From: Scott McMorrow (firstname.lastname@example.org)
Date: Thu Jan 27 2000 - 12:03:47 PST
It depends ....
On the dI/dT of the 1.8 V core when the device is switching and the
noise tolerance required by the device.
Usually there are no return path issues involved with core voltages.
(Other than the normal currents returning on the ground plane.)
But there are the usual noise and decoupling issues. If you can design
a power distribution circuit for the core that has a wide enough bandwidth
to provide for the power needs of the device, then it doesn't matter whether
it is a plane or a power strip. However, planes have much lower inductance
and much higher capacitance and so extend the bandwidth of the
power supply circuit.
Personally, I like to use a large plane fill area on a layer that is normally
a signal layer for these types of small power distribution islands. This keeps
the inductance low and maximizes the interplane capacitance. Of course, the
fill area should be adjacent to a ground plane to achieve the desired effect,
and no other signal layers should be adjacent, or at least no traces should
cross the fill area.
-- Scott McMorrow Principal Engineer SiQual, Signal Quality Engineering 18735 SW Boones Ferry Road Tualatin, OR 97062-3090 (503) 885-1231 http://www.siqual.com
"phelan, tony" wrote:
> If I have a device whose I/O are 3.3V and internal logic is powered from > 1.8V do I need a power plane to supply that 1.8V or will a power strip to > that device suffice? Another way of asking this I guess would be: Are all > return path currents for the internal 1.8V signals handled internally to the > device. If so then there would be no advantage to making the 1.8V supply a > plane? > Any comments on this would be appreciated. > > Tony Phelan > EMC Corporation > > **** To unsubscribe from si-list: send e-mail to email@example.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. > si-list archives are accessible at http://www.qsl.net/wb6tpu > ****
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