buck boost gsm power supply
That depends on your application. I assume you are talking about the power supply for a GSM modem with GPRS. If you need to downsize a voltage from 5-7V to 3,3, you can use linear power supply. But if the input voltage is higher, you will need switch mode power supply, ex. buck converter with MC34063 or similar. Just make sure you design the SMPS with a good transient response. During transmission, the modem can consume up to 2A from the power supply, and this is a problem if your power supply will have a poor transient response. The voltage will drop and can shut down your modem. Also, use low ESR caps for filtering. Pay attention to the PCB also, use thick traces between the ps and modem and as short as possible.
/pisoiu