Providing high current on battery pin from USB gets it booting

[Andy Green]

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Somebody in the thread at some point said:

|> It all comes down to where is all the current going at startup.  We'll
|> get unconfused real quick when we work that out.  The regulators are
|> current limited during this phase and shouldn't be able to add up to
|> ~10W that is suspected.  It seems the culprit is via Vsys path not via
|> battery directly.
|
| The auto_converter powering IO_3V3 is designed to deliver high
current, and in
| step-down mode probably even might draw way more than 2A, to charge C1707
| (47u) and dunno what else. I wouldn't rely on current-limiter of this
| regulator on startup.
| To test it might be a simple setup to feed some 3V to IO_3V3 from
external
| powersupply, thus precharging all these Cs - dunno if this would terribly
| spoil anything on startup. However the test is so simple it might be
worth
| it.

Gah I read this as I just finished doing that test on STBY_1V2, 1V8 and
IO_3V3 rails from a second A5 that was powered first -- the first two
make no difference, but we get a good start on the unit under test if I
gave it "3V3 transfusion" from the other A5.  So good idea!

This is clearly a big clue, but our PMU variant says that it current
limits the Auto regulator to 400mA during its startup.

- -Andy
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