SIM900 Reference Design Guide Notes
efficiency.
When the module is powered by a DC-DC converter, then should pay more attention to the
switching noise suppression design, otherwise the RF performance of SIM900 will be interfered
by the switching noise of DC-DC, and cause some RF performance degraded, for example,
modulation spectrum, switching spectrum will exceed the limit. As a solution, a large current
ferrite bead FB101 (0805 size package, rated current > 2A, low DC resistance ) can be added
between the DC-DC output and the VBAT in series. By default, FB101 can be mounted with a
0ohm resistor, and when need, it can be instead with a large current ferrite bead. For a typical
application in automotive, following Figure 2 shows a reference circuit.
Figure 2: DC-DC Power Supply
When the input is from the USB port of a computer, the average current is 500mA according to
the USB specification, so a super capacitor must be added near the module VBAT pins to
compensate the peak current in transmit burst. A low ESR tantalum capacitor is usually used. The
value for the capacitor should be not less than 470uF.
2.1.2 Battery Charge design
SIM900 do not support the battery charge function. When the Li-ion battery is needed in the
application, a charger IC will be implemented. If the battery is only for backup (the AC adaptor is
mostly powered), we suggest you choose a charger IC with power-path management for battery
lifespan. For example: BQ2407x series from Texas Instruments. Following figure 3 is the
reference circuit using BQ24075.
SIM900_AN_Reference Design Guide_V1.01 01.27.2010
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