[gta02-core] More audio considerations

Werner Almesberger werner at openmoko.org
Mon Oct 12 23:58:02 CEST 2009

Rask Ingemann Lambertsen wrote:
> s/GTA02/GTA01/

Oops ! And I was wondering why I had never noticed that one ;-))

>    I'd say reassign the PMU LDOs. I can't imagine that their overcurrent
> protection won't be sufficient to prevent damage.

Yes, it does look good. The ~500 mA limit we effectively get should
prevent any major trouble if the output is shorted or otherwise has
an impedance that's outside the normal operating range.

We have the option of supplying LDO34 from IO_3V3 instead of Vsys.
LDO3 is IO_2V8, well within range of the maximum drop of 250 mV.

For the amplifiers, the WM8753L manual cautions on page 71 that
"If HPVDD and /or SPKRVDD are lower than AVDD, the output signal may
 be clipped.", but I think that's just a distortion of amplitudes
that shouldn't be there in the first place.

If we supply from IO_3V3, this has the following effects:

- volume does not change with type and state of power source

- better energy efficiency, because we drop from Vsys to 3.3 V with
  the DC-DC converter, not the resistive drop in the LDO

- more load on IO_3V3 (AUTO current budget is 1100 mA)

So if we move the amplifier power to IO_3V3, basically all
power-hungry subsystems except IO_1V8, CORE_1V3, and SD_3V3 would go
through IO_3V3, taking about 400 mA for LDOs from the AUTO budget.
Still, doesn't look too scary at first sight.

- Werner

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