Hardware Design V1.01(091226)

SIM900 Hardware Design
Figure 5: VBAT voltage drop during transmit burst
3.3.1 Power Supply Pins
Three VBAT pins are dedicated to connect the supply voltage and fifteen GND pins are dedicated to connect
ground. VRTC pin can be used to back up the RTC.
3.3.2 Minimizing Power Losses
When designing the power supply for your application please pay specific attention to power losses. Ensure that
the input voltage VBAT never drops below 3.4V even in a transmit burst where current consumption can rise to
typical peaks of 2A. If the power voltage drops below 3.4V, the module may be switched off. The PCB traces
from the VBAT pins to the power source must be wide enough to decrease voltage drops in the transmitting burst
mode.
3.3.3 Monitoring Power Supply
To monitor the supply voltage, you can use the “AT+CBC” command which include a parameter: voltage value
(in mV).
The voltage is continuously measured at intervals depending on the operating mode. The displayed voltage (in
mV) is averaged over the last measuring period before the AT+CBC command is executed.
For details please refer to document [1]
3.4 Power Up and Power Down Scenarios
3.4.1 Turn on SIM900
SIM900 can be turned on by various ways, which are described in following chapters:
z Via PWRKEY pin: starts normal operating mode (please refer to chapter 3.4.1.1);
z Via PWRKEY pin and PWRKEY_OUT pin：starts normal operating mode
SIM900_HD_V1.01 26.12.2009
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