Fred Bartoli wrote:> 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. >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 :-) -- 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
On 10/22/2012 10:46 PM, krw@att.bizzzzzzzzzzzz wrote:> On Mon, 22 Oct 2012 18:02:03 -0700, Joerg<invalid@invalid.invalid> wrote: >> >> 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. > > 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
Reply by ●October 23, 20122012-10-23
On Tue, 23 Oct 2012 14:58:07 -0700, Joerg <invalid@invalid.invalid> wrote:>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)You *could* just add to, or fight, their current source. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Reply by ●October 23, 20122012-10-23
On Tue, 23 Oct 2012 14:58:07 -0700, Joerg <invalid@invalid.invalid> wrote:>Jim Thompson wrote: >> On Tue, 23 Oct 2012 14:19:16 -0700, Joerg <invalid@invalid.invalid> >> wrote:[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. What is the ramp-up time? I gather that is the variable part to match your resonant load? An alternative would be to change the comparator threshold... if you can get to it. Is this adjustment on the fly, or is it set once and forget? ...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
On Mon, 22 Oct 2012 15:06:40 -0700, miso <miso@sushi.com> 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.Helluva good idea! Since it's a timer and not particularly linearity sensitive you could use a bipolar mirror. ...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
On Tue, 23 Oct 2012 15:32:19 -0700, John Larkin <jlarkin@highlandtechnology.com> wrote:>On Tue, 23 Oct 2012 14:58:07 -0700, Joerg <invalid@invalid.invalid> >wrote: > >>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) > >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. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Reply by ●October 23, 20122012-10-23
On Tue, 23 Oct 2012 15:47:25 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:>On Mon, 22 Oct 2012 15:06:40 -0700, miso <miso@sushi.com> 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. > >Helluva good idea! Since it's a timer and not particularly linearity >sensitive you could use a bipolar mirror. > > ...Jim ThompsonOr, given the "brutal discharge", and it's comparator-tripped, just add pull-up current to vary the timing. Then you don't have headroom issues on the mirror. ...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
John Larkin wrote:> On Tue, 23 Oct 2012 15:32:19 -0700, John Larkin > <jlarkin@highlandtechnology.com> wrote: > >> On Tue, 23 Oct 2012 14:58:07 -0700, Joerg <invalid@invalid.invalid> >> wrote: >> >>> 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) >> 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. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
Jim Thompson wrote:> On Tue, 23 Oct 2012 14:58:07 -0700, Joerg <invalid@invalid.invalid> > wrote: > >> Jim Thompson wrote: >>> On Tue, 23 Oct 2012 14:19:16 -0700, Joerg <invalid@invalid.invalid> >>> wrote: > [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. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20122012-10-23
On 23 Okt., 22:42, Joerg <inva...@invalid.invalid> wrote:> langw...@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. Al=l> >>> 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 frequenc=y> >>> 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 t=oo> >>> 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 th=e> >> 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 immediatel=y> >> gets sea-sick when you do anything DC to that pin." > > >> Had you tried *sourcing* a small (a few uA) current into CT to increas=e> >> 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 ... >I wonder what that strange ramping up current sink at the end of each cycles is? -Lasse
