Electronic – Establish connection between analog and GSM modem

I used some generic modules, so here are some values to work with:

• voice call: 250 - 300 mA (unactive unused bursts: 20 - 50 mA)
• GPRS (2Tx, 3Rx slots): 340 - 410 mA

This assumes having a matched antenna. The current consumption itself depends on the transmitted power and is thus dependend on the network. If you have a highly unmatched antenna, a voice call can draw up to 2500 mA.

My experience (across four different cellular modems, both GSM and CMDA), is that they are intended to be run directly off the battery. For the AirPrime SL808x series (as an example as I haven't worked with the SL6087), VCC_3V6 (which is used for the power amplifier), is rated at 3.3v min to 4.3v max, with a typical value of 3.6v -- this closely matches a nominal 3.7v Li-Po battery which may reach 4.2 volts on a full charge.

The digital section of the cell module (logic levels), on the other hand, runs off a 1.8v rail which is internally generated. It is also output on the VREF_1V8 pin (1 ma max).

So you don't really need a high power buck-boost regulator at all. If you need more than 1 ma to power your logic level conversion circuitry (assuming 3.3v logic levels elsewhere), you will need to provide an LDO regulator to generate 1.8v from the 3.3v rail.