On Oct 23, 1:28=A0pm, Allan Herriman <allanherri...@hotmail.com> wrote:> On Mon, 22 Oct 2012 14:22:49 -0700, Joerg 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? > > > Even the ones I found in the 100pF range are either obsolete or not > > recommended for new designs. > > > 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. > > > Doing it with caps and a mux chip or two has its own challenges. The > > ADG-series from AD is around 11pF per pin, otherwise their Rdson is too > > high. Talking about the a rock and a hard spot here. > > > Oh, and cost is not very important. If a diode or two or three are > > needed that cost $5 a pop that's ok. > > Reading between the lines on the LTC3721-1 datasheet, it seems that the > CT pin generates the upward part of the timing ramp with a (roughly) > 150uA current source. =A0The downward part of the ramp is from a much > stronger current sink. > > In an earlier post, Miso suggested sinking some current out of the CT pin > to reduce the frequency, to which you replied "... the chip immediately > gets sea-sick when you do anything DC to that pin." > > Had you tried *sourcing* a small (a few uA) current into CT to increase > the frequency? > > Regards, > AllanI tried simulating LTC3721 and the charge waveform looks a bit funny, it is ~160uA but at the end of each cycle there's a different ramp upto somtimes -25mA before the 50mA reset pulse adding some current, 10V - 50k, ~doubles the frequency and does not seem to make it go crazy -Lasse
Large capacitance varicaps, where are thee?
Started by ●October 22, 2012
Reply by ●October 23, 20122012-10-23
Reply by ●October 23, 20122012-10-23
On Mon, 22 Oct 2012 14:22:49 -0700 Joerg <invalid@invalid.invalid> wrote in Message id: <aelrpnF24rtU1@mid.individual.net>:>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? > >Even the ones I found in the 100pF range are either obsolete or not >recommended for new designs. > >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. > >Doing it with caps and a mux chip or two has its own challenges. The >ADG-series from AD is around 11pF per pin, otherwise their Rdson is too >high. Talking about the a rock and a hard spot here. > >Oh, and cost is not very important. If a diode or two or three are >needed that cost $5 a pop that's ok.How about http://spraguegoodman.com/pdfs/500.pdf
Reply by ●October 23, 20122012-10-23
Phil Hobbs wrote:> On 10/22/2012 8:55 PM, Joerg wrote: >> legg wrote: >>> On Mon, 22 Oct 2012 15:29:00 -0700, Joerg <invalid@invalid.invalid> >>> wrote: >>> >>>> miso wrote: >>>>> On 10/22/2012 2:22 PM, Joerg wrote: >>> <snip> >>>>> If you are not conflating sawtooth with triangle wave, then maybe you >>>>> could shunt some current from the osc pin, which would slow down the >>>>> frequency. Pick your external cap for the highest frequency needed, >>>>> then >>>>> bleed with some DAC based circuit or even a resistive trimpot. The >>>>> 10-turn PCB mounted resistors still exist. >>>>> >>>>> The difficulty will be in the compliance of the current source used >>>>> for >>>>> bleeding. >>>> >>>> That won't be a problem but the chip immediately gets sea-sick when you >>>> do anything DC to that pin. >>> >>> What do you think a varactor will do, with a sawtooth bias? >>> >> >> 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. >> > > BB201s are 95 pF at 1 volt, and come in duals, so you can get 200 pF-ish > out of one SOT23. You have to watch out for varactor specs--some > headline their maximum capacitance, others their minimum. >I can't go below 3V because that's my working voltage, with margin. So realistically I'd get 10-60pF range, times two. Not bad at all. I am just concerned that it might go unobtanium like all the others. Much of the BB series already is, and some places like Allied are cleaned out of this one right now. The solution I have now is a mux plus 1% caps. I was surprised how cheap they've become. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
legg wrote:> On Mon, 22 Oct 2012 17:55:55 -0700, Joerg <invalid@invalid.invalid> > wrote: > >> legg wrote: >>> On Mon, 22 Oct 2012 15:29:00 -0700, Joerg <invalid@invalid.invalid> >>> wrote: >>> >>>> miso wrote: >>>>> On 10/22/2012 2:22 PM, Joerg wrote: >>> <snip> >>>>> If you are not conflating sawtooth with triangle wave, then maybe you >>>>> could shunt some current from the osc pin, which would slow down the >>>>> frequency. Pick your external cap for the highest frequency needed, then >>>>> bleed with some DAC based circuit or even a resistive trimpot. The >>>>> 10-turn PCB mounted resistors still exist. >>>>> >>>>> The difficulty will be in the compliance of the current source used for >>>>> bleeding. >>>> That won't be a problem but the chip immediately gets sea-sick when you >>>> do anything DC to that pin. >>> What do you think a varactor will do, with a sawtooth bias? >>> >> 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. > > The whole tuning range of the hyperabrupt parts was achieved by a 6-8V > span of bias. If the ramp amplitude is a signifigant percentage of > this, you risk rectification and other effects. >The capacitance will modulate which will cause slight non-linearity. Would be acceptable in this case. My worry is the supply chain, not the technical side. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
miso wrote:> On 10/22/2012 3:29 PM, Joerg wrote: >> miso wrote: >>> On 10/22/2012 2:22 PM, Joerg 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? >>>> >>>> Even the ones I found in the 100pF range are either obsolete or not >>>> recommended for new designs. >>>> >>>> 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. >>>> >>>> Doing it with caps and a mux chip or two has its own challenges. The >>>> ADG-series from AD is around 11pF per pin, otherwise their Rdson is too >>>> high. Talking about the a rock and a hard spot here. >>>> >>>> Oh, and cost is not very important. If a diode or two or three are >>>> needed that cost $5 a pop that's ok. >>>> >>> >>> There are plenty of hoarders that have those caps. I picked them up >>> years ago when I figured they would go extinct. They same goes for 10 >>> turn pots of the panel variety with the dials on them. (Good old Mike >>> Quinn in San Leadro by the Oakland airport would save panels with those >>> pots and junk the rest of the product.] Most radios are synthesized >>> these days, so there is no market for those caps. >>> >> >> Yeah, I know but I was hoping there'd be at least one other market. >> Seems like there ain't. >> >> >>> If you are not conflating sawtooth with triangle wave, then maybe you >>> could shunt some current from the osc pin, which would slow down the >>> frequency. Pick your external cap for the highest frequency needed, then >>> bleed with some DAC based circuit or even a resistive trimpot. The >>> 10-turn PCB mounted resistors still exist. >>> >>> The difficulty will be in the compliance of the current source used for >>> bleeding. >> >> >> That won't be a problem but the chip immediately gets sea-sick when you >> do anything DC to that pin. >> > > Is this pin where you see the sawtooth? Seems to me with a high > impedance current source, you can steal the current. Now a triangle > waveform is a different case. > > So you have tried a high impedance current source? >Yup. The chip then starts to misfire, big time, at least on the simulator. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
Don Lancaster wrote:> On 10/22/2012 3:31 PM, Joerg wrote: >> Nico Coesel wrote: >>> Joerg <invalid@invalid.invalid> 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? >>>> >>>> Even the ones I found in the 100pF range are either obsolete or not >>>> recommended for new designs. >>>> >>>> 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 >>> >>> How about using a bad quality ceramic capacitor? >>> >> >> They don't make Z5U this small. Only for larger capacitors. >> > > > And the problem with using an alternator diode instead is...? >Real estate. Those things are huge, plus they barrier is quite fuzzy. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
Nico Coesel wrote:> Joerg <invalid@invalid.invalid> wrote: > >>>> Anyhow, it's not a show stopper, I can use a small varicap plus mux. The >>>> mux is like an umpteen-speed gear shifter, not very pretty. If big >>>> varicaps were still available it would become very easy. But I guess >>>> even if I found one the risk is too great that it's going to be >>>> obsoleted in a few years. >>> That's too ugly to contemplate. >> >> 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. > > 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. >This one has to be fast design and fast turnkey. But there was some good old bench time, with their old system so I can figure how the load reacts and stuff. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
krw@att.bizzzzzzzzzzzz wrote:> On Mon, 22 Oct 2012 18:02:03 -0700, Joerg <invalid@invalid.invalid> wrote: > >> krw@att.bizzzzzzzzzzzz wrote: >>> On Mon, 22 Oct 2012 16:30:01 -0700, Joerg <invalid@invalid.invalid> wrote: >>> >>>> krw@att.bizzzzzzzzzzzz wrote: >>>>> On Mon, 22 Oct 2012 14:22:49 -0700, Joerg <invalid@invalid.invalid> 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. >>>>> Use a regulator with an external clock. That's what we do (all of our >>>>> switchers are synchronized). >>>>> >>>> Well, I need one from LTC because it must be simulated. The load is >>>> really ugly yet must be well regulated with some unorthodox loop >>>> elements in there. Linear only has the 3721 and 3723 for push-pull. Some >>>> older ones as well but they have the same engine type in them. >>> Nuts. >>> >> What's nuts about this? > > Single source because you want a crappy behavioral model to make you feel > good. There *are* other vendors out there, with perfectly good products. >But no simulator models, plus they would also be single-sourced. Some don't have current-mode control and that's a no-no here. So far pretty much all my designs came out almost verbatim as simulated. That's the kind of time-to-market clients want. [...] -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
legg wrote:> On Mon, 22 Oct 2012 16:38:28 -0700, Joerg <invalid@invalid.invalid> > wrote: > >> legg wrote: >>> On Mon, 22 Oct 2012 14:22:49 -0700, Joerg <invalid@invalid.invalid> >>> 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? >>>> >>>> Even the ones I found in the 100pF range are either obsolete or not >>>> recommended for new designs. >>>> >>>> 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. >>>> >>>> Doing it with caps and a mux chip or two has its own challenges. The >>>> ADG-series from AD is around 11pF per pin, otherwise their Rdson is too >>>> high. Talking about the a rock and a hard spot here. >>>> >>>> Oh, and cost is not very important. If a diode or two or three are >>>> needed that cost $5 a pop that's ok. >>> SVC383 ex-Sanyo, is still in the ON catalog, with no product notices >>> re last run or anything. Dual 500pf. >>> >>> http://www.onsemi.com/pub_link/Collateral/ENN6264-D.PDF >>> >> Thanks, but I've been through that as well. Here is the bad news: >> >> http://www.digikey.com/product-detail/en/SVC383T-TL-E/869-1202-2-ND/2165536 >> >> Quote "Obsolete item; call Digi-Key for more information". > > I receive all ON product notices (spec, process, material, source, > discontinuance, resurection etc etc etc) changes, and there's nothing > on SVC383. > > Did you ask the ON recommended vendor - Rochester Electronics? They > also have no stock, but they shouldn't give you a phoney reason for > this. Digikey still lists their supplier for SVC389 as Sanyo > Semiconductor (USA), so their info for both parts may be dated. Did > you actually ask Digikey? Looks like the usual minimum purchase of a > reel, for items not recently stocked. >No, they list ON Semi as the supplier.> The issue is more likely a Sanyo-based supply chain one - strictly > sales related. I'd sniff around previously reliable Sanyo distribution > outlets (not excluding Digikey). SVC389 was also an RS Component line > item. >FindChips lists only Digikey as the supplier and they state "obsolete" (on the ON brand, not Sanyo). No other distributors listed, and I tend to go by the more major ones. ON Semi's tuning diodes tab does not list the SVC383: http://www.onsemi.com/PowerSolutions/parametrics.do?id=827 Keyed it into the search field and it didn't find it. That's too much "writing on the wall" for me to use a part, no matter what a sales guy would tell me over the phone. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
Tim Williams wrote:> Have you looked at Big Fat Schottkies? The 30V 40A types are in the 10nF > at zero bias, and obviously will handle the puny 50mA reset pulse. :) > > Can't say I ever got great results using schottky capacitance, I think > their Q is crap or something (which is something to think about when > everyone touts "zero reverse recovery losses"!). It also varies rather > quickly with voltage, maybe too rapidly for the chip. Well, you can > bracket the C range with series/parallel caps anyway, and maybe a higher > voltage schottky, or a beefy junction diode, will have a more gentle > curve. >Tried it, the results were less than enthusing. -- Regards, Joerg http://www.analogconsultants.com/
