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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.
Thanks pisoiu for your reply.
I am using GSM/GPRS modem machine to machine interface & input voltage range is 6-8V & operating voltage is 3.6V .Are u talking about linier power supply such as 78XX,317 or lm2575?will they provide sufficient peak current to modem?
LM2575 is switch mode, quite at limit, but I think will work. The current draw is maximum 2A but not continuous, only during GSM TX bursts (and this only in the worst case, low GSM signal). Same for linear 78XX and 317, but in TO220 pack. In both cases you will have to use before the modem good filtering with low ESR caps and good layout techniques (short and thick traces). Usually all the modem datasheets contain informations about this issue, take a look inside them.