Neo_M680 GPRS Module Hardware User Guide
Copyright © Neoway Technology Co., Ltd
11
Figure 3-4
Reference design of power supply controlled by p-MOSFET
Q2 is added to eliminate the need for a high enough voltage level of the host GPIO. In case that the GPIO
can output a high voltage greater than VDD_IN_3.9V - |V
GS(th)
|, where V
GS(th)
is the Gate Threshold Voltage,
Q2 is not needed.
Reference components:
Q1 can be IRML6401 or Rds(on) p-MOSFET which has higher withstand voltage and drain current.
Q2: a common NPN transistor, e.g. MMBT3904; or a digital NPN transistor, e.g. DTC123. If digital
transistor is used, delete R1 and R2.
C3: 470 uF tantalum capacitor rated at 6.3V; or 1000 uF aluminum capacitor. If lithium battery is
used to supply power, C3 can be 220 uF tantalum capacitor.
Protection
Place a TVS diode (V
RWM
=5 V) on the VBAT power supply to ground, especially in automobile
applications. For some stable power supplies, zener diodes can decrease the power supply overshoot.
MMSZ5231B1T1G from ONSEMI and PZ3D4V2 from Prisemi are options.
Trace
The trace width of primary loop lines for VBAT on PCB must be able to support the safe transmission of 2A
current and ensure no obvious loop voltage decrease. Therefore, the trace width of VBAT loop line is
required 2 mm and the ground should be as complete as possible.
Separation
As shown in Figure 3-1, the GPRS module works in burst mode that generates voltage drops on power
supply. And furthermore this results in a 217 Hz TDD noise through power (One of the way generating
noise. Another way is through RF radiation). Analog parts, especially the audio circuits, are subjected to
this noise, known as a "buzz noise" in GSM systems. To prevent other parts from being affected, it's better
