At 06:44 PM 2/24/2008, you wrote:
>I am working on an application where the standard R/C timing
>forovercurrent protection in a device is not sufficient.... For those
>really interested we're talking about a L6206, driving a load where
>there is an internal capacitor.
>
>What happens is that the L6206's overcurrent protection kicks in 
>during the initial capacitor inrush.  The amount/duration of the 
>current is not
>sufficient to damage the L6206 (it's within the ratings), but it is 
>enough to trigger the overcurrent for long enough that the device shuts
>down and the load never fully powers up.   I can tweak the R/C 
>values and get it to power up, but then short circuits will occasionally kill
>the device, due to the short cycle.   And so on.
>
>My thought is to put an external circuit between the overcurrent 
>detect output and the enable pin which allows me to more finely adjust what I
>want the overvoltage circuit to do.  The goal is to be able to delay 
>the shutdown by around 5us or so (adjustable) and then keep it off for a few
>seconds.  I was thinking about the 8 pin pics - however, with a 
>500ns instruction time, this sure doesn't look reasonably likely to work for
>me - especially if I want to control both OCD output/input 
>pairs.   I was also looking a bit at the AVR equivalent since it 
>looks like the
>instruction time could be as low as 125ns, but still, I'm not sure 
>that's fast enough.   There is also the possibility of using an external
>XTAL, but I'd like to avoid that.
>
>Because this is for a design which is fairly late in it's design 
>cycle (somehow we didn't catch this until "beta"), I don't have a lot of space
>- and I'd like a small solution.   I was wondering if there was 
>something else I missed...  I will be playing a bit with a 556-like
>circuit, but intuitively it feels that this would have similar 
>issues to the R/C which won't work for me.
>
>-forrest

Do you *really* need to lock it out for seconds? A shorter time, such as
100msec would be a lot easier-- just a pot + R from OCDa to discharge a
ceramic cap of say 47nF and (eg. 100R pot + 10R) and charge the cap 
through a 2M
resistor to Vdd. That would give you a lock out time constant of about
100msec, and a trigger time constant of about 5usec +/- 2usec. No active
parts at all.

Otherwise, you might also look at the HC123 and similar parts.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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