The power supply range of SIM900 is from 3.2V to 4.8V. The transmitting burst will cause voltage drop and thepower supply must be able to provide sufficient current up to 2A. For the VBAT input, a bypass capacitor (lowESR) such as a 100 ?F isstrongly recommended; this capacitor shouldbe placed as close as possible to SIM900VBAT pins. The following figure is the reference design of +5V input power supply. The designed output for thepower supply is 4.1V, thus a linear regulator can be used.