next-generation PMU discussion

[joerg at openmoko.org]

Am So  20. April 2008 schrieb Andy Green:
> Somebody in the thread at some point said:
> | [ Context: we're looking for a PMU for GTA04 and beyond. One possible
> |   choice is to use NXP's PCF50633  again. ]
> |
> | Andy Green wrote:
> |> NXP is "the devil we know", but there is no integral codec.
> |
> | In the past, we had lots of trouble with the NXP PMUs, since their
> | default settings (such as which regulators are active in what state,
> | and what voltage they default to) are mask-programmed in the chip,
> 
> | However, since GTA04 will have an MPU, we're considerably better
> | off there. What we still have to worry about is:
> |
> | - feeding the MPU (unfortunately, the upper end of the MSP430's
> |   voltage range is only 3.6V. Otherwise, we could just have
> |   connected it to Vsys.)
> 
> | So making the PCF50633 fit wouldn't be quite the pain it was in
> | GTA02.
> 
> Agreed... VSYS is not an ideal way though because the MSP430 is happy to
> run at 1.8V if you don't want to blow the flash or 2.2V if you do.
> These are really big savings over 3.6V in power terms... 1mA at 2.2V is
> 2.2mW while at 3.6V it is 3.6mW, over time that'll add up.

Not taken in account the figure of:
I(AM at 1MHz)@2.2V = 330uA typ
I(AM at 1MHz)@3.0V = 500uA typ

so 
2.2 x 330*10E-6 = 726uW
3.0 x 500*10E-6 = 1500uW
is more like a exp2 rise, not linear.
for 3.6V its probably
3.6 x 600*10E-6 = 2160uW
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.


> I guess if we go this way we have to examine the 50633 defaults closely
> to see what the other options are if any.


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. All the other chips will have additional FET-low-R switches (one for 
each powerline that need it) to protect them from OV until MPU has set up the 
PMU properly (and maybe even tested for PowerGood), then enables power to the 
real good stuff. Not nice but reliable and sturdy. Cost for us: 1 MPU-GPIO, 
~4 FETs. Some VoltageDetection for PowerGood if we can spend.


/jOERG