Reply by jasen October 14, 20062006-10-14
On 2006-10-09, rickman <gnuarm@gmail.com> wrote:
> Tim wrote: >> If I have understood you correctly, your system has two operating modes >> a) low power and b) high power. Could you thus use two power supplies >> that could be enabled and disabled using FET switches according to your >> system requirements by this CPLD? In low power mode you would use LDO >> and in high current mode you would use switched mode power supply+LDO. >> >> I have not designed switched mode power supplies, but I guess if you >> reduce the switching frequency, you will improve efficienfy with the >> expence of ripple which could be "filtered" by this LDO. Could you get >> away with for example 1 kHz switching frequency? Or could an adjustable >> switching frequency be more feasable? >> >> Please note, this was all pure speculation and I haven't tried this at >> home :) > > Thanks for the ideas. > > Typically our devices have several power consumption levels. But I > can't use multiple PS circuits for better efficiency because they don't > make any that I have found that are efficient at low currents.
so don't use them at low currents. only enable them when you need high current. Bye. Jasen
Reply by Jim Granville October 10, 20062006-10-10
dagmargoodboat@yahoo.com wrote:

> Jim Granville wrote: > >>dagmargoodboat@yahoo.com wrote: >> >> >>> Interleaving two, out of phase, to get your 600KHz. >>> >>> Efficiency vs: Load Current graph in the bottom right corner of pg. 6 >>>shows only ~50% efficiency @ 10 mA, rising to 75% @ 14V input and >>>100mA. (http://datasheets.maxim-ic.com/en/ds/MAX5096-MAX5097.pdf) >> >>Yes, something seems not consistent in their data. >>They state 14Vin 5V out, Io=0, typ 680uA (9.52mW) >>But then show 62% on the graph for 14V/5V/10mA, [50mW], >>which suggests 30.6mW of losses. (2.2mA?) > > > That is a little puzzling, until you notice the buck mode Iq (they > call > it Is) spec at the top of pg.3 shows 720uA typ. for the 330KHz version, > >>>NOT<< switching. That's really what they mean too, since the feedback > > voltage Vadj is specified to be 1.4v--higher than the 1.2v reference > voltage. > Sneaky. > > The problem is that banging those FETs at 330KHz is going to burn > some gate drive, which is why everyone likes to pulse-skip at low > powers. > > Actually, I rather wonder whether this part might indeed start pulse > skipping at low loads, but I'm too lazy to check. > > >>Pity they don't plot Iq vs Io for BUCK mode.
I've found a SMPS data sheet where they DO plot Iq vs Io, ( for a LTC3835 ), and that does show a variation in loss, under 10mA. On this device the losses corner is appx 0.5mA : below that, the loss is effectively independant of Io, and above that, losses climb with io. -so the maxim device might take 680uA at 0mA, and 2.2mA at 10mA Io. Silly NOT to carry the graphs below 10mA, as someone might think they have more wastage than they really do. -jg
Reply by October 10, 20062006-10-10
Jim Granville wrote:
> dagmargoodboat@yahoo.com wrote: > > > Interleaving two, out of phase, to get your 600KHz. > > > > Efficiency vs: Load Current graph in the bottom right corner of pg. 6 > > shows only ~50% efficiency @ 10 mA, rising to 75% @ 14V input and > > 100mA. (http://datasheets.maxim-ic.com/en/ds/MAX5096-MAX5097.pdf) > > Yes, something seems not consistent in their data. > They state 14Vin 5V out, Io=0, typ 680uA (9.52mW) > But then show 62% on the graph for 14V/5V/10mA, [50mW], > which suggests 30.6mW of losses. (2.2mA?)
That is a little puzzling, until you notice the buck mode Iq (they call it Is) spec at the top of pg.3 shows 720uA typ. for the 330KHz version,
>>NOT<< switching. That's really what they mean too, since the feedback
voltage Vadj is specified to be 1.4v--higher than the 1.2v reference voltage. Sneaky. The problem is that banging those FETs at 330KHz is going to burn some gate drive, which is why everyone likes to pulse-skip at low powers. Actually, I rather wonder whether this part might indeed start pulse skipping at low loads, but I'm too lazy to check.
> Pity they don't plot Iq vs Io for BUCK mode. > > -jg
Regardless, efficiency-wise it beats an LDO by quite a bit, and for all inputs. Best, James Arthur
Reply by Jim Granville October 10, 20062006-10-10
dagmargoodboat@yahoo.com wrote:

> Interleaving two, out of phase, to get your 600KHz. > > Efficiency vs: Load Current graph in the bottom right corner of pg. 6 > shows only ~50% efficiency @ 10 mA, rising to 75% @ 14V input and > 100mA. (http://datasheets.maxim-ic.com/en/ds/MAX5096-MAX5097.pdf)
Yes, something seems not consistent in their data. They state 14Vin 5V out, Io=0, typ 680uA (9.52mW) But then show 62% on the graph for 14V/5V/10mA, [50mW], which suggests 30.6mW of losses. (2.2mA?) Pity they don't plot Iq vs Io for BUCK mode. -jg
Reply by October 10, 20062006-10-10
Uwe Bonnes wrote:
> In sci.electronics.design rickman <gnuarm@gmail.com> wrote: > > > Thanks for the pointer. It would have been perfect, but the sync is > > only good up to 500 kHz and we need 600 kHz. But this is exactly the > > type of part I would like to have. > > Sync it to 600khz/2!
Interleaving two, out of phase, to get your 600KHz. Efficiency vs: Load Current graph in the bottom right corner of pg. 6 shows only ~50% efficiency @ 10 mA, rising to 75% @ 14V input and 100mA. (http://datasheets.maxim-ic.com/en/ds/MAX5096-MAX5097.pdf) James Arthur
Reply by Uwe Bonnes October 10, 20062006-10-10
In sci.electronics.design rickman <gnuarm@gmail.com> wrote:

> Thanks for the pointer. It would have been perfect, but the sync is > only good up to 500 kHz and we need 600 kHz. But this is exactly the > type of part I would like to have.
Sync it to 600khz/2! -- Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt --------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
Reply by rickman October 10, 20062006-10-10
Jim Granville wrote:
> John wrote: > > Rick, > > > > > > > >>This is the second problem. If I try to find an LDO with Vin up to > >>16.5 volts, output current up to 100 mA and dropout voltage of 200 mV, > >>I come up short. > > > > > > Will this work for you? : http://www.national.com/pf/LP/LP2980.html > > > > I'm using the AIM-3.3V of this part in a design and it sounds like it's > > almost a perfect fit. Perhaps parallel two of them or use an external > > FET to get the 100 mA continuous. > > and this one looks a very good reality-check for any alternative > discrete designs [4-40V ip] : > > http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5224 > > Still new, claims just 680uA for no load inductor mode, and it can > select LDO mode, for ~40uA Iq (100mA pk). > Has Reset and Sync to top off the features.... > > I see it also has a snap-action on rising Vin, which is something > very few regulators do - benefit it is avoids brownout areas, and > many devices have poor brownout behaviour.
Thanks for the pointer. It would have been perfect, but the sync is only good up to 500 kHz and we need 600 kHz. But this is exactly the type of part I would like to have.
Reply by Jim Granville October 10, 20062006-10-10
John wrote:
> Rick, > > > >>This is the second problem. If I try to find an LDO with Vin up to >>16.5 volts, output current up to 100 mA and dropout voltage of 200 mV, >>I come up short. > > > Will this work for you? : http://www.national.com/pf/LP/LP2980.html > > I'm using the AIM-3.3V of this part in a design and it sounds like it's > almost a perfect fit. Perhaps parallel two of them or use an external > FET to get the 100 mA continuous.
and this one looks a very good reality-check for any alternative discrete designs [4-40V ip] : http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5224 Still new, claims just 680uA for no load inductor mode, and it can select LDO mode, for ~40uA Iq (100mA pk). Has Reset and Sync to top off the features.... I see it also has a snap-action on rising Vin, which is something very few regulators do - benefit it is avoids brownout areas, and many devices have poor brownout behaviour. -jg
Reply by John October 10, 20062006-10-10
Rick,

 
> This is the second problem. If I try to find an LDO with Vin up to > 16.5 volts, output current up to 100 mA and dropout voltage of 200 mV, > I come up short.
Will this work for you? : http://www.national.com/pf/LP/LP2980.html I'm using the AIM-3.3V of this part in a design and it sounds like it's almost a perfect fit. Perhaps parallel two of them or use an external FET to get the 100 mA continuous. John.
Reply by Jonathan Kirwan October 9, 20062006-10-09
On 9 Oct 2006 09:36:50 -0700, "rickman" <gnuarm@gmail.com> wrote:

>Mark Borgerson wrote: >> I suppose that you could simply use a comparator and gate to halt >> the switcher---through a shudown input or by gating the clock. >> >> This could all be moot if you are constrained on size---you will >> probably need a pretty large capacitor. However the super caps >> are getting better all the time. > >I'm not sure how this would be a better approach to this problem. It >seems to be pushing the limits of what you can do with a switcher. I >can't recall the startup times for switchers, but this sounds to me >like it could be a very tricky circuit to get to work correctly over >all conditions, especially when you add in something like a super cap >that has very limited temperature range. > >I think the point is moot now. The last thing I needed to check to see >if it was viable was the drive for the PFETs. When I calulated the >drive current required, I have a choice of switching them fast, or >switching them with low current, I can't do both. I did not know that >discrete FETs were so limited for speed. The part I had selected has >an input capacitance of 600 pF! > >This type of circuit might be practical inside a chip, but using >discrete components makes it slow and unwieldy.
FAB capacity has been brokered for some time, now, and the pricing isn't scary. And also offered to universities at still lower prices. You might be able to get a decent number of your chips made at fairly okay costs and then shop them around. Just a thought. Jon