1. Home
  2. Hobby Electronics Basics
  3. USB and digital power supply

USB and digital power supply

I'm working on a project using a PCM2902 (TI) DAC chip.

In the data sheet, on page 27 (typical circuit connection), it shows pin 27 as Vddi which connects to the USB D+ line via a 1.5K resistor.

I read where USB data is derived from a -200mv and a +200mv above a certain reference on the data lines, so I guess the voltage at Vddi is always there but varies with this (reference + 200mv) fluctuation?

But isn't that a (very?) crude method of powering the digital part of the chip?

Wouldn't it be preferable to have a seperate supply, even a seperate regulator, always on....on the Vddi pin?

It may be a stupid question....but just trying to learn.

Thanks.

Reply to
Michael
Loading thread data ...

Yes Mike, you got it the other way around. VDDi is an output of a voltage regulator. Look at page 10, the supply is via the VBUS terminal. The resistor on the data line pulls up the level to signal the controller, that a device has been connected.

--
ciao Ban
Bordighera, Italy
Reply to
Ban

Ok....I see, thanks.

All this talk of seperate analog and digital supplies and yet internally the chip is fed from the same bus.

?

I had started to read where seperate ground runs aren't nearly as important as seperate power planes.

I guess you don't have that option with this chip.

Reply to
Michael

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.