[coreboot] LPCflasher Project

[Joseph Smith]

On Sat, 15 Nov 2008 16:09:58 -0500, <steve at fl-eng.com> wrote:
> Host USB ports are supposed to use a MosFET with internal current sense
for
> power protection. The FET signals the O/S when its current threshold is
> being
> exceeded and then the O/S suspends the port and removes power.
> 
> If you apply +5v INTO the host port, you can create current spikes across
> the
> FET which would damage the FET, trip the O/S into thinking a power fault
> has
> occured and shut down the port. Even in this state, the reverse bias
diode
> in
> the FET will continue to allow the external voltage to be fed into the
the
> motherboard.
> 
> The FET/Motherboard external voltage will keep fighting each other as the
> ripple in the motherboard +5v and your external +5v criss-cross each
> other.
> 
> This would probably be bad....
> 
That's what I was thinking, thanks.
> 
> You can diode OR all of your mother board/external supplies together and
> then run them into a buck/boost regulator. THat way the buck/boost will
> maintain a local +5v supply at your board "regadless" of external
> voltages.
> 
> Thoughts?
> 
Ah, so I can use a Buck-Boost regulator to compensate for the diode voltage
drop. That makes sense. In that case I could just cut out the simple LED
circut, use the Buck-Boost regulator to boost the 5V to 8V and put a LED
inline as my diode and the output should be a solid one way 5V power line
right?

-- 
Thanks,
Joseph Smith
Set-Top-Linux
www.settoplinux.org