RE: [Balloon] B3 supply voltage?

Unfortunately the limitation to 4v6 on P2 is real.

The problem comes from the V7 output on the MAX1586
Which turns out not to be a regulator (when you RTM properly) but a switch,
this is fine when VBATT comes from a nice Lithium backup battery but not so
nice when its fed from 5v. The series diode drops a bit as does the switch
in the MAX1586, but combined they still give an upper Vin of 4v6 because
VBATT into the PXA has an upper limit of 3.75v or 0.2/0.3v over VDD_IO.

This is fixed on E1 with an optional 3v0 LDO if you aren't using a battery.

The red glow on the reset LED is also fixed on E1.

David.

-----Original Message-----
From: balloon-bounces@balloonboard.org
[mailto:balloon-bounces@balloonboard.org] On Behalf Of Steve Wiseman
Sent: 23 September 2006 22:40
To: balloon@balloonboard.org
Subject: [Balloon] B3 supply voltage?

While thoroughly violating the USB spec, I've started powering my B3 from
my PC's USB port (nominally 5V, IIRC, the USB standard permits 4.5-5.5V).
The red Reset light glows dimly when I do this, since the LED's pulled up
to VRaw, rather than 3V, but the board works just fine.
The schematic states 4.6Vmax on VRaw (which turns into VBatt). I can't
remember why, for the life of me. The MAX1586 (Xscale power) claims to be
OK up to 5.5V, and the TPS75003 (FPGA power)is good to 6.5V. Is this a
hangover from an earlier design iteration, or a real limit? (being able to
be USB powered has a certain simple charm - I'm now running with a 1N4001
in series with the PSU, to stop the red glow and reduce the damage a
backwards-power blunder would cause...

Steve


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