MC34063 with small inductors

Just stick a CD4060 in there. That oughta fix it ;-) ...Jim Thompson

=A0 =A0 ...Jim Thompson

=A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

Won't a 555 do the trick?

Even better !-) ...Jim Thompson

Won't a 555 do the trick?

I think it needs an ideal diode. :)

Hell no. You need a PIC!

Jan can do it! He's the PIC master! 8-p

Tim

and a few 1K pots

What's the frequency? It's spec'd for 100K max.

Those old things are slow. With 24 in, 12 out, on time is ~5 usec at

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.

John

Too slow. IIRC there's somewhat faster (and more expensive) parts that might be pin-compatible at OnSemi.

Best regards, Spehro Pefhany

Try the NCP3063 from On Semi, it's a faster pin compatible for 34063

Grant.

Need a 556 to get 400mA :)

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.

Because it's marginal timing-wise, places high stress on the inductor and switch, and is likely inefficient.

-- Cheers, James Arthur

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

It's a pretty crude PWM chip but what can ya do?

These things run discontinuous nearly all the time. That would be ok.

Can't do a relayout on this one, else I'd already be home :-(

Although it does run in SPICE.

Thanks, got to take a look.

Thanks! Technically I can't change it but if needed it's needed. It's only 50% faster but maybe that helps here.

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.

If I could just find one that fits into the cramped space :-(

The NCP3063 that others have suggested might work but it's not breathtakingly faster, just by 50%.

Inductor stress would be ok, just has to stomach the current. I do get

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.

They are still quite cheap, not as cheap as 34063 of course.

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