Fred Bartoli wrote:> Joerg a �crit : >> 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. >> > > Medium size Mosfets? >As varicaps? -- Regards, Joerg http://www.analogconsultants.com/
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
Allan Herriman 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. The 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? >Yes, and on the simulator everything went to pots when I did. Now I don;'t know how the real part behaves because I don't have one and got no time for elaborate tests. So I'll just use a mux and a bunch of caps. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
Joerg a �crit :> Fred Bartoli wrote: >> Joerg a �crit : >>> 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. >>> >> Medium size Mosfets? >> > > As varicaps? >Sure. Use Coss (G-S shorted) which usually is highly non linear and has a low enough esr in the 10s to 100s of mR. The figures you need are harder to find because the process have improved a lot, but still, poking a bit leads for example to: http://www.fairchildsemi.com/ds/FD/FDMS86150.pdf Now, you can also do something entirely different like: | | +--------------. | | --- | --- | | | | | | | +5V Tune >-----. +-----. | V | | | | | .-. | | | .-. 10K/4K7 | | \| | |/ | | | | Q1 |---+------| Q2 | |22K '-' <| | |> '-' | | - | | +------+ ^ +------+ | | | | | | 100R.-. | .-. | 100n --- | | === | | --- 100n --- | | GND 100R| | --- | '-' '-' | | | | | === === === === GND GND GND GND With a dual BJT and a low enough cap (to compensate for the BJTs offset) you might save the Q2 emitter bias components. You might even want to further lower the cap value since you have available gain. -- Thanks, Fred.
Reply by ●October 23, 20122012-10-23
Fred Bartoli wrote:> Joerg a �crit : >> Fred Bartoli wrote: >>> Joerg a �crit : >>>> 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. >>>> >>> Medium size Mosfets? >>> >> >> As varicaps? >> > > Sure. Use Coss (G-S shorted) which usually is highly non linear and has > a low enough esr in the 10s to 100s of mR. > > The figures you need are harder to find because the process have > improved a lot, but still, poking a bit leads for example to: > http://www.fairchildsemi.com/ds/FD/FDMS86150.pdf > > > Now, you can also do something entirely different like: > > > | > | > +--------------. > | | > --- | > --- | > | | > | | > | | +5V > Tune >-----. +-----. | V > | | | | | > .-. | | | .-. > 10K/4K7 | | \| | |/ | | > | | Q1 |---+------| Q2 | |22K > '-' <| | |> '-' > | | - | | > +------+ ^ +------+ > | | | | | > | 100R.-. | .-. | > 100n --- | | === | | --- 100n > --- | | GND 100R| | --- > | '-' '-' | > | | | | > === === === === > GND GND GND GND > > > With a dual BJT and a low enough cap (to compensate for the BJTs offset) > you might save the Q2 emitter bias components. > You might even want to further lower the cap value since you have > available gain. > >Thanks, that FET idea is a good one, should be possible to find one that has the kink at lower more civilized voltages. It might be a bit drifty with temperature though and that would not be so cool for my case. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
On Tue, 23 Oct 2012 12:45:14 -0700, Joerg <invalid@invalid.invalid> wrote:>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.Everything is single sourced now so that's an irrelevant argument.>Some >don't have current-mode control and that's a no-no here.Almost all do. Everything I've used is current-mode.>So far pretty >much all my designs came out almost verbatim as simulated. That's the >kind of time-to-market clients want.You trust behaviorals? Amazing.
Reply by ●October 23, 20122012-10-23
langwadt@fonz.dk wrote:> On Oct 23, 1:28 pm, 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. The 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, >> Allan > > I 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 >Interestingly, that's what I tried a few days ago and it didnt' work. Now it does, no funny waveforms. Hmm ... -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
Joerg a �crit :> Fred Bartoli wrote: >> Joerg a �crit : >>> Fred Bartoli wrote: >>>> Joerg a �crit : >>>>> 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. >>>>> >>>> Medium size Mosfets? >>>> >>> As varicaps? >>> >> Sure. Use Coss (G-S shorted) which usually is highly non linear and has >> a low enough esr in the 10s to 100s of mR. >> >> The figures you need are harder to find because the process have >> improved a lot, but still, poking a bit leads for example to: >> http://www.fairchildsemi.com/ds/FD/FDMS86150.pdf >> >> >> Now, you can also do something entirely different like: >> >> >> | >> | >> +--------------. >> | | >> --- | >> --- | >> | | >> | | >> | | +5V >> Tune >-----. +-----. | V >> | | | | | >> .-. | | | .-. >> 10K/4K7 | | \| | |/ | | >> | | Q1 |---+------| Q2 | |22K >> '-' <| | |> '-' >> | | - | | >> +------+ ^ +------+ >> | | | | | >> | 100R.-. | .-. | >> 100n --- | | === | | --- 100n >> --- | | GND 100R| | --- >> | '-' '-' | >> | | | | >> === === === === >> GND GND GND GND >> >> >> With a dual BJT and a low enough cap (to compensate for the BJTs offset) >> you might save the Q2 emitter bias components. >> You might even want to further lower the cap value since you have >> available gain. >> >> > > Thanks, that FET idea is a good one, should be possible to find one that > has the kink at lower more civilized voltages. It might be a bit drifty > with temperature though and that would not be so cool for my case. >I thought you had feedback, so you should not have pb. 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? -- Thanks, Fred.
Reply by ●October 23, 20122012-10-23
krw@att.bizzzzzzzzzzzz wrote:> On Tue, 23 Oct 2012 12:45:14 -0700, Joerg <invalid@invalid.invalid> wrote: > >> 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. > > Everything is single sourced now so that's an irrelevant argument. >Why did you then flag the single source issue if you think it's irrelevant? 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.>> Some >> don't have current-mode control and that's a no-no here. > > Almost all do. Everything I've used is current-mode. >I am replacing a TI regulator that isn't, and this is one of the reasons.>> So far pretty >> much all my designs came out almost verbatim as simulated. That's the >> kind of time-to-market clients want. > > You trust behaviorals? Amazing.Depends on who made them. From Linear Technology, yes, usually I trust them. Many years of successful design with their models gives me confidence. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
On Tue, 23 Oct 2012 14:19:16 -0700, Joerg <invalid@invalid.invalid> wrote:>krw@att.bizzzzzzzzzzzz wrote: >> On Tue, 23 Oct 2012 12:45:14 -0700, Joerg <invalid@invalid.invalid> wrote: >> >>> 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.[snip] Where is the sawtooth located? Minimum trough voltage? Supply voltage(s)? ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●October 23, 20122012-10-23
Jim Thompson wrote:> On Tue, 23 Oct 2012 14:19:16 -0700, Joerg <invalid@invalid.invalid> > wrote: > >> krw@att.bizzzzzzzzzzzz wrote: >>> On Tue, 23 Oct 2012 12:45:14 -0700, Joerg <invalid@invalid.invalid> wrote: >>> >>>> 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. > [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) -- Regards, Joerg http://www.analogconsultants.com/
