Automotive Circuit Protection

[Nic van der Walt]

>I can see why the reverse polarity diode is a good idea, but I was
>hesitant to add another diode drop in the power supply.  Under cranking
>conditions, the battery voltage might drop pretty low.  Since the 2940
>is an LDO regulator, things will probably be fine for a 5V supply.  I
>need an 8V supply for one application, and that might prove to be a
>problem.  I think you're right, though, so I'll put the diode in there. 
>I can't find any information (or sources for) the SMS2100.  What's
>better about it?  Any links or equivalent part numbers?

The SMS2100 is a shotky diode from Diotec. Cheap and easy to source in
most parts of the world. 2A continious, 100V standoff, 0.4V maximum drop.

As for the brownout during cranking, if the margin is smaller than a diode
drop you are going to have other problems anyway. Do you need to operate
during cranking with a bad battery? 

I'm more concerned about the negative spikes you get while cranking. The
TVS is there to catch positive spikes, you need to block the negative
ones.
 
>> The cap before the regulator should be at least 100uF 63V Electrolytic.
>> It's as much part of the protection as the TVS.
>
>I just found some application notes from a TVS manufacturer.  They have
>one specifically about automotive applications.
>
>http://www.gensemi.com/product/AppNotes/appnotes.htm

We have GenSemi transorbs on about 25K automotive units. Never had any
problems.
The SGS Thompson parts also work very well.

Temperature rating of the caps is a hotly debated issue. Is this an underhood
or in-cab application? I have used the Nichicon PJ series will very good
results, although you can go to PW if size is important.

>Here's the GP30 diode I mentioned:
>
>http://www.gensemi.com/Pdfs/gp30a.pdf
Will work well, just note that the forward voltage is 1.1V.

>I'd like to try a switching supply, but that will probably have to wait
>for the next revision.  I've got too many unknowns as it is now and need
>to get this thing cranked out.  One thing I worry about with a switcher
>is the effect on analog circuits.  Some people I know have had problems
>with this and went to a linear regulator for the analog stuff and a
>switcher for the rest.  I don't know the details.  What are your
>experiences?

If you use a National simple switcher or a Linear tech switcher all you
need to do is use the recommended parts. If the caps are the right rating
and big enough you will have no problems. The Nichicon PJ's are really good
for use on switchers.

The problems people see with switchers are usually because they use scrap box
caps, or terrible PCB layout. If you want I can look at the design and layout
before you make a proto, if there are any problems that will make the switcher
not work I should be able to spot it.

N.
Regards,
Nic van der Walt

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 Cellpoint Systems SA               http://www.cellpt.co.za
 GSM Technology for Positioning and Telematics                
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