Large capacitance varicaps, where are thee?

Nuts.

Setting a timer is too many MIPS?

Analog dudes certainly are everywhere I've worked. If they want resources, they get them. In fact, they get almost all the MIPS. ;-)

That's too ugly to contemplate.

That would make a whole big Christmas ornament :-)

In the off chance that you haven't thought of it yet:

xxx3055 or other fairly low-voltage "moose" power BJT -- use the B-C junction.

One of the "used to be common, now it's boutique" obsolete semiconductor houses.

OK. This is obscenely bizarre, and any design and implementation details are left to your poor tortured brain:

You can make a gyrator to turn a capacitance into an inductance via the good graces of an op-amp. The effective inductance depends on the capacitance and the gain.

So -- consider yourself challenged to make a similar gizmo that either takes a capacitance and multiplies it by a variable amount, based on a gain that you can adjust, or that takes an inductance in a "gyrator" circuit that presents a capacitive load (and make it stable, heh heh heh).

D'oh.

Use the Miller capacitance of a gain-modulated stage. A series cap, a transistor with a controlled bias voltage, a B-C cap (to be "Millered"), and your analog circuits brilliance to actually make it work.

Have fun getting it through a circuit design review.

And finding a voltage supply big enough for the amp.

I get my name on the patent if it works.

Yikes, looks like the Zetex varicaps are gone.

Skyworks has a varicap that swings about 80 pF, maybe 50 usable with your swing. Maybe combine that with a few switched fixed caps?

Maybe use some big grungy power diode that wasn't born to be a varicap?

There must be a way to do your function without needing a variable capacitor. What switcher chip are you using?

Now you've got me thinking about varicaps in series with fixed caps, with the diodes sometimes working as varicaps and sometimes forward conducting like PIN diodes. Could get messy to program.

Start with the biggest cap value needed, 2000 pF. Connect one end of that to chip terminal A, the one that expects a cap to ground.

Now pick off the voltage from A, run through an MDAC, buffer that, and drive the low end of the cap. With mdac gain = 0, A sees the full 2000 pF. With MDAC gain = 0.999, it sees 2 pF. Gain 0.5 nets 1000 pF. Like that.

But the buffer does have to handle all the AC current in the cap.

Ooh Ooh! Not a Miller capacitance! You want an Anti-Miller capacitance, because then your amplifier output voltage range can be the same as the input voltage range!

i_in = (Vin - Vo) * C * s

Vo = K * Vin

i_in = Vin * (1 - K) * C * s

So, the effective capacitance at Vin is (1 - K) * C. Use a variable-gain amplifier, and don't blame me if you go insane before you get it to remain stable under all operating conditions.

|| .-----||------. | || | | | | | Vin | |\ | | | \ | o-------o----|K >-----' Vo | / i_in --> |/ (created by AACircuit v1.28.6 beta 04/19/05

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Hey, maybe some cheap photodiode could be used as a varicap? (I have no ideas as to the cheap part.)

George H.

SVC383 ex-Sanyo, is still in the ON catalog, with no product notices re last run or anything. Dual 500pf.

RL

text -

Good idea. Phil might know of a large-area part.

What do you think a varactor will do, with a sawtooth bias?

RL

It's going to present a capacitance to it which can be varied via a higher voltage. Works nicely, I've got it running on LTSPice with an old Siemens varicap model. But ... only Avnet Europe and Wuhan have stock left.

What's nuts about this?

There will be a loop going. That needs a tad more resources than just a timer.

Yeah, I usually did get my MIPS so far. But every time I step into a uC guys cubicle they ask "So you want to eat up the last timer now?" or something.

Well, I can't spend time bench-testing a new switcher chip that doesn't have a decent (meaning behavioral) SPICE model. My stuff usually goes CAD to production these days, with no lab bench time before the prototypes are done. On this one I only had to bench test the load and almost got sick when I looked at the results.

And the problem with using an alternator diode instead is...?

Lasse had suggested something similar but I am dealing with a sawtooth and a brutal reset pulse.

About 25V is the highest I can provide, else it'll be more complicated than my mux is now.

Sounds scary :-)

Plus it would need to stomach a 50mA reset pulse and if the behavior ain't 100% capacitive the switcher chip gets upset.

Sounds like all the fun went away from your job :-) I re-instated my etching setup for testing chips. OTOH fast turnaround services aren't that expensive. In most cases some undocumented features get revealed.

Ach! .

Yup :-(

I thought about massively paralling but then the low end gets tough, and it starts to consume real estate. Got it done with a mux now.

Probably, as long as it's not a big fat one. But the issue is that I'd need to be able to simulate because this one has to wrap up soon.

LTC3721-1.

Also, one has one watch EMI with such a long string. Anything that is forward conducting will pick up stuff, the nastier ones are usually GSM cell phones.

My wife just cooked homemade Sauerbraten, with dumplings and all the trimmings. Yummmmm!