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Newbie LM3886 Circuit
Newbie LM3886 Circuit
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Old Today, 01:01 PM   #61
simonra is offline simonra  United Kingdom
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Here's the updated layout; I've added R8, R4(Rf2) & C5(Cf). The way it is at the moment I've got a pretty long loop (SPKR+ -> R4 -> C5 -> Vin-).

If that looks like it'll be a problem I'll have to shuffle a lot of the components about to find room. I'm thinking of moving the speaker out connector to the top of the board and taking HBR with it. Perhaps also moving C6 down to the bottom right so I can snug C7 up against C8 - does that sound like a good plan?
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Old Today, 01:41 PM   #62
simonra is offline simonra  United Kingdom
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And here's version 10 of the layout, does that look better?
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Old Today, 04:54 PM   #63
tomchr is offline tomchr  Canada
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Suggestion on Rev. 10: Instead of using a trace to connect C4 to the input ground plane, why not just extend the plane north in the layout and scoot C4 over? That would give you lower ESR and ESL on C4. As I've said before... You're paying for the copper anyway. You might as well use it.

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Originally Posted by simonra View Post
I plan to solder Cc directly across the input pins on the underside of the pcb - that's why it's disappeared from the layout.
Why? The whole point of getting a PCB made is that you avoid point-to-point and fly-by-wire components. There's plenty of room for Cc. For example, you could rotate C2 and place Cc next to R6.

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Originally Posted by simonra View Post
I have to admit that I'd completely confused myself about Rf2 and Cf and also mis-read the responses in this thread.
If you add Cc, you'll get overshoot in the transient response. Rf2 and Cf tame this overshoot.

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Originally Posted by simonra View Post
When you say to add the resistor to the "outside" of the input cap does that mean between the signal input to the board (the molex connector) and the cap?
"Outside" would be the source side of the input cap. So across the Molex input connector.

Be mindful of the spacing on the thermal reliefs. If you push the limits on the supply voltage, you'll have 84 V between nodes in places. Now, I'm not suggesting you run your circuit at those voltages, but I would at least make sure the board doesn't arc over before the ABS MAX limit on the IC is reached.

For my recommendations on the supply voltage, have a look here: Taming the LM3886 - Output Power.

Tom
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