On Mon, Sep 07, 2009 at 04:25:49PM -0300, Werner Almesberger wrote: > Dave Ball wrote: > > > Should the modem be powered from VBAT or VB_GSM? I've just noticed > > that I had assumed the latter, but looking at U1750, VB_GSM is on > > the battery side of the switch, where VBAT is actually the switched > > supply. > > At least the RF amplifier has to be supplied directly from the > battery, because of its high current peaks.
Why? You can get power switches with a low enough on resistance. Two 6-TSOP examples of 3.3 A, 55 [email protected] V and 2.9 A, 60 [email protected] V respectively (and even with the same pinout): http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=NTGD1100LT1GOSCT-ND http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=SI3865BDV-T1-E3CT-ND SuperSOT-6 package, 2.8 A, 55 [email protected] V: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=FDC6331LCT-ND The Telit GE865 Hardware User Guide has examples of how to power the modem from an LDO as well as from a step down converter. Why would it be a problem to have a power switch between the battery and the RF amplifier when you are supposed to get away with an LDO? The power switches listed above are quite a bit better than the 1.5 A, 110 [email protected] V RT9702A one the GTA02 uses (RT9702A). About half the on resistance and about twice the current rating. We also have a 100 uF capacitor on the modem power vs. the 22 uF in the GTA02A6 and only 10 uF in the GTA02A5. -- Rask Ingemann Lambertsen Danish law requires addresses in e-mail to be logged and stored for a year _______________________________________________ gta02-core mailing list [email protected] https://lists.openmoko.org/mailman/listinfo/gta02-core
