MC34063 with small inductors
in [Design]
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From: Grant on 7 Apr 2010 02:01 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 Grant. -- http://bugs.id.au/
From: Kevin McMurtrie on 7 Apr 2010 03:30 In article <hpgmiq$81r$1(a)speranza.aioe.org>, "tm" <noone(a)msc.com> wrote: > "brent" <bulegoge(a)columbus.rr.com> wrote in message > news:91110477-cc99-4dc0-844c-e4322b214131(a)n13g2000vbn.googlegroups.com... > On Apr 6, 9:10 pm, Jim Thompson <To-Email-Use-The-Envelope-I...(a)On-My- > Web-Site.com> wrote: > > On Tue, 06 Apr 2010 18:00:41 -0700, 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. > > > > >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 ;-) > > > > ...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 athttp://www.analog-innovations.com| 1962 | > > > > The only thing bipartisan in this country is hypocrisy > > Won't a 555 do the trick? Need a 556 to get 400mA :) > > I think it needs an ideal diode. :) -- I won't see Google Groups replies because I must filter them as spam
From: dagmargoodboat on 7 Apr 2010 11:20 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. 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. Or just get a faster part. > 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. -- Cheers, James Arthur
From: Joerg on 7 Apr 2010 11:59 John Larkin 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? > > What's the frequency? It's spec'd for 100K max. > During ramp-up it runs around 130kHz, then drops to around 30kHz depending on load. There ain't much flexibility because of the way the 34063 comparator works. It can issue a cycle but it can never stop one. That's why it behaves like one of those old aircraft rotary engines during landing, rat-tat-tattatta-tat ... poof ... rat-tat-tat ... It's a pretty crude PWM chip but what can ya do? > Those old things are slow. With 24 in, 12 out, on time is ~5 usec at > 100 KHz. Ripple current into 47 uH is 1.25 A p-p. Not good. It will go > seriously discontinuous. Does Spice really approve? > These things run discontinuous nearly all the time. That would be ok. > If you can't change the layout, push the frequency above spec and go > with 68 uH maybe. Best of luck. > > Or kluge in one of the new National packaged switchers, the dpak ones > with inductors and all. > Can't do a relayout on this one, else I'd already be home :-( -- 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 12:00
Spehro Pefhany wrote: > 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. ... Although it does run in SPICE. > ... IIRC there's somewhat faster (and more expensive) parts that > might be pin-compatible at OnSemi. > Thanks, got to take a look. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |