A5 Power routing "minor issues" for some modules
Neng-Yu Tu (Tony Tu)
tony at openmoko.com
Tue Feb 26 15:02:42 CET 2008
> Hi folks -
> Thisafternoon I noticed that
> - vibrator
> - USB Host function (plug USB device into gta02)
> - whole GSM subsystem
> are all powered specifically from Vb, which can be absent or unusably
> reduced during operation without a battery due to charger state machine
> actions. Every now and again the charger wants to stop charging and
> make a measurement of the battery's own voltage.... no battery in there
> means no usable Vb then. If we don't run the charger, there won't be
> anything put on Vb at all in the USB-no-battery case.
> It's not a huge deal though because you can run from USB with a battery
> in there okay, and it will be powered from the USB 99% of the time,
> including charging the battery.
> A solution can be to move these guys to take power from the VB_SYS rail
> which is the output of the power source summing stuff. The chip has a
> 2.2A limit here on the chip from the battery to VB_SYS.
> However I talked it over with Tony and he remembers bad things happening
> with a similar idea for a switched GSM power rail on GTA02 A2, we dug
> one out and it used a TPS2023...
> It is rated quite similar to the Vb <-> VB_SYS switch in the pcf50633
> (also specified for 2.2A max) and likely has a similar Rdson for the
> switches (33mR for the TPS2023). It's really not clear why that
> solution made trouble, the standard reasons would be insufficient caps
> and maybe too long a loop for the power, but eyeballing the board it's
> not too far away from the GSM stuff (nor is the pcf50633 in the GTA02-A5
> case), but that doesn't say anything about the routing.
Following link explains why I am so worry about GSM part change. GSM
might have instant pulse of 2A power consumption.
Standard USB only 500mA max. And the switch has very bad performance
under low temperature. And for mobile device design, usually it would
not power up if battery not exist, even adapter with power plugged in.
Let us look at an example of a GSM mobile unit that transmits at 5.0W,
but due to internal loss in the phone, 10.84W is required from the
battery. The GSM mobile unit is a 5V system that has a quiescent current
of 0.2A and draws 2.17A when transmitting voice or data packets.
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