User's Manual

SIM900B Hardware Design
The circuit design of the power supply depends strongly upon the power source where this power is drained. The
following figure is the reference design of +5V input source power supply. The designed output for the power
supply is 4.1V, thus a linear regulator can be used. If there’s a big difference between the input source and the
desired output (VBAT), a switching converter power supply will be preferable because of its better efficiency
especially with the 2A peak current in burst mode of the module.
A single 3.6V Li-Ion cell battery type can be connected to the power supply of the SIM900B VBAT directly. But
the Ni_Cd or Ni_MH battery types must be used carefully, since their maximum voltage can rise over the absolute
maximum voltage of the module and damage it.
Figure 4: Reference circuit of the source power supply input
The following figure is the VBAT voltage ripple wave at the maximum power transmit phase, the test condition is
VBAT=4.0V, VBAT maximum output current =2A, C
A
=100µF tantalum capacitor (ESR=0.7Ω) and C
B
=1µF.
Max:300mV
VBAT
Burst:2A
IVBAT
4.615ms
577us
Figure 5: VBAT voltage drop during transmit burst
3.3.1 Power Supply Pins
Eight VBAT pins of the board-to-board connector are dedicated to connect the supply voltage; six GND pins are
recommended for grounding. VRTC pin can be used to back up the RTC.
SIM900B_HD_V1.03 24.06.2010
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