I want to integrate an AirPrime SL6087 GSM modem into a design, this is my first try at working with a GSM module. The datasheet states that the modem draws 2A for 1mS every 4mS (max) at 3.6V, the problem is that I intend to run this project from a single Li-Po battery and want to use a buck-boost regulator so I can keep the unit alive for as long as possible (running it down all the way to 3V). The only regulator I can find capable of this is the LTC3113 but there doesn't seem to be stock anywhere. The LTC3112 is also a possibility but it's 2.5A output current is rated at 5V input not down to 3V.
My question is this, there's a lot of small GSM products on the market, how do these go about keeping the power supply stable while running on a single Li-Po battery? I have this nagging feeling that I'm over complicating this...
The complete datasheet for the module is only available after logging in, registration is free however. Here's a quick recap (page 20):
In connected mode, the RF Power Amplifier current (2.0A peak in GSM/GPRS mode) flows with a ratio of 2/8 of the time (around 1154uS every 4.615mS)
Answer
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.
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