tvs diode. That way the transient get shorted by the capacitor as well.
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Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 


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