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VIN Track Current Rating + USB Current Request


#1

Hi guys,

Could you let me know what current the VIN track is rated for? Or at least, please let me know what copper thickness the board is made on so that I can calculate it myself.

Also, on a sidenote, an unrelated question: How much current is the default USB connection set up to request?

This is for the Panda and the Panda II.


#2

I’m not sure you will get a “real” answer here.

USB can supply a MAXIMUM of 500ma therefore you should not draw more than that at any time. You need ~2/5 of that for the Panda itself.

But where you will get into trouble is that most USB ports on computers will drop their voltage if they have to supply that much, and that can cause you issues like brown-outs or unexplainable behaviour.

Simple answer - if you have anything that needs more than 300ma, then you had better figure out a better power supply, including regulators that can handle the current you need (the onboard ones are not very capable especially at higher voltage drop and higher current)


#3

Dear Brett,

I am aware of the physical limitations of the USB port, and of non-perfect voltage sources in general, and am very happy with that side of things.

My issue is that, as far as I understand this - and this is not my specialty - When enumerating USB ports, the slave device requests a certain amount of current, and then should it exceed that current the master will cut the power supply and reset the device.

I wish to know how much does a Panda request, and if it is below 500mA, is there a simple way to increase that. I know for a fact if a device does not specify, it will have a default cut off current of 100mA.

Thanks for your time though.


#4

Given there’s no control over any of this (you can check the GHI SDK; there’s no ability to influence that), then the only place you can find this kind of information will be in the chip datasheet. But given you know that the Fez draws around 200ma, you know that either the host is not cutting off the current or it’s requesting more - and it wouldn’t make any sense for a device like this to request the 500ma maxiumum - GHI can’t know what peripherals you’re going to want to try to attach.

So let me ask this, why is this important? What is critical in your application that you would do something different if this was restrictive?


#5

If I were able to connect to the USB pins on the controller, I would attach a battery on my project, and have a FET switching it on when the USB is plugged in. However, since this is not an option, I’m looking at whether it is possible to power the device through USB. Now from an electronic point of view, if the current is not software capped, my project would run just fine from most USB ports - Hence I’m asking.

My other question is for a different part of the project, where a big juicy battery has to be connected to the board and some of its peripherals. Now I’m trying to work out whether I can rout the charge through the Panda board and the existing connector, or have a separate connector routing the power to the Panda Board.