Application Interface; Power Supply Design; Vbat Design - SimCom SIM900 Reference Design Manual

SIM900 Reference Design Guide Notes

2 Application Interface

2.1 Power Supply Design

2.1.1 VBAT design

The power supply of SIM900 is from a single voltage source of VBAT which normal operating
range is form 3.4V to 4.5V. The peak working current can rise up to 2A in maximum power
transmiting period, which will cause a voltage drop. So the power supply must be able to provide
sufficient peak current, if not, the voltage may drop lower than 3.4V, and the module will auto
power down. Typically, VBAT can be set to 4V.
SIM900 can be used in a wide range of application, the power supply design is deeply depending
on the power source.
When the input is a 5V/2A adapter, a LDO linear regulator can be used in the design because the
drop out between input and output is not so big. Figure 1 is the recommended circuit with
MIC29302. Please also pay attention to the heat dissipation of the LDO. Usually, pouring a copper
plane on the PCB is an effective way to the heat sink problem of the power IC.
Figure 1: LDO Power Supply
NOTE 1:
The bypass capacitor C102 in Figure 1 is selected strongly depending on the rated current of the
power source and the power IC .If both of the rated current are 2A, a low ESR tantalum capacitor
(220uF or smaller) close to the VBAT pin is enough. Or you should change C102 to a big value
according to the practical application. The rated current should not be less than 800mA (5V
output).
The power IC and the bypass capacitor should be placed near the module, and the VBAT trace
should be routed as wide and short as possible to reduce the PCB copper resistance when layout.
When the input voltage is 12V or more high, a DC to DC converter is the best choice as its high
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SIM900_AN_Reference Design Guide_V1.01 01.27.2010