Providing high current on battery pin from USB gets it booting

Joerg Reisenweber joerg at
Tue Jul 1 02:54:57 CEST 2008

Am Di  1. Juli 2008 schrieb Andy Green:
> 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.

had no look at the particular power-up sequence. So just an idea: precharge 
via a diode from some low-power lower-voltage more-early-in-sequence 

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