[coreboot] LPCflasher Project

Joseph Smith joe at settoplinux.org
Sat Nov 15 22:33:24 CET 2008

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
> 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
> in
> the FET will continue to allow the external voltage to be fed into 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

Joseph Smith

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