MC34063 with small inductors
in [Design]
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From: Joerg on 7 Apr 2010 12:02 Grant wrote: > On Tue, 06 Apr 2010 18:00:41 -0700, Joerg <invalid(a)invalid.invalid> wrote: > >> Hello Folks, >> >> Have to correct a design with a 34063 in there. Buck, VIN 16-30V, VOUT >> slightly under 12V, 400mA max output. And no, I don't like it much :-) >> >> One of the issues is a saturating inductor. Can't change the layout and >> to be able to cram one in there I probably have to drop down to 47uH, >> maybe 68uH if lucky. >> >> SPICE says it's ok but none of the datasheets or app notes recommends >> going that low. Any reason why not? > > Try the NCP3063 from On Semi, it's a faster pin compatible for 34063 > Thanks! Technically I can't change it but if needed it's needed. It's only 50% faster but maybe that helps here. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 7 Apr 2010 12:10 dagmargoodboat(a)yahoo.com wrote: > On Apr 6, 8:00 pm, Joerg <inva...(a)invalid.invalid> wrote: >> Hello Folks, >> >> Have to correct a design with a 34063 in there. Buck, VIN 16-30V, VOUT >> slightly under 12V, 400mA max output. And no, I don't like it much :-) >> >> One of the issues is a saturating inductor. Can't change the layout and >> to be able to cram one in there I probably have to drop down to 47uH, >> maybe 68uH if lucky. > > With 30v in, 12v out, that's 18v across 47uH, producing 380mA/uS thru > the inductor. > > So, for a 400mA load, you'll need a 1uS on-time, multiplied by 1/(duty > factor) to yield 400mA average. > > At 100KHz you could charge the inductor 0-to-1.2A in roughly 3uS, let > it discharge 1.2A-to-zero in 4.5uS, stand 2.5uS at zero current, and > get an average of 400mA. The lighter the load, the shorter the on- > time needed, and the faster the switcher will have to be. > > That might barely work. Sure is ugly. > The 34063 doesn't quite work that way, it is not a fixed frequency device. When the output is above threshold it just stops for a while. When it falls the comparator issue another cycle. Since the oscillor itself does run at fixed frequency (unless the current limit comes on) it starts the conduction somewhere, sometimes in the middle of a cycle. That results in rather ugly looking ripple. > Inductance goes with turns^2 while saturation goes with ampere-turns, > so higher inductance works much to your favor. If you can keep the > thing continuous, that lowers peak currents in the inductor, and > saturation concerns with it. > If I could just find one that fits into the cramped space :-( > Or just get a faster part. > The NCP3063 that others have suggested might work but it's not breathtakingly faster, just by 50%. > >> SPICE says it's ok but none of the datasheets or app notes recommends >> going that low. Any reason why not? > > Because it's marginal timing-wise, places high stress on the inductor > and switch, and is likely inefficient. > Inductor stress would be ok, just has to stomach the current. I do get 1.5A peaks which pushes the 34063 a bit but dissipation is "only" around 500mW. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 7 Apr 2010 12:14 Jim Thompson wrote: > On Tue, 06 Apr 2010 18:00:41 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> Hello Folks, >> >> Have to correct a design with a 34063 in there. Buck, VIN 16-30V, VOUT >> slightly under 12V, 400mA max output. And no, I don't like it much :-) >> >> One of the issues is a saturating inductor. Can't change the layout and >> to be able to cram one in there I probably have to drop down to 47uH, >> maybe 68uH if lucky. >> >> SPICE says it's ok but none of the datasheets or app notes recommends >> going that low. Any reason why not? > > Just stick a CD4060 in there. That oughta fix it ;-) > Hold your coffee, but I did one with a CD40106. Still in mass production, 15 years and counting. That was probably the lowest cost switcher I ever did. Pretty darn efficient as well, nothing gets hot. Oh, and this was one of the many cases where the client already had a design but had a Maxim chip in there. Which, predictably, turned into unobtanium. That called for the big knife. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Devereux on 7 Apr 2010 12:27 Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> writes: > On Tue, 06 Apr 2010 18:00:41 -0700, the renowned Joerg > <invalid(a)invalid.invalid> wrote: > >>Hello Folks, >> >>Have to correct a design with a 34063 in there. Buck, VIN 16-30V, VOUT >>slightly under 12V, 400mA max output. And no, I don't like it much :-) >> >>One of the issues is a saturating inductor. Can't change the layout and >>to be able to cram one in there I probably have to drop down to 47uH, >>maybe 68uH if lucky. >> >>SPICE says it's ok but none of the datasheets or app notes recommends >>going that low. Any reason why not? > > Too slow. IIRC there's somewhat faster (and more expensive) parts that > might be pin-compatible at OnSemi. http://www.onsemi.com/PowerSolutions/product.do?id=NCP3063 http://www.onsemi.com/PowerSolutions/product.do?id=NCP3064 They are still quite cheap, not as cheap as 34063 of course. -- John Devereux
From: Jim Thompson on 7 Apr 2010 12:41
On Wed, 07 Apr 2010 09:14:29 -0700, Joerg <invalid(a)invalid.invalid> wrote: >Jim Thompson wrote: >> On Tue, 06 Apr 2010 18:00:41 -0700, Joerg <invalid(a)invalid.invalid> >> wrote: >> >>> Hello Folks, >>> >>> Have to correct a design with a 34063 in there. Buck, VIN 16-30V, VOUT >>> slightly under 12V, 400mA max output. And no, I don't like it much :-) >>> >>> One of the issues is a saturating inductor. Can't change the layout and >>> to be able to cram one in there I probably have to drop down to 47uH, >>> maybe 68uH if lucky. >>> >>> SPICE says it's ok but none of the datasheets or app notes recommends >>> going that low. Any reason why not? >> >> Just stick a CD4060 in there. That oughta fix it ;-) >> > >Hold your coffee, but I did one with a CD40106. Still in mass >production, 15 years and counting. That was probably the lowest cost >switcher I ever did. Pretty darn efficient as well, nothing gets hot. > >Oh, and this was one of the many cases where the client already had a >design but had a Maxim chip in there. Which, predictably, turned into >unobtanium. That called for the big knife. I did off-line switchers for GenRad using only LM339's and TL431's for the control system... plus Opto Couplers for loop isolation. ...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 | The only thing bipartisan in this country is hypocrisy |