I'm a bit confused on some information I have read regarding USB
over-current limiting.
I was wondering if someone could offer their USB expertise on a question I
have.

I've been looking at the various USB specifications that have come out (revs
1.0, 1.1 CB, and 2.0).
From reading the specifications, it sounds like you must limit
short-circuits the USB supply to 5.0 amps or less.

But then I was reading a PolySwitch technical paper that says "short circuit
or other fault condition, current output must be limited to below 5A within
60 seconds."

I can't back up this statement from PolySwitch by reading the USB specs --
there is no mention of UL60950 or 60 seconds that I can find in the USB
specs.

My questions are as follows:

1) Does anyone know what the *maximum* steady-state *short-circuit* current
that is permissible under the USB specifications?

At first I thought it was 5.0 A or less, but now I'm not so sure anymore.

2) Is there a time-related factor associated with the short-circuit?

I realize that PTC protection devices are commonly used in USB ports and
they have a finite tripping time.

I have some excerpts from the sources below.

Thanks for any help. I appreciate it. Thank for your time.

Best regards,

Ken Pergola




EXCERPTS FROM DOCUMENTS BELOW:
------------------------------


From PolySwitch Technical paper:
--------------------------------

"Using Protected Power Switch and PolySwitch Devices in USB Meeting USB,
IAPC, Cost, User, and Performance Requirements"

"This paper will discuss USB circuit protection requirements
Where current limiting is required, the requirements of UL
60950 must be met. For USB, this means that in a short circuit
or other fault condition, current output must be limited to
below 5A within 60 seconds."



From USB Specification:
-----------------------

7.2.1.2.1 Over-current Protection
---------------------------------

The host and all self-powered hubs must implement over-current protection
for safety reasons, and the hub must
have a way to detect the over-current condition and report it to the USB
software. Should the aggregate current
drawn by a gang of downstream facing ports exceed a preset value, the
over-current protection circuit removes
or reduces power from all affected downstream facing ports. The over-current
condition is reported through the
hub to Host Controller, as described in Section 11.12.5. The preset value
cannot exceed 5.0 A and must be
sufficiently above the maximum allowable port current such that transient
currents (e.g., during power up or
dynamic attach or reconfiguration) do not trip the over-current protector.
If an over-current condition occurs on
any port, subsequent operation of the USB is not guaranteed, and once the
condition is removed, it may be
necessary to reinitialize the bus as would be done upon power-up. The
over-current limiting mechanism must be
resettable without user mechanical intervention. Polymeric PTCs and
solid-state switches are examples of
methods, which can be used for over-current limiting.

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