MC34063 Vcc level

[throbscottle]

I'm using a MC34063 to generate a negative supply for a vacuum fluorescent display, but it needs to be activated after the main 5v rail is up. I thought the easy way to do this is use the 5v supply to turn on the chip's Vcc connection, but THEN I thought, can I use it as the Vcc supply directly? There's no documentation I could see saying I can or can't. The main supply will be between about 7v and 12v.
What do you lot think?

 

[augustinetez]

The datasheet I have says the MC34063 will run from 3-40V so can't see any reason to not run it from the 5V rail.

With the right value components, it should be ok supplying the negative rail as from memory the VF display requires only very little current from the -Ve supply.

Only thing to watch for is to decouple it well so it doesn't feed any noise back on to the 5V rail.

 

[throbscottle]

You have echoed my own thoughts, though I hadn't thought about decoupling.
Negative supply works well, btw
Just to be clear, I'm talking about the output transistors being supplied from the main (up to) 12v supply, which also feeds the 5v regulator, and supplying Vcc with just 5v - not supplying the whole thing from 5v. That is what you thought I meant, I hope!

 

[augustinetez]

Perhaps just monitor the output with different voltages on the 12V rail to confirm the MC34063 is regulating it properly.

Other than that, can't see it being a problem - the datasheet shows an application of the MC34063 running from a lower regulated supply and the switching transistor from the main rail.

 

[throbscottle]

Well, the result of that is the chip is fried. Don't know if it's due to this particular experiment though, it's the 2nd one on this lash-up to fail. No idea what caused the 1st one to go. Wonder if it's because I've got a bunch of cheap eBay ones?

 

[throbscottle]

Who's datasheet have you got btw? I have the TI one but the content is ancient.

 

[rjenkinsgb]

The Onsemi version always shows the output load power and the device power from the same source:

https://www.onsemi.com/pdf/datasheet/mc34063a-d.pdf

Page 8 shows a negative out from +5 in setup.

 

[augustinetez]

Who's datasheet have you got btw? I have the TI one but the content is ancient.

STI Datasheet from 2013 page 23

 

[throbscottle]

Ahh, not quite what I meant. However I've now got two datasheets more helpful than the ti one.
Anyway, I tried again with an old one that was pulled from something and it works fine. Regulation is better too.

 

[throbscottle]

I eventually found that my idea doesn't work since Vcc forms the other side of the Ipk detection connection. Apart from which, it went bang. Blew the top right off...