[gta02-core] Modem schematic / RF help needed

Rask Ingemann Lambertsen ccc94453 at vip.cybercity.dk
Fri Sep 11 01:01:50 CEST 2009

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 mOhm at 4.5 V and 2.9 A, 60 mOhm at 4.5 V
respectively (and even with the same pinout):

SuperSOT-6 package, 2.8 A, 55 mOhm at 4.5 V:

   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 mOhm at 5.0 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
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