[SI-LIST] : On-board Switching Power Supply

Ravinder Ajmani (ajmani@us.ibm.com)
Tue, 20 Oct 1998 11:53:19 -0400

Hi Everyone,
My application requires a -4V power supply for an off-board analog circ=
uit.
The off-board circuit is connected to the main PCB through a flexible c=
able. I
can't get a linear regulator which will provide -4V from +5V power supp=
ly. My
two main requirements are: 1) Very low ripple on the -4V supply, and 2)=
keeping
the power supply noise isolated from the main board.
In my initial design, I used top two layers of the six-layer main PCB f=
or the
switching power supply circuitry, with the second from top layer acting=
as a
ground plane for the power supply only. My stackup is S-S-P-G-S-S and =
this
can't be changed due to other reasons. I isolated the power supply cir=
cuit
from the rest of the PCB, using two ferrite beads for +5V input and -4V=
output,
and two 0 ohm resistors for the ground connections.
The power supply has a fair amount of ripple (25 mV) at full load even =
with a
large capacitor (680 uF) placed on the load side of the ferrite bead. =
Some of
my colleagues think that I should not isolate the power supply circuit =
ground
from the main PCB ground, but instead I should use one continuous groun=
d
plane. According to them, this will reduce the ripple because of the r=
educed
bounce on ground. My worry is that this may inject noise into the grou=
nd
plane, which could affect the functioning of nearby mix-signal ASIC, wh=
ole
analog input signals are at 100 mV or less.
Any ideas????

Regards, Ravinder
PCB Development and Design Department
IBM Corporation
Email: ajmani@us.ibm.com
***********************************************************************=
****
Always do right. This will gratify some people and astonish the rest. =
....
Mark Twain
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