Hi Everyone,
I started a thread earlier about USB bus powering an instrument that needs to draw more than 100mA and less than 500mA. The USB controller is part of a processor and not really negotiable. It is a high speed device.
I think this problem is quite common but I have not found a great solution. So I am putting it out here for ideas.
Here are the constraints:
- The device will draw more than 100mA before it can request 500mA.
- Even if power could be constrained to 100mA, there is no direct way to support suspend current.
I have also looked at a variety of USB Power Managers but I don't see how these get around the problem.
Several ideas have been considered:
Idea 1:
Use a two port bus powered hub. Connect a low power USB device like a PIC or FTDI, etc that requests 500mA. This device just controls a switch that supplies almost 500mA to the second device. The second device looks to the USB bus like a self powered device.
This takes a 2 port bus powered high speed hub and another microcontroller or USB peripheral. Lots of parts to solve a basic problem.
Idea 2:
Just use a bus powered 2 port hub. Since it is a bus powered hub, it presumably already has permission to draw 500mA. Connect the target device to this internal hub. It gets 500mA less the hub current.
I assume that this probably breaks some USB rules, but at least eliminates the extra USB device from Idea 1.
Idea 3:
This is where you tell the rest of us, the clever way to solve this problem......
If either Idea 1 or Idea 2 are practical, what parts make the most sense to implement the circuits. The idea is to keep the wasted power to a minumum and the size and cost down.
Thanks
Al Clark Danville Signal