Smart Machine Smart Decision
4
Application Interface
4.1
Power Supply
The power supply range of SIM900-DS 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
SIM900-DS 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.
GND
Figure 5: Reference circuit of the LDO power supply
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.
Figure 6: Reference circuit of the DC-DC power supply
The single 3.6V Li-ion cell battery can be connected to SIM900-DS 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 150m
Ω
.
The following figure shows the VBAT voltage drop at the maximum power transmit phase, and the test condition
is as following:
VBAT=4.0V,
A VBAT bypass capacitor C
A
=100µF tantalum capacitor (ESR=0.7
Ω
),
Another VBAT bypass capacitor C
B
=1µF.
SIM900-DS_Hardware Design_V1.00
17
2013-01-20
