Dear meBG,
meBigGuy said:
One problem you have to face is that in order to draw power from a real USB2 port you need to have a USB data interface that will enumerate the device. Otherwise the port will shut off its power. Once you enumerate you can draw 100ma, and must request permission to draw 500ma. Some hubs may supply power anyway, but I don't have any practical suggestions as to which ones.
thank you for your reply.
The idea to take voltage from a USB port came to me because in the vicinity of the devices in / out there is a PC and thought it was easy to pilot a power saving circuit via the USB port without having to use discrete power device.
Assuming you solve the USB part, you have 5V@500ma to work with. The MAX4910 is a single supply part and it handles audio in the VCC to VCC-5.5 volt range. So, if you power it with 3V it can switch +-2.5 easily.
That's right, my idea was this.
I don't know what you are switching, and how much power you want to draw. Maybe you are planning to have a USB interface anyway to control the switching. If so, then you have 100ma or 500ma at 5V to work with.
The problem is simple: in a room for audio conferencing there is a sound system with powered mixer, a PC with Skype and, for the moment, a mechanical switch (orribile!) that allows the user to interact with people connected on Skype and ask them to intervene as needed.
AS you can see from the figure below, the switch has the only function to connect the PC line IN (Micro PC) to :
- the Mixer AUX output of the mixer, standard config (so that anyone who is connected with Skype will follow the lecture in the classroom) - or
- the Micro Op. of the operator, special config (to give instructions to those who connected, or ask to speak)
The Head PC - Head Op. line is to allow the operator to follow the classroom lecture.
If you really want bipolar power then you can build a switching inverter to create -5V or -3.3V. Those can be simple capacitive switchers or inductor based switchers. Depends on your requirements.
The charge pump (capacitor) based switcher is simpler if you can live with 100ma. For example:
http://www.linear.com/product/LTC1983
Chances are you want to regulate the +-5V down to +-3.3V or whatever to eliminate noise from the USB supply and from the switcher.
I would simply replace the mechanical switch with an electronic one, and I thought that the IC MAX series 49xx are appropriate when supplied with dual voltage in order to avoid clipping of audio signals.
Any idea at this point is the wellcome!
Max
