SIM900D Hardware Design
battery is between DDLO and UVLO. In DDLO charge, SIM900D gives out 1mA current to the battery. And in
UVLO charge, SIM900D provides about less than 200mA current to the battery.
DDLO charge terminated when the battery voltage reaches 1.5V. UVLO charge terminated when the battery
voltage is up to 3.3V. Both DDLO and UVLO charge are controlled by the SIM900D hardware only.
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. Fast charge delivers a strong and constant current
(about 550mA) through VBAT Pin to the battery until battery voltage reach 4.2V.
Trickle charge:
After fast charging, the battery voltage is close to the whole battery capacity, trickle charge begins. In this state,
the SIM900D charges the battery under constant voltage.
3.5.4 Operating modes during charging
The battery can be charged during various operating mode. That means that when the GSM engine is in Normal
mode (SLEEP, IDLE, TALK, GPRS IDLE or GPRS DATA mode), charging can be in progress while SIM900D
remains operational (In this case the voltage supply should be sufficient). Here it’s named Charging in Normal
mode as Charge mode.
If the charger is connected 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 (Off and charging). The
following table gives the difference between Charge mode and Charge-only mode
：
Table
8
: 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
registered 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.
Then SIM900D will exit the SLEEP mode.
SIM900D_HD_V1.02
09.04.2010
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