GSM/GPRS Module Series
M66 Hardware Design
M66_Hardware_Design Confidential / Released 23 / 80
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 burst transmission. 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 burst
transmission. The width of trace should be no less than 2mm and the principle of the VBAT route is the
longer route, the wider trace.
VBAT
C2
C1
+
C3
C4
GND
100uF
100nF
10pF
0603
33pF
0603
Figure 4: Reference Circuit for the VBAT Input
3.3.3. Reference Design For Power Supply
The power design for the module is very important, since the performance of power supply for the module
largely depends on the power source. The power supply is capable of providing the sufficient current up to
2A at least. If the voltage drop between the input and output is not too high, it is suggested to use a LDO
as module’s power supply. If there is a big voltage difference between the input source and the desired
output (VBAT), a switcher power converter is recommended to use as a power supply.
The following figure shows a reference design for +5V input power source. The designed output for the
power supply is 4.0V and the maximum load current is 3A. In addition, in order to get a stable output
voltage, a zener diode is placed close to the pins of VBAT. As to the zener diode, it is suggested to use a
zener diode whose reverse zener voltage is 5.1V and dissipation power is more than 1 Watt.
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