Jamie Morken wrote:
Joerg wrote:
[email protected] wrote:
On Feb 10, 7:43 pm, Joerg <
[email protected]>
wrote:
[email protected] wrote:
All,
I am designing amulti-channelboard and I have individual linear
regulators for each channel running off a common switcher.
Now, my concern is that due to the split power planes, which are like
thin fingers running through the board, I will cause potential
differences based on the current in different channels, especially,
because I have a common ground plane. Now, I was thinking my choices
are:
1) Split the ground plane as well, so I would have thin finger like
power and ground plane separated by narrow notch but connected at the
linear regulator end and at the input end. (All the channels are
w.r.t
common ground plane.)
Splitting a ground plane is usually a recipe for disaster. I have
yet to
see a case where that really worked and it's been decades now. In
audio
designs it can work but only until a strong RF field shows up, upon
which all hell breaks loose.
OTOH if nobody split planes anymore I'd have less work so it does have
some upsides ;-)
The trouble is, that I have common (to all channels) control lines
runnning across my planes (though separated by 2 layers) and even
if I
isolate the grounds, some noise is bound to take the signal lines.
That's one of the reasons why it's a recipe for disaster ;-)
2) Keep split power planes, but add a narrow slice ground plane
between the power plane 'fingers' in between them on the same
layer as
the power planes.
Copper pour on the same plane as power? You can do that but via it
through here and there. And no thermal reliefs for those vias unless
something needs to be soldered in there.
3) Keep 1, but put connections between ground planes at regular
intervals.
That's like saying "Let's split it, oh, wait, maybe split them only a
little". I would not split the ground at all.
Anyone have thoughts on this. The voltages are +5 or less and it is
all analog..there are digital lines and a digital section, but they
come into the analog area through an isolator.
Make sure your power is clean and the switcher doesn't chatter
through.
Use shielded versions for the magnetics if it's close by. Design your
stuff with reasonable PSRR or bypass extra good where you can't, like
for transistor stages.
--
Regards, Joerg
http://www.analogconsultants.com/- Hide quoted text -
- Show quoted text -
Well, actually, I am not splitting the ground planes, in the sense
that they are connected at the same potential at all the linear
regulators and at the input. But with my control lines, crossing
across my parts, my split is not going to have much effect. I could
put de-coupling caps across the notch to allow high frequencies to
pass, but again they might go through my control lines and not bother
with the caps...basically it is big ?..so yeah, I think I am not going
to split the ground...best to do it when I have free time as a test
project.
For my thin ground slice in between the power planes (on the same
layer0, I put vias every 50 mils or so and no themal reliefs and it is
a relatively unpopulated location, so that will help. My switcher and
linear regulator are LT parts 3430/1763, but I have been told by just
about every amplifier part vendor (and LT themselves!) that linear
regulators don't attenuate frequencies above 100 kHz very well, so I
might keep a 100 kHz switching frequency and maybe put ferrites at the
linear regulator input.
Easy fix if you need good rejection: Use a npn-Darlington with enough
heat dissipation capability to drop around 1.5V at your maximum
current. Collector to switcher output, emitter to linear regulator
input, 1K resistor from base to collector, 47uF cap from base to
ground. Plus a couple discharge diodes. That'll behave like having a
shoe size 15 electrolytic in there.
Hi,
Is this npn-darlington solution good at rejecting HF better than a
linear regulator like the LM317? Also where do the discharge diodes go
in this circuit?
Yes, for smaller transistors and small currents. Mostly I don't even use
Darlington, just a transistor with a good guaranteed minimum beta. One
diode goes from emitter to collector (cathode to collector which would
be at the input rail), the other from base to collector (also cathode to
collector). The diodes make sure the capacitors don't zap the
transistor(s) in case someone drops the pliers and abruptly shorts out
the input rail.
Remember that this drops 700-800mV or so, or twice that in case of a
Darlington. And it is not too useful if the output of this stage must be
very stable. Of course, then you could run a secondary loop back to the
switcher but that becomes esoteric and is not for the faint of heart.
