Cerb40 Power Requirements

Hi, all. I’m new to GHI and want to use the Cerb40 in a project. It seems like an interesting low cost controller, but I’m having trouble identifying the power requirements.

The schematic shows a LM1117 voltage regulator supplying 3.3 V, but it is attached directly to the USB port as well as a pin marked VBUS on the schematic, and USB on the silkscreen. There is a VCC pin, but it seems to be VCC in relation to the micro, not the system. Then there is a pin marked VBAT directly attached to the micro. This is a bit confusing when there seems to be no datasheet available for this system.

When not powered by USB, can I assume I use the VBUS pin for power? Does it support the entire 6 to 12V range of the LM1117 regulator? What happens when I plug a USB cable in and VBUS is at 12V?

Thanks in advance.

@ bigtwisty - Welcome to the forum.

VBAT is for a battery for RTC clock.
When not plugged in to USB I would supply 3.3V via 3V3 Pin

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Thank you for the quick reply!

This design already requires an 8V regulator. I can add another 3V3 regulator if I have to, but I’d like to use the one on the board if I can. Is this not recommended for the Cerbo40 II?

I think it is ok to power via VBUS as you have mentioned.
But when connected to USB I would turn of that power source. Or cut off VBUS line from usb connector if you want to always power from your source.

Hey there Bigtwisty, welcome to the forum!

VBUS is the 5v line from the USB. That is the input to the LM1117. VCC is the 3v3 output of the LM1117 and does power the processor.

As far as whether you can support 12v, I would say your bigger issue is heat dissipation. The VREG will generate heat the further it has to lower the voltage so the further it is away from 3v3, and the more current is drawn. Not sure what the current draw is on the CERB but I would try not to use 12v but 9v might be more reasonable max.

As for supplying power on both - don’t. There’s no protection.

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Thanks, Brett. That’s kind of what I figured, but wanted some confirmation before I built one.

I’m assuming the micro probably draws 100mA or less off the 3V3. With a 12V supply, that would be about 870 mW nominal. With no heat sink, that would raise this chip about 118 degrees C. Yowsah! Yeah, I’d say 12V is out…

An 8V source could still raise the chip about 64 degrees, but that’s still within the rated junction temp.


@ bigtwisty - +1 for Good math

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