Power Supply - SimCom SIM900B Hardware Design

3.2

Power Supply

The power supply range of SIM900B is from 3.2V to 4.8V. The transmitting burst will cause voltage drop and the
power supply must be able to provide sufficient current up to 2A. For the VBAT input, a bypass capacitor (low
ESR) such as a 100 µF is strongly recommended; this capacitor should be placed as close as possible to SIM900B
VBAT pins. The following figure is the reference design of +5V input power supply. The designed output for the
power supply is 4.1V, thus a linear regulator can be used.
If there is a high drop-out between the input and the desired output (VBAT), a DC-DC power supply will be
preferable because of its better efficiency especially with the 2A peak current in burst mode of the module. The
following figure is the reference circuit.
The single 3.6V Li-ion cell battery can be connected to SIM900B VBAT pins directly. But the Ni-Cd or Ni-MH
battery must be used carefully, since their maximum voltage can rise over the absolute maximum voltage of the
module and damage it.
When battery is used, the total impedance between battery and VBAT pins should be less than 150 mΩ.
The following figure shows the VBAT voltage drop at the maximum power transmit phase, and the test condition
is as following:
SIM900_Hardware Design_V2.01
Figure 2: Reference circuit of the LDO power supply
Figure 3: Reference circuit of the DC-DC power supply
VBAT=4.0V,
A VBAT bypass capacitor C
Another VBAT bypass capacitor C
=100µF tantalum capacitor (ESR=0.7Ω),
A
=1µF.
B
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Smart Machine Smart Decision
2012-05-07