SimCom SIM900D EVB, Проектирование аппаратного обеспечения

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SIM900D Hardware Design
functionality
mode (without
remove power
supply) without removing the power supply. In this case, the RF part of the module will not work or
the SIM card will not be accessible, or both RF part and SIM card will be closed, and the
serial port is still accessible. The power consumption in this case is very low.
Charge-only
mode The module will systematically enter Charge-only mode when a charger and battery are
connected to a switched-off SIM900D. In the mode, the module does not search for network
and has limited AT commands available. The module also can enter Charge-only mode from
Charge mode during normal operation by normally shutdown the module.
Charge mode
during normal
operation The module will automatically go to this mode when a charger is connected to a Normal
operation mode module of which battery voltage is not lower than 3.2V. Normal operation
mode includes: SLEEP, IDLE, TALK, GPRS IDLE and GPRS DATA.
3.3 Power Supply
The power supply of SIM900D is from a single voltage source of VBAT= 3.2V...4.8V. In some case, the ripple in
a transmitting burst may cause voltage drops when current consumption rises to typical peaks of 2A. So the power
supply must be able to provide sufficient current up to 2A.
For the VBAT input, a local bypass capacitor (about 100 µF, low ESR) is recommended. Multi-layer ceramic chip
(MLCC) capacitors can provide the best combination of low ESR and small size but may not be cost effective. A
lower cost choice may be a 100 µF tantalum capacitor (low ESR) with a small (0.1µF to 1µF) ceramic in parallel,
which is illustrated as following figure. The capacitors should be placed as close as possible to the SIM900D
VBAT pins. The following figure is the recommended circuit.
C
A
C
B
VBAT
+
Figure3: Reference circuit of the VBAT input
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 is 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.
The single 3.6V Li-Ion cell battery type can be connected to the power supply of the SIM900D VBAT directly.
SIM900D_Hardware Design_V1.04 2010.10.09
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