MC34063 with small inductors

I don't know yet. The package just arrived but I have a much more pressing problem from an other client to deal with right now. Also a chip that seems to not exactly do what I think it should. For that one we have lab bench scope plots and the manufacturer is investigating. This one simulates alright but chokes in real life.

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I have a NCP3063 Buck on a Breadboard now. I'm using it to test cross-regulation on a coupled inductor (+/- 5V out). It only terminates on the falling edge of Ct.

See scope shot. Ct ramp and switch node same as Jorge's sim.

You can see the glitch on the Ct ramp when the switch changes states.

Joerg I used the MC33063 for a 15W DCM buck before, it was the first SMPS I did on a PCB in college and it ran fine. Mind you it needed 3 x

1mF of output capacitance to get the ripple down to something reasonable. The inductor was 12uH for 5V out at 3A; 15 to 20Vin, 60kHz. The peak current was a little over 6A ;-) using an external P-Fet.

I think the only reason they don't show or recommend DCM is because the already high and modulated ripple you get with Hysteretic type converters is much worse in DCM particularly at higher power levels. This isn't flattering for their IC so they just show CCM examples. Just my thoughts.

Here is the output ripple 12Vin, 5V out, 0.4A. This is with a 220uH inductor and a 330uF PSA POLYMER output cap. Doing a proper PCB and a small LC filter (bead MLCC) would get rid of those HF oscillations

This is using the NCP3063 at 175kHz

Thanks, Hammy. If you never see it turn off before the peak ramp reversal then it behaves according to the datasheet. That would mean the SPICE model isn't quite up to snuff or LTSpice misinterprets something in there.

3000uF on the output? Yikes! I don't even have that much on the rectifier after the mains transformer.

The RF is fairly easy to lowpass out but the lower frequency ripple from the slightly hysteretic behavior of this chip drops in spectrum a lot the higher the inductance and the larger the output cap. But in this app we can live with it since that rail is only used for relay drivers. There's another regulator downstream for sensitive stuff.

I don't think I'll be able to keep it in DCM considering the cramped space I've got on the board.

Thanks for posting the scope shots, looks encouraging :-)

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Is it happening simultaneous with _on-set_ of ramp-down? ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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      The only thing bipartisan in this country is hypocrisy

On the 34063? I have simulated the dickens out of that and the comparator never shuts off the switch at onset, just occasionally in the middle of a ramp-up which it ain't supposed to be able to do. And in Hammy's case doesn't do.

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But I can't convince any of these companies to hire me to do high quality modeling... they want me to submit a quote for about the equivalent of one day's work, when it would generally take a week to do a real quality job :-(

I guess they really don't care.

Have you tried other models for the 34063? So far I've seen two, but haven't had time to try either one yet.... I Usenet while running sims that might pay off :-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
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| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
      The only thing bipartisan in this country is hypocrisy

Sometimes I have those request as well but never from exisiting clients. If the conversation starts with "We've got something that should be rather trivial for you ..." I can smell it, and usually must decline.

Grand total of four, same problem.

Same here. But now I've got a li'l RF project for dessert, solder iron time :-)

And the last Weller ETS tip just bit the dust, have to see if I can revive that with some sandpaper for now so it'll hang on until Saturday. Did I already say that I hate RoHS solder?

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You use it even for test circuits?

Oh no, never. But I do have clients who market internationally so there's the occasional RoHS board crossing my lab bench.

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You just can't fix stupid. :(

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Lead free solder is Belgium's version of 'Hold my beer and watch this!'

te:

d

Geez these threads get messy. First of all, have you called apps?

Generally if there is a limit, there is science behind that limit. But in the chip biz, you have to pick limits that WILL work for the tens of millions of parts you crank out. Probably more for this part. So you get it to work in spice, but they fab a "slow" boat of wafer that is still within spec, and the thing doesn't work. Now if your volume is low and you temperature test your boards, I wouldn't be too concerned with using an external component a bit out of the spec limit.

time

=A0Burp,

n and

I

ping

ICE.

Propagation delay? When the ramp comparator trips, it'll take a minute or two to turn the timing ramp current source around. So to speak. :-)

-- Cheers, James Arthur

That was a good one. Maybe there's a bureaucrat in the chip and the comparator needs request a permit every time it trips :-)

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Are you trying to run it faster than spec? Faster rise times capacitively tripping a comparator? ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
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      The only thing bipartisan in this country is hypocrisy

Not yet, another more pressing problem came up. Other client, other chip. That one behaves ok in the sims but doesn't behave like the datasheet says in real life. Hurumph ...

Again, I am _not_ operating the thing outside its spec limits.

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and

No, in that mode it's actually running way slower than spec, around

30kHz while spec says 100kHz possible.

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We're using some tiny 3 MHz switchers now. I think I've seen datasheets on 10 MHz switchers.

The power isolators inside the ADI powered data isolators run at 300 MHz, I think.

John

he

Two orders of magnitude faster than the MC34063. Sweet. Aren't core and skin losses deadly?

I've done 300KHz, discrete, but 50KHz-ish is usually more efficient.

-- Cheers, James Arthur

The compensation is that you need very little copper and ferrite at 3 MHz.

John