SIM900D Hardware Design
Fast charge:
If there is a charger supply and battery present and the battery is not in DDLO and UVLO, SIM900D will enter
fast charge state. Fast charge is controlled by the software to make the current/voltage regulation. The charging
scheme for the Li-Ion battery is constant current (about 550mA) first, followed by constant voltage charging once
4.2V is reached. Charging is stopped when the charging current at constant voltage has decreased down to the
pre-set current.
3.5.4 Operating modes during charging
The battery can be charged during various operating mode. That means that charging can be in progress while
SIM900D is in Normal mode (SLEEP, IDLE, TALK, GPRS IDLE or GPRS DATA mode). In this case the
voltage supply should be sufficient. Here Charging in Normal mode is named as Charge mode.
If the charger is connects to the module’s VCHG Pin and the battery is connected to the VBAT Pin while
SIM900D is in POWER DOWN mode, SIM900D will go into the Charge-only Mode .
The following table gives the difference between Charge mode and Charge-only mode
：
Table8: Charge operating modes
How to activate mode
Features
C
h
arge M
o
de
Connect charger to module’s VCHG Pin
and connect battery to VBAT Pin of
module while SIM900D is in Normal
operating mode, including:
IDLE, TALK mode; SLEEP mode etc;
z
GSM remains operational and registers
GSM network while charging is in
progress;
z
The serial interfaces are available in
IDLE, TALK mode, the AT command
set can be used fully in this case;
In SLEEP mode, the serial interfaces are
not available. Once the serial port is
connected and there is data in transfer.
SIM900D will exit the SLEEP mode.
C
h
arge-onl
y Mode
Connect charger to module’s VCHG Pin
while SIM900D is in POWER DOWN
mode.
IMPORTANT: Here Charge-only mode is
charging when power down, it means that
not all software tasks are running.
z
Battery can be charged when GSM
engine is not registered to GSM
network;
z
Only a few AT commands is available,
as listed below.
Note: VBAT can not provide more than 5mA current while SIM900D module is during the DDLO charge state.
In other words it is strongly recommended that VBAT should not be the main power supply in the application
subsystem if SIM900D DDLO charging state occurs.
SIM900D_Hardware Design_V1.04
2010.10.09
31
