RE: [OT] iPad query...

["Dave McLaughlin" <dave_mclaughlin@xxxxxxxxxxxxx>]

Hi Paul,

I think that is more to do with the USB port itself. This has to be
negotiated for supply current.

The other issue we may have is the actual hardware in the iPad controlling
the max input current to the charger circuit. I recently designed such a
circuit into a battery powered display and I could set the max input
current
of the charger via a resistor. I have this set for 1A for use with an
external supply but I can also use a USB port to charge and the current is
then assume to be 500mA max input.

If we are lucky, the charging circuit in the iPad will automatically use
the
max input of whatever is plugged into it, say 500mA for USB and 2A for the
external supply. The real issue is if the charging circuits current limit
is
set to 500mA if it detects that there is a valid USB connection and in that
case, even having he 2A supply plugged in, it won't draw more than 500mA
and
we are back to square one.

With my design I can set the output current limit of the desk power supply
to 3A and yet the charger only pulls a max of 1A as this is the maximum
rate
that the battery can be charged at.

As I don't have access to an iPad it would be really handy if someone could
do the test with 2 butchered USB cables. I can provide a drawing if you
need
one?

Dave...
---
Very funny Scotty, now beam down my clothes!!!
---



From: ukha_d@xxxxxxx [mailto:ukha_d@xxxxxxx] On Behalf Of
Paul Gordon
Sent: 18 March 2011 15:23
To: ukha_d@xxxxxxx
Subject: RE: [ukha_d] [OT] iPad query...


For some reason, I thought that "was" the case... - i.e. that
iPad/Pod/Phone
devices' charging ports aren't just "dumb" power-in sockets...
but that they
negotiate with the charging device to agree the charging current between
them... - which is why many docks cause the iDevice to display a message to
the effect of "Charging is not supported with this accessory"...

I have a number of cheap iPod docks which exhibit exactly this behaviour...
- seemingly the device *does* charge, even though the iDevice says it
wont...

Food for thought...

Paul G.




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