Atmega32u4(Pro Micro) Why is it OK to power with 5v usb, but it needs 7v at Vin?

Other Hardware 3rd Party Boards
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Hi, I'm working on a project where I'm using an LM317 set to 7v to power the Pro Micro. It's working on the breadboard just fine, but I also need a 5v rail for my other devices. They are all low power devices, so I could use one or more digital pins, but it just doesn't seem efficient. The Atmega32u4 is going to end up on a custom PCB so I'm thinking ahead.

Why would it be stable and run just fine with 5v(if that)from USB, but need 7v at Vin? I understand that the onboard regulator needs a higher voltage input to output 5v. Using USB it doesn't, but it seems stable otherwise.

I'm using an external voltage reference, so I'm good there. What would you do in my situation? Thanks in advance! I'm googling different ideas, but not coming up with much.

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You don't need to power it from Vin. You can apply regulated 5 volts to the 5V pin instead.

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Vin is an unregulated input. If you have (or can easily arrange) 5V regulated, you might as well feed it into your arduino via the usb or +5V connections.

The only reason to provide ~7V regulation outside the arduino is if your "native" supply is much greater than 7V, in which case you'd be splitting power dissipation across more area...

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noobdude:
Hi, I'm working on a project where I'm using an LM317 set to 7v to power the Pro Micro. It's working on the breadboard just fine, but I also need a 5v rail for my other devices. They are all low power devices, so I could use one or more digital pins, but it just doesn't seem efficient. The Atmega32u4 is going to end up on a custom PCB so I'm thinking ahead.

Why would it be stable and run just fine with 5v(if that)from USB, but need 7v at Vin? I understand that the onboard regulator needs a higher voltage input to output 5v. Using USB it doesn't, but it seems stable otherwise.

I'm using an external voltage reference, so I'm good there. What would you do in my situation? Thanks in advance! I'm googling different ideas, but not coming up with much.

Think of it this way: You have a bucket that you want to skim 5.0 gallons from. Some will evaporate, some will spill, etc... so the bucket needs to start out filled with MORE than 5.0 gallons in order for you to be able to scoop out a clean, uninterrupted 5.0 gallons.

Now, thinking about the voltage regulator chip on the Arduino board, it has to "skim off" 5.0 volts to power your board. But, there are small losses here and there (voltage drops across the output pass transistor, diode junction drops across various op-amp and voltage reference transistors, etc...) so in order to get 5 volts out, you need to put IN 5 volts plus all that gets lost, plus maybe 1 more volt "for good luck".

There are LDO (low drop out) regulators that are carefully designed to minimize these losses, but still even the best one has some losses and require 1/2 to 1 volt or more "extra" input above what they output.

In reality, your board probably only needs 6 volts input, but they specify 7 volts just to account for manufacturing tolerances and to keep users from complaining "I put in 6 volts like you said and it doesn't work WAH WAH WAH!!!".

When you run the board from the USB port, a switch (a FET) connects the USB 5 volt line directly to the Arduino power input. But when you use external power, it has to go through the regulator and therefore needs to be 5 volts "plus" a bit more. The power from the USB port DOES NOT go through the regulator. That's why it's OK as-is.

Make sense?

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As I recall from the schematic they use a voltage divider on Vin, to halve it, and then compare it to the 3.3V output of the 3.3V regulator. If it is less than 3.3V (ie. if Vin is less than 6.6V) then it switches to the USB input and ignores Vin.

Thus you need 6.6V or more, which is why they recommend 7V.

But as the others said, just plug 5V into the 5V pin on the board. I do that all the time (eg. connecting two Arduinos together, with only one of them powered).

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I wired it to VCC and it totally works. :slight_smile: @Krupski, that makes perfect sense. I'll check out the schematic and see how they wire it and read the datasheet as well as more on the web. Have to figure out a way to do this on a custom PCB, but that's down the road.

Thanks everyone else for the replies as well. I read over and over that you shouldn't wire 5v to the 5v(vcc in my case)pin, but it's working great. Thanks for the confirmation!