OpenPCD problems, opcd_test can't get serial #

[Milosch Meriac]

Hi Robert,

Robert Fitzsimons schrieb:
> I also had a quick look at the openpcd schematic again and noticed the
> OPA354 opamp, most people won't use the RC632 AUX output but the opamp
> is still being powered.  The datasheet sets the typical quiescent
> current of the opamp at about 4.9 mA.  I'm wonder if this using more
> current then that, I get a little bit under 5V from the mount point with
> a volt meter but I can't test the current used while it's in circuit.
> 
> So I'm wondering if there would be any benefit in removing the opamp?

No - I'm pretty sure that the problem has to do with the too big
capacitors (around 500uF total). This results in too high
rush-in-currents. This high current triggers the over-current detection
of the USB hub. USB allows only capacitors in the range of 10uF.
I guess the best way forward is to introduce a active MOSFET based
rush-in current limiter and disable the 5V rail until enumeration is
done and then also ramp it up in a current limited way. The static power
consumtion is perfectly fine for the device already.
What I can do is to add an MOSFET for the OPAMP power supply - so it can
be disabled while it's not used.

Warmest regards,

Milosch
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