Notches in ground planes for multi-power multi-channel board

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 ;-)

That's one of the reasons why it's a recipe for disaster ;-)

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.

That's like saying "Let's split it, oh, wait, maybe split them only a little". I would not split the ground at all.

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.

Best just to reference and decouple the output regulators to the load at the ground plane of the load. input decoupling will only work with an intervening impedance at the unregulated input.

If the switcher outputs had individual returns (ie were isolated) you might do some funny stuff with the grounds before the regulators, but not with a common return. You might consider runnung the switcher return to an intelligent board location, before it hits the ground plane.

RL

Keep a single, solid ground plane. Break the power plane up into individual pours, each fed from its own regulator.

That's the best general advice I can give without getting involved in the detailed design of your product (and you probably can't afford me.)

John

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.

I am also considering using ceramic 10 uF caps instead of the tantalum datasheets reference. Also some 0508/0402 caps.

Unfortunately, my switcher is teh standard run of the mill, I could use a DC-DC converter with isolation probably. My switcher return is almost immediately goes to the ground plane as per the manufacturer (LT) recommendation, however I can connect the grounds of the linear regulator and the switcher to a common location and then send it to the ground plane.

Well

Thanks, that is very helpful, I will stick with one plane and hopefully the ground 'slice' between the power planes will help as well. Also, I am wondering if putting decoupling caps between two power plane pours will help further

Do NOT slice the ground plane. Bypass each power pour to ground.

Over and out.

John

Only one of the following can be true:

Some splits are worse than others, but case [A] is almost always the best choice unless you are doing something really specialized, know what you are doing, and are willing to build and test the proposed topology to prove that case [A] doesn't beat it. And even then case [A] tends to be better when conditions you didn't envision arise.

Replace the caps with copper (no notches). Use those caps to bypass high frequencies your power plane(s) to your ground plane.

Unlike splitting the ground plane, having a split power plane with separate onboard regulators for different sections is usually a good idea, depending on your cost constraints.

It is always better to kill as much switching noise as possible before feeding it into an analog regulator.

That's a trade-off too complex to discuss here. Higher switcher frequencies are better at blowing past linear regulators, jumping through the air, etc. but are easier to kill at susceptible IC or transistor. Lower frequencies are just the opposite.

If you are building a switcher on the board, use the layout from the app notes and data sheets for your switcher. If it is the usual separate box, putting ferrites on the wires is a cheap way of stopping some of the noise. Please note that choosing ferrites and deciding where to put them is an engineering task in itself. How big? What frequency? Differential mode (bead per wire) or common mode (power and return through one bigger bead)? Saturation problems? Look for app notes that will help with those decisions.

Why? Do you really think that you know more than the engineer who wrote the datasheet?

It is almost always a good idea to put a 0.1 ceramic bypass cap at every power pin of every IC, depending on your cost constraints.

In general, that space is better used to make the power planes bigger.

Not a good plan. Put those decoupling caps between each power plane and the ground plane instead.

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.

Also, consider using the LM317. With a cap at the adjust pin it'll maintain a low output impedance all the way up to a MHz:

Personally I'd resist the temptation to use an LDO. I've seen them bring a lot of grief.

Tantalums in bypass applications are quite evil IMHO. Seen too many orange-greenish clouds wafting up after the explosion and so on.

Careful, that can develop it's own nasty form of life. Just like a subway station in a seedy part of town.

Many linear regs are unstable with ceramics on the output. The suggestion to use tantalums is their Zen way of saying so without saying so.

John

Keep one solid ground plane.

Using a linear regulator on each channel is a good way to isolate power supply cross talk, so, keep that in.

You will have more problems with that switcher getting into things. The switcher needs to be properly laid out on the pcb or you will get switching noise into places you don't want it. Linear Tech. has the layout of their reference designs for most or all of their switchers. If you can't find it on LT's web site, call your LT rep. LT has excellent customer support for the small guy. It is a good idea to use LT's layout as a reference. If you can, use one of the MHz plus switchers, then, apply ferrite beads to input and output of the switcher.

It'd be handy if someone started collecting a "decoder ring" between what data sheets say and what they really mean.

"It is suggested to use tantalum caps on the output of this regulator" --> "This regulator is unstable with too low of ESR output caps."

Ever use chips by FTDI? They take the approach that if you want to do something "easy" based on the register descriptions... but due to chip bugs it doesn't actually WORK that way... they never mention there IS a bug, but instead provide circuitous example code that works around it!

Thou shalt not use LDOs ;-)

Every time someone says they don't believe in LDOs a designer at Analog Devices, Linear, or Maxim dies. :-)

I suspect you've rolled a few discrete transistor-based LDO designs in your time, Joerg!

Eventually one becomes sensitized to this sort of new-speak. Even more fun is when you have to research the part numbers that they call out on the eval board, because *their* esr's are the only hint about what might work. Micrel is especially good at this trick.

This sort of obscurity means that many, many users spend many, many man-weeks and board spins discovering these facts over and over.

John

And if I'd sing my song about Maxim their CEO might clutch his chest ;-)

Yes, I did. But then you know how it'll behave. Problem with modern chips is that they don't give out much information. For example, TI refused to release the SPICE model for one of their TPS chips after some went unexplicably "exotherm" on our boards. They also refused to run our simple circuitry around it. So I gave it the ax, for good. I sure won't use one anytime soon.

data

Even if they give you an ESR range it'll be in the figure section and thus typical, not guaranteed. Personally I do not use LDOs with non-guaranteed ESR ranges. Which pretty much excludes most of them. The ones that aren't excluded (stable down to zero and stuff) are often out of budget range.

it

If there is a better device from the competition just move on.