SIM900D Hardware Design
mode
registered to GSM network and only limited AT commands can be accessible, the following
way will enter Charge-only
mode
:
z
From POWER DOWN mode: Connect charger to the module’s VCHG Pin and Battery
connect VBAT Pin while SIM900D is power down.
z
From Normal mode: Connect charger to the module’s VCHG Pin and Battery connect
VBAT Pin, then power down the module by “AT+CPOWD”
Charge mode
during normal
operation
Start charging while the module is in normal mode including: 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= 3.4V...4.5V. 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 is recommended. A 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’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.
The single 3.6V Li-Ion cell battery type can be connected to the power supply of the SIM900D 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 for the module and damage it.
SIM900D_HD_V1.02
09.04.2010
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