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Large capacitance varicaps, where are thee?

J

Joerg

Jan 1, 1970
0
Fred said:
Joerg a écrit :

I thought you had feedback, so you should not have pb.

Yes, but it has to let go for extended periods of time.

For the tempco I don't expect it to be any different from a varicap. Now
you have the production spread to cope with.
It'll almost surely be wider than a varicap which is done for that.

Anyway the spread given in the datasheet between typical and Abs max is
a 3:2 ratio to the nth decimal place, so I'd expect it to be much better
( much like the high leakage current of CMOS gate).

Anyone having a better informed figure for the actual power MOSFETs
capacitance spread?

3:2 would be just fine. Now it would be nice if the SPICE model
contained this effect :)
 
R

rickman

Jan 1, 1970
0
You think you're alone? Get the manufacturer to guarantee the design.

What good is that? So the board doesn't work like the simulation. Is
the manufacturer going to fix the chip? They might fix the model and
you still miss your schedule.

Rick
 
J

Joerg

Jan 1, 1970
0
John said:
Jim Thompson wrote:
[email protected] wrote:

[email protected] wrote:

[email protected] wrote:

[email protected] wrote:

Folks,

Looked at Digikey and some others. Where are those huge capacitance
varicaps? The ones with several hundred pF of range for AM radios. All
gone lalaland by now?
http://www.angelfire.com/electronic2/index1/VariableCap.html

About half way down.

Even the ones I found in the 100pF range are either obsolete or not
recommended for new designs.
No kiddin? ;-)

What I am trying to do: I need to control a switcher chip in frequency
because I've got a very resonant load to deal with. Unfortunately it
sets the frequency with a timing cap. I'll have to somehow vary that
between 750pF and 2000pF. Can also be digital but then with a
granularity of 5pF. The sawtooth voltage across it is 2.5Vpp.
[snip]

Where is the sawtooth located? Minimum trough voltage? Supply
voltage(s)?

At the timing pin CT, where the timing cap hangs off of.

The voltage swing is zero at start of ramp, ramps up to 2.3V, where it
is being quite brutally reset by discharging the cap, about 50mA. Now it
starts to ramp up again. So I guess they have a comparator in there that
sense when 2.3V is reached.

The Supply voltage for that part is either about 9V or 5V (they don't
say, internal to the chip)
You *could* just add to, or fight, their current source.

Hmmm, just put in the cap for the highest frequency, and sink some
charging current to pull the frequency down. Sounds like a DAC of some
sort, and maybe as simple a sink as an NPN with an emitter resistor.
Of course, you have to disappear 5/8 of the ramp current to cover your
stated frequency range.

The other direction, adding current to push the frequency up, is
numerically safer. You should be able to compute the existing charging
current pretty easily... or measure it... to see if that looks safe.

Heck, an opamp swinging from 0 to, say, +10, and a resistor to the cap
node, would probably do it. That can add a lot of current and remove a
little, a nice compromise.

Looks like a current source (not sink) will work, for some strange
reason it didn't last week. But LTC would have to bless this, to avoid
some unforeseen pathology in there.
 
J

Joerg

Jan 1, 1970
0
Jim said:
Jim said:
[snip] The sawtooth voltage across it is 2.5Vpp.
[snip]

Where is the sawtooth located? Minimum trough voltage? Supply
voltage(s)?
At the timing pin CT, where the timing cap hangs off of.

The voltage swing is zero at start of ramp, ramps up to 2.3V, where it
is being quite brutally reset by discharging the cap, about 50mA. Now it
starts to ramp up again. So I guess they have a comparator in there that
sense when 2.3V is reached.

The Supply voltage for that part is either about 9V or 5V (they don't
say, internal to the chip)

The brutal discharge makes cap multipliers difficult. I think you're
stuck with (digitally) switching caps in and out.

Or current sourcing, if LTC blesses that. Pronlem ist, my main guy is on
a seminar all week and I'd need to know this week. They are really good
when it comes to support though.

What is the ramp-up time? I gather that is the variable part to match
your resonant load?

20usec nominal. I must vary the frequency to appease the load and the
frequency is set by the ramp time, which is set by the current.

An alternative would be to change the comparator threshold... if you
can get to it.

Nope, can't, and the slope is used elsewhere like for slope compensation
to avoid the possible RHP zero buckaroo. So if you truncate it too much
other things can go kaputt.

Is this adjustment on the fly, or is it set once and forget?

Sometimes on the fly, then pauses during certain phases of the game,
then comes back to regulate.
 
Why did you then flag the single source issue if you think it's irrelevant?

Because you refuse to consider any other supplier!
For some of my clients this is relevant. I have used PWM chips in the
past that are 2nd-sourced. From Richtek and TI, for example. But those
tend to be older designs.

Jellybeans, perhaps. Nothing leading edge.
I am replacing a TI regulator that isn't, and this is one of the reasons.



Depends on who made them. From Linear Technology, yes, usually I trust
them. Many years of successful design with their models gives me confidence.

I sure don't. I've never seen a behavioral that's good outside it's *NORMAL*
operating conditions. That's never where the demons are.
 
What good is that? So the board doesn't work like the simulation. Is
the manufacturer going to fix the chip? They might fix the model and
you still miss your schedule.

They pay. Big time. Model? You're nuts.
 
J

Joerg

Jan 1, 1970
0
Because you refuse to consider any other supplier!

That's not the definition of single source :)

I do use others, just not for really critical stuff like this where
simulation is key.

Jellybeans, perhaps. Nothing leading edge.


Are you saying5 that TI doesn't have current mode regulators? <boggle> Almost
everything I've looked at in the last year is current mode.

Huh? That's not what I said. I said I am replacing a TI regulator that
is not current-mode. It has to go because that one isn't up to the job.
Of course they have developed better ones by now. But no behavioral
models so one cannot simulate in a reasonable time frame.

I sure don't. I've never seen a behavioral that's good outside it's *NORMAL*
operating conditions. That's never where the demons are.

Then you haven't tried hard enough with LTSpice :)

By now they know me at LTC for lots of unorthodox uses.
 
R

rickman

Jan 1, 1970
0
They pay. Big time. Model? You're nuts.

Hah! Pay!!! LOL!!!

Good luck getting a chip maker to guarantee YOUR design no matter what
you do.

Rick
 
That's not the definition of single source :)

You only consider one source. That *is* a single source. Nuts!
I do use others, just not for really critical stuff like this where
simulation is key.

With a behavioral, you can only simulate what the model builder anticipated.
Again, that's rarely where the demons be. At least with full models, you have
a chance.
Huh? That's not what I said. I said I am replacing a TI regulator that
is not current-mode. It has to go because that one isn't up to the job.
Of course they have developed better ones by now. But no behavioral
models so one cannot simulate in a reasonable time frame.

Yet you will only use LTC because they're current mode. said:
Then you haven't tried hard enough with LTSpice :)
Nonsense.

By now they know me at LTC for lots of unorthodox uses.

The models are useless outside their limited operational area. IOW, useless
for guaranteeing operation.
 
T

Tim Williams

Jan 1, 1970
0
Joerg said:
They do reset the cap with some stiff FET in there once it reaches
2.35V. But I do not have any strange ramping, it looks very clean. When
I tried the same before the ramp looked totally sick and it was a
circuit I hadn't touched. Weird.

It wasn't on trapezoidal integration, was it? :-o Sounds like the kind
of gobbledeygook you get from funny simulation settings (or funny models
that need more than default fudging!).

Tim
 
J

Joerg

Jan 1, 1970
0
[...]
I do use others, just not for really critical stuff like this where
simulation is key.

With a behavioral, you can only simulate what the model builder anticipated.
Again, that's rarely where the demons be. At least with full models, you have
a chance.

Yep. Last time I did that one single run too about four (!) hours.

Yet you will only use LTC because they're current mode. <boggle>

You are not correctly reading what I write. I am using LTC because there
are models that allow fast simulation. _Not_ because they are current
mode. I only require that any switcher that replaces the old one is
current mode.

The models are useless outside their limited operational area. IOW, useless
for guaranteeing operation.

Sorry, but I have proven that statement wrong many times. For that to
work you do need to establish good connections to their engineers
though. I have.
 
J

Joerg

Jan 1, 1970
0
Tim said:
It wasn't on trapezoidal integration, was it? :-o Sounds like the kind
of gobbledeygook you get from funny simulation settings (or funny models
that need more than default fudging!).

Well, like with all unorthodox uses of such chips I want to get a
blessing from their IC designers. Usually takes about a week, then they
say yea or nay. Those guys know.
 
[...]
I do use others, just not for really critical stuff like this where
simulation is key.

With a behavioral, you can only simulate what the model builder anticipated.
Again, that's rarely where the demons be. At least with full models, you have
a chance.

Yep. Last time I did that one single run too about four (!) hours.

<shrug> I've used entire mainframes for weeks on a single simulation. CPUs
weren't as cheap, nor as powerful, as they are now. Not everyone had one
sitting on their desk, either.
You are not correctly reading what I write. I am using LTC because there
are models that allow fast simulation. _Not_ because they are current
mode. I only require that any switcher that replaces the old one is
current mode.

That's not what you said. You said it like current mode was something unique.
It surely isn't.
Sorry, but I have proven that statement wrong many times. For that to
work you do need to establish good connections to their engineers
though. I have.

YOu *think* you've simulated everything that can go wrong. Obviously a model
can't be used outside it's limits. Behaviorals have very limited useful area
and they're rarely documented.
 
J

Joerg

Jan 1, 1970
0
[email protected] wrote:

[email protected] wrote:

[email protected] wrote: [...]

I do use others, just not for really critical stuff like this where
simulation is key.
With a behavioral, you can only simulate what the model builder anticipated.
Again, that's rarely where the demons be. At least with full models, you have
a chance.
Yep. Last time I did that one single run too about four (!) hours.

<shrug> I've used entire mainframes for weeks on a single simulation. CPUs
weren't as cheap, nor as powerful, as they are now. Not everyone had one
sitting on their desk, either.

Well, I do not have a mainframe available here.

That's not what you said. You said it like current mode was something unique.

Where?


It surely isn't.


YOu *think* you've simulated everything that can go wrong. Obviously a model
can't be used outside it's limits. Behaviorals have very limited useful area
and they're rarely documented.

The same goes for full models. There are limits to everything virtual.
However, with behavioral models I've had years of successful designs.
And yeah, I did discover the occasional model bug and also the
occasional real bug on an IC. Where sim and real life did not jibe.
Usually it meant that the datasheet didn't either. That's part of life.
 
R

rickman

Jan 1, 1970
0
You are an idiot.


You're batting 1000 today. Go back to your g5eometry lessons, with Bloggs.

I'm waiting for Joerg to get to this point with you. You get to where
you can't reply so you start insulting people. I don't get why you
bother.

rick
 
I'm waiting for Joerg to get to this point with you. You get to where
you can't reply so you start insulting people. I don't get why you
bother.

Joerg has a brain and is a nice guy, unlike you, who are neither.
 
J

Joerg

Jan 1, 1970
0
[email protected] wrote:

[email protected] wrote:

[email protected] wrote:

[email protected] wrote:
[...]

I do use others, just not for really critical stuff like this where
simulation is key.
With a behavioral, you can only simulate what the model builder anticipated.
Again, that's rarely where the demons be. At least with full models, you have
a chance.

Yep. Last time I did that one single run too about four (!) hours.
<shrug> I've used entire mainframes for weeks on a single simulation. CPUs
weren't as cheap, nor as powerful, as they are now. Not everyone had one
sitting on their desk, either.
Well, I do not have a mainframe available here.

You have about 100x the CPU power there.

Probably even more. But that kind of power is no accessible to everyone.
I am hoping that cloud deals arise some day where you can "rent" CPU
time at more reasonable deals. That would really be useful to folks who
have to run tough sims.

"Some don't have current-mode control and that's a no-no here." ^^^^



Almost all do today. If you weren't making some point about it being somehow
unique, it was a null statement.

Makes no sense to discuss this point further.

Sure, but they're almost always more general than a behavioral. Most
behaviorals don't even model their own supply current. ...


LTC's usually do.

... Very few actually tell you *what* they're modeling.

True. Although if you have a good relationship with a manufacturer's
engineers they'll help you there. In my case this has enabled several
unorthodox designs and they were, consequently, rewarded with increased
business driven their way.

I've had years of successful designs with little simulation at all.

Our grandparents drove around without safety belts and survived :)

Irrelevant. I'm not talking about bugs.


If you know the limitations of the models I find them extremely useful.
I do miss the good old bench time though, because nowadays there's a lot
less of that than 20 years ago. I have relied on simulators my whole
career but of course in the 80's the computers weren't up to snuff for
the real heavy lifting.
 
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