larry moe 'n curly wrote:
The only known issue with things like NEC chips, is the ports
"blow out" easily, via static electricity. So that's the only
warning against owning them. Lots of NEC owners, may have a port
or two that no longer works, and the failure is believed to be
due to static sensitivity.
The purpose of 100uF caps, in USB port designs, is to handle inrush
surge (charging the capacitor inside the USB peripheral itself). An
Intel report, showed an oscillogram of the current flow transient,
and there can be a 5 amp spike when a USB peripheral is plugged in.
At that time, Intel recommended the host port be protected with
100uF bypass, as that holds sufficient charge such that the host power
bus doesn't dip too much. Motherboards typically use electrolytics
for the purpose, which are fine for the job. If I was designing
it, I'd put in an electrolytic without a second thought, whereas
with tantalum, I'd need to do further research.
At least some flavors of tantalum, are sensitive to current spikes.
Adding a series limiting resistor, as this article suggests, would
defeat any advantage a tantalum might offer.
http://www.avx.com/docs/techinfo/surgtant.pdf
There is some upper limit, on the size of bypass cap inside a
USB peripheral, and the host side cap has to be correspondingly
larger. And from that, that is where the 100uF comes from - the
ability to plug a second USB device into a USB stack, without
the first device being upset and losing state.
With respect to the NEC USB2 port failures, this is purely from
anecdotal reports, of cards where one or more ports no longer
functioned, and the others worked fine. The USB cards they make
now for $10, generally only have the main chip, and a few electrolytics.
The National part is one of those "current policeman" type devices.
The datasheet says it trips at 1 amp, which means a sustained
(after the transient) current flow of 1 amp, cause the flag
to be asserted, and the external port to have +5V power cut.
That part doesn't touch D+ and D-, and if you wanted to add
static discharge protection, that would be an entirely
different research project.
The static protection solutions here, are rated in terms of
stray capacitance, and that is what degrades high frequency
signal performance.
http://www.semtech.com/esd-protection/USB-ESD-Protection/index.html
There is a nice eye diagram on page 14, showing what a USB port
with transient protection on it looks like. The PESD5V0X1BL
are $0.16 a piece, and you'd need two per port for a total
of $0.32 added per port (on a $10 retail USB card). You'd have
to bump up the price at retail to $15 to cover it. I think
most fly-by-night USB card makers, would rather let the
NEC ports blow, than do that.
http://www.nxp.com/documents/application_note/AN10753.pdf
