Re: 3.3V on a 5V bus

[Andy Ingraham <ingraham@wrksys.ENET.dec.com>]

Jens-Peter K. Jensen asks:

> My question is that for a 5.5V bus with 4 PCI slots, what maximum current
> should I expect to be drawn from the 3.3V, i.e. what type of regulator
> should I use.

This is a sticky question.

The 3.3V supply is optional in the 5V version of the PCI bus, yet
with no loss of functionality.

Furthermore, section 4.3.4.1 in the PCI spec says that PCI expansion
cards for use in the 5V bus, must generate their own 3.3V from one of
the other supplies, due to the fact that the 3.3V supply might not be
active.

Therefore, in theory at least, the maximum 3.3V current you can expect
to see, is zero.  Any card for a 5V bus that draws current from the
3.3V supply, might be said to be technically non-compliant.

The absolute maximum current would have been 7.6A per card, for cards
at the 25W limit that drew all of it from the 3.3V supply.

Regardless, you need to bypass the 3.3V pins near the connectors.

Regards,
Andy Ingraham
\2I2