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Question on buck converter layout

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thinkfat:

--- Quote from: Sparky on January 07, 2020, 10:16:55 pm ---
--- Quote from: thinkfat on January 07, 2020, 10:22:50 am ---Let me check, I have a drawing where I measured out the positions of the connector and the holes. I've attached a DXF, you should be able to import it into Kicad and use it as a template for the hole positions and the FPC. I've marked the position of pin 1 of the FPC connector with a small line. The dimensions didn't plot for whatever reason, but if you overlay this graphic onto your PCB you should be OK.

PS: I would not use a linear regulator. The module draws a lot of power while heating the OCXO. You're limiting yourself to a pretty small input voltage range, maybe 6 to 7 volts. Decent cooling required.
PPS: MPN for the FPC connector: Molex 54132-5033. You can order it from Mouser.

--- End quote ---

Super helpful!  :)   Thank you so much!  Yes, I'm a little worried about the high current and heat dissipation.  The LDO I selected was LMS1587 and the input will be 9V 2A supply (hard to find ~7-8V adapter).  My simple test board is to evaluate the LDO.  If it doesn't work I will change to something else.

--- End quote ---

Oi, that's going to burn. For one, 2A is not enough, you'll need in the area of 2.2 to 2.5A during warm up. Second, you're looking at around 8W to 10W power dissipation, i.e. waste heat. That's a lot. I'd make up my mind about the thermal design. You'll need a good sized heatsink, rated at least 4°C/W or better to keep it below 60°C (assuming 20°C ambient temperature). Those do exist for TO-220 packages so you can definitely pull that off, but they're, like, really chunky! You can of course run the thing hotter, just keep in mind that the junction-to-case thermal resistance is in the area of 2.5°C/W, so the die will be 25°C hotter than the heat sink for sure.

Sparky:

--- Quote from: thinkfat on January 07, 2020, 11:19:46 pm ---Oi, that's going to burn. For one, 2A is not enough, you'll need in the area of 2.2 to 2.5A during warm up. Second, you're looking at around 8W to 10W power dissipation, i.e. waste heat. That's a lot. I'd make up my mind about the thermal design. You'll need a good sized heatsink, rated at least 4°C/W or better to keep it below 60°C (assuming 20°C ambient temperature). Those do exist for TO-220 packages so you can definitely pull that off, but they're, like, really chunky! You can of course run the thing hotter, just keep in mind that the junction-to-case thermal resistance is in the area of 2.5°C/W, so the die will be 25°C hotter than the heat sink for sure.

--- End quote ---

Oh, 2.5A... that's a bit higher than I was thinking.  I thought it was closer to 1.5A during heating and dropping to just below 1A in normal operation.  It does sound like I need to revise the design.

May I ask which switcher you used so I can review its spec's; that would be a help to know what has performed well.  Thanks! :)

thinkfat:
The info is in this thread, you could use a LM2596 or LM2676 instead as well.

thinkfat:
You could also take into account that the gpsdo will only require maximum power for one or two minutes and choose a heatsink with just enough thermal capacity to keep the temperature low enough to survive that. Might involve solving a differential equation, though.

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