GSM/GPRS Module Series
M35 User Manual
M35_User_Manual Confidential / Released 25 / 85
3.3. Power Supply
3.3.1. Power Features of Module
The power supply is one of the key issues in designing GSM terminals. Because of the 577us radio burst
in GSM every 4.615ms, power supply must be able to deliver high current peaks in a burst period. During
these peaks, drops on the supply voltage must not exceed minimum working voltage of module.
For M35 module, the max current consumption could reach to 1.6A during a transmit burst. It will cause a
large voltage drop on the VBAT. In order to ensure stable operation of the module, it is recommended that
the max voltage drop during the transmit burst does not exceed 400mV.
Vdrop
4.615ms
577us
IBAT
VBAT
Burst:1.6A
Figure 3: Voltage Ripple during Transmitting
3.3.2. Decrease Supply Voltage Drop
The power supply range of the module is 3.3V to 4.6V. Make sure that the input voltage will never drop
below 3.3V even in a transmitting burst. If the power voltage drops below 3.3V, the module could turn off
automatically. For better power performance, it is recommended to place a 100uF tantalum capacitor with
low ESR (ESR=0.7Ω) and ceramic capacitor 100nF, 33pF and 10pF near the VBAT pin. The reference
circuit is illustrated in Figure 4.
The VBAT route should be wide enough to ensure that there is not too much voltage drop during transmit
burst. The width of trace should be no less than 2mm and the principle of the VBAT route is the longer
route, the wider trace.
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