next-generation PMU discussion

[Werner Almesberger]

joerg at openmoko.org wrote:
> with standard battery we get a standby of 1200 / 0.6 = 2000h max. Sounds quite 
> neglectable, but like Andy said, every bit of current counts. It's all 
> summing up.

Whoa, that's the MPU going full blast all the time. SETI at MPU, here
we come ;-)

If the MPU can sleep every once in a while, we get a low-power mode
that's about three orders of magnitude more battery-friendly.

> We can handle this. Have the MPU at some booster that's to be leveled down 
> from e.g. 3.0V default to sth like 1.8V, what is task of the MPU to set up 
> PMU.

Yeah, with an LDO, we'd just burn the power elsewhere, so it should
be a switching supply if we really care about power consumption there.
Not sure if there's anything really nice and small out there, though.

Maxim have a bunch of boost converters that don't look too bad, e.g.,
the MAX867EUA+T [1] (3.3V, 3x5mm footprint). Needs external caps,
inductor, and diode. 

The tiniest I could find is the TI TPS62700YZFR [2] (3.09V or
adjustable 1.6x1.5mm), which doesn't even need an external diode. But
it's not cheap, and it burns 300uA on its own, basically negating any
gains we could get.

[1] http://datasheets.maxim-ic.com/en/ds/MAX866-MAX867.pdf
[2] http://focus.ti.com/lit/ds/symlink/tps62700.pdf

> All the other chips will have additional FET-low-R switches (one for 
> each powerline that need it) to protect them from OV

I don't think the PCF50633 ever pulls strongly to GND. It normally
uses a weak pull down if the regulator is off, though.

> Some VoltageDetection for PowerGood if we can spend.

So far, we got away with what's inside the PMU :-)

- Werner