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

[Joerg]

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.

And by golly, use shielded inductors on those. Else you might sing the
blues at the EMC lab.

 

[John Larkin]

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.



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

I recently did a 3.3 to 2.5 volt "regulator" to power VccAux of a
Xilinx FPGA. I used a zener diode. In series. Backwards.

John

 

[Joel Koltner]

I recently did a 3.3 to 2.5 volt "regulator" to power VccAux of a
Xilinx FPGA. I used a zener diode. In series. Backwards.

Nice... but why not just a regular silicon diode? Better regulation or
something?

I do like the trick of using LEDs as a combination means of setting up biases
as well as indcating power/activity/whatever.

 

[Joerg]

Nice... but why not just a regular silicon diode? Better regulation or
something?

Or two Schottkys. But I guess the zener costs less.

I do like the trick of using LEDs as a combination means of setting up biases
as well as indcating power/activity/whatever.

Be careful. Suddenly someone releases another kind of red (Oh, it looks
much nicer ...) and your biases are all off the rocker.

 

[John Larkin]

Nice... but why not just a regular silicon diode? Better regulation or
something?

I do like the trick of using LEDs as a combination means of setting up biases
as well as indcating power/activity/whatever.

The voltage drop came out right, 0.8 exactly. Plus, the melf package
we have in stock exactly bridged the pads of the LDO that didn't work.

Yeah, an led can make a useful bias voltage, and free light.

John

 

[Joerg]

The voltage drop came out right, 0.8 exactly. Plus, the melf package
we have in stock exactly bridged the pads of the LDO that didn't work.

What did the LDO do? The usual, such as oscillating?

 

[John Larkin]

What did the LDO do? The usual, such as oscillating?

It dropped out! Actually, I used an LM1117, which has a dropout of
around 1 volt. My bad.

The diode seems remarkably stable over a wide range of cap esr's.

John

 

[Fred Bartoli]

John Larkin a écrit :

It dropped out! Actually, I used an LM1117, which has a dropout of
around 1 volt. My bad.

The diode seems remarkably stable over a wide range of cap esr's.

Then try a TD or a GUNN

 

[John Larkin]

John Larkin a écrit :

Then try a TD or a GUNN

I used to love TD's. Too bad nobody makes them any more; GPD used to,
but I don't think they still do. The fabrication process was insane,
but it's something that an amateur could potentially do.

John

 

[Joerg]

I used to love TD's. Too bad nobody makes them any more; GPD used to,
but I don't think they still do. The fabrication process was insane,
but it's something that an amateur could potentially do.

Happened to all the boutique diodes. AFAIR you had found a source for
lower cost SRDs a while ago. But the thing is, if Digikey doesn't carry
something chances are it'll go lalaland soon.

 

[Jim Thompson]

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.

There's been a trend in I/C design going on, actually, for quite some
time... driven by you parts consumers...

You want to buy a circuit function that requires NO ADDITIONAL PARTS
or PINS.

Therefore your LDO can't have the proper feed-forward compensation to
adequately retain stability with capacitive loads... the manufacturer
fudges by specifying ESR.

So it's your own fault ;-)

...Jim Thompson

 

[Phil Hobbs]

The voltage drop came out right, 0.8 exactly. Plus, the melf package
we have in stock exactly bridged the pads of the LDO that didn't work.

Yeah, an led can make a useful bias voltage, and free light.

John

Nice and quiet, too, if you can spend some current on it, especially if
you use a series string to make higher voltages instead of multiplying
it up.


Cheers,

Phil Hobbs

 

[Phil Hobbs]

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.

Capacitance multipliers work dramatically better than VRs for cleaning
up SMPS crap.

Cheers,

Phil Hobbs

 

[JosephKK]

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

Hey gotta keep the economy going.

 

[JosephKK]

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

You simply must give us the libretto now.

 

[Joerg]

There's been a trend in I/C design going on, actually, for quite some
time... driven by you parts consumers...

You want to buy a circuit function that requires NO ADDITIONAL PARTS
or PINS.

Yeah but you's guys, the chip designers, typically don't ask us guys
before releasing a chip to the public. I would not mind at all if an RC
or whatever has to be connected if that makes an LDO unconditionally
stable. After all we are all used to that from PWM chips for switchers.

Before doing the architecture have your people call my people ;-)

Therefore your LDO can't have the proper feed-forward compensation to
adequately retain stability with capacitive loads... the manufacturer
fudges by specifying ESR.

So it's your own fault ;-)

<harumph ... grumble>

 

[Joerg]

You simply must give us the libretto now.

In the army our drill sergeant told two guys to stop singing. One of
them is yours truly.

 

[Jim Thompson]

Yeah but you's guys, the chip designers, typically don't ask us guys
before releasing a chip to the public. I would not mind at all if an RC
or whatever has to be connected if that makes an LDO unconditionally
stable. After all we are all used to that from PWM chips for switchers.

Takes an extra pin.

Before doing the architecture have your people call my people ;-)

My people don't decide the architecture. You should know that well.
MARKETING decides ;-)

<harumph ... grumble>

<sno-o-o-o-ort>

...Jim Thompson

 

[JosephKK]

In the army our drill sergeant told two guys to stop singing. One of
them is yours truly.

So when do we get the libretto?

 

[Joerg]

So when do we get the libretto?

Like in singing a requiem? No way my vocal cords would even get up there.