[Kat Manton]

Hi Dave,

Originally Posted by davegsm82

You've just mentioned one of my old favourites, the 34063.

I think it might turn into one of mine. In a past life I was once paid to spend a fortnight building a small pile of dead components and making smoke. Devices like this make designing switching converters almost trivial, that and being able to simulate them in LTSpice. I think more of my circuits are going to run from a single 9V or 12V DC supply if I can generate extra rails this easily!

Originally Posted by davegsm82

It should be noted that they are generally not good at boost applications, however they should be fine in this position.

I think it'd be fine in a boost application with a near-constant load. In this application, VPP to the device is switched; in my worst-case simulations with the original circuit, using a pulsed load of 100mA, the output voltage 'sagged' a bit before the converter noticed and pumped it back up. (Torturing it in LTSpice is quite entertaining!)

Hence my idea of following it with a linear regulator; now, the output of the 34063 circuit drops a bit when the load is connected, but not enough for the linear reg. to drop out.

Originally Posted by davegsm82

I have found in practice that there is a MASSIVE variation in brands of this IC, I've made boost converters using generic chinese versions and the smoke was very eager to escape, however using an ST34063 (ST Micro's) or a KA34063 (Fairchild Semi) the circuit performed just fine.

Thanks for that. I was about to order a pile of them from China; now I think I'll order a smaller number of 'known-brand' ones; it'll probably work out cheaper if the smoke stays inside more of them

Originally Posted by davegsm82

These things are the basis of almost every in-car charger since the early 2000's, so are reliable technology. My in car charger was stripped and I fitted a inductor which didn't saturate as soon as you were drawing ANY current.

Does everyone skimp on inductors? You'd think copper was expensive...

Originally Posted by davegsm82

Have a look at this datasheet, http://www.intusoft.com/onsemipdfs/AN920-D.pdf

Its the 'Good' app note for that IC and explains quite a lot (you don't need to go into the maths), page 12 shows a ripple reduction technique which i've used successfully in the past.

I've got that one; it's available from the ON Semiconductor link in post #5.

The ripple reduction technique you mentioned formed the inspiration for my idea for having the step-up converter track the output of the linear regulator.

Imagine the zener in Fig. 13 is variable and set to the output voltage of the linear regulator. That's effectively what the first op-amp section does, forcing the 34063 to track the output voltage, 1.25V above it.

As the differential amplifier has unity gain, the ripple voltage is presented in full to the comparator input of the 34063 (as it is when a zener is used), rather than being divided down as it would be with the basic potential divider.

So I get reduced ripple and the voltage remains 1.25V above the output voltage, enough for the linear regulator to work. Am I allowed to call that bit 'elegant'? I'm rather pleased with how it worked out.

Originally Posted by davegsm82

Since i've not seen the programmer schematic, make sure that the short circuit/saturation current resistor (Rsc between pins 6-7) is actually fitted

It's fitted in the original circuit (my reverse-engineered diagram is attached to post #3) but is 0.5R. I've re-calculated everything and ended up with 0.22R. Luciano ended up with 0.33R; I think I've aimed for a higher maximum current (over-engineering everything is a habit of mine!)


Hi Martin,

Originally Posted by Dekatron

Here is a good calculator for the MC34063 that I have used a few times: http://dics.voicecontrol.ro/tutorials/mc34063/

I found that calculator, but TBH I found it pretty straightforward to work through the calculations as detailed in AN920, then simulate it in LTSpice and see if it works as expected.

Originally Posted by Dekatron

The MC34063a has never failed me so something must be wrong with the wiring or components to have it overheat like you describe.

There's definitely something wrong with a component, but I knew that in between ordering it and it arriving; I found Luciano's Converting Willem Programmer PCB3b to PCB4.5 page.

The 100uH inductor is a tiny bobbin type, way too small, you can make it out in the photo attached to post #1 in between the 8-pin DIL 34063 and the TO220 7805, upper right.

It's saturating with only a 25mA load; my test in post #8 was just proving Luciano was right. I found one post in another forum describing how the inductor started smoking when someone tried to program an older '27C' EPROM.

It's a known problem and is partly why I redesigned the VPP supply; there's no room on the programmer PCB for a bigger inductor so I thought I'd build the whole thing on another board, that meant there's room for more bits, one thing led to another...

I've got wire and I've got suitable toroid cores, I just need to put the two together if I can stop procrastinating...

Kat