USB-OVP

[Joerg Reisenweber]

Am Fr  29. August 2008 schrieb Uwe Klein:
> On 8/29/08, Andy Green <andy at openmoko.com> wrote:
> 
> 
> > Fair enough, I could tell you it won't survive 12V on USB port.  Nor
> >  will any other USB device I heard of, I think it is OK.
> 
> There are these nice little (Zehner) diodes that are designed
> to fail into a shorted mode.
> TransZorb, TranSil ....
> Gives protection against overvoltage and inverted supply.

Nope, Tranzorb won't help here because the energy this component had to absorb 
is too high, so it would burn out. Transzorbs are for ESD surge suppression 
only.
But we already found a decent way to protect USB of GTA03 with a replacement 
for U4905 (GTA02-schematics), which gives OVP up to 25V continuous. Component 
cost around +/-zero, compared to original design :-). I'm happy with that!
For GTA02 though I would recommend to strictly avoid connecting it to *any* 
source with even transients >5.5V, as this is ABSOLUTE maximum of PCF50633 
USB-VBUS voltage.
At least I tell you now, not "could tell you..." ;-)
Also I tell you now there are very few universal power supplies of the type 
you might use with powered USB-hubs (barrel connector), that guarantee a 
voltage of within +/-10% of selected output voltage under all operation 
conditions, like e.g. sudden switchoff of 1A-charging mode.
However meanwhile main rant here seems to be about additional cost for a 
~$0.05 diode to clamp reverse voltage and trip the polyfuse we have in new 
GTA03 design. Obviously there is a notion customer wouldn't like to pay this 
price for increased ruggedness of USB-power input. Also there have been 
objections about the safety of such a design, not taking in account that it 
can not be less safe than what we had originally.

cheers
jOERG
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