USB and digital power supply

[Michael]

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.

 

[Ban]

Michael wrote:

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.

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

 

[Michael]

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.



"Ban" <bansuri@web.de> wrote in message
news:6DxPe.104623$fm.6928218@news4.tin.it...

Michael wrote:
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.


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