User's Manual

720 Hardware User Guide

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 VCCIN - |VGS(th)|, where VGS(th) is the Gate

Threshold Voltage, Q2 is not needed.

Reference components:

 Q1 can be IRML6401 or low Rds(on) pMOSFET, which has higher, withstand voltage and

drain current.

 Q2: a common NPN tripolar transistor, e.g. MMBT3904; or a digital NPN tripolar transistor,

e.g. DTC123. If digital tripolar transistor is used, delete R1 and R2.

 C3: 470-F tantalum capacitor rated at 6.3V, or 1000 F aluminum capacitor. If lithium

battery is used to supply power, C3 can be 220-F tantalum capacitor.

Power Supply Protection

Add TVS diodes (VRWM=5 V) on the VBAT power supply, especially in automobile applications.

For some stable power supplies, Zener diodes can decrease the power supply overshoot.

SMF5.0AG from ONSEMI is an option.

Line Rules

The width of primary loop lines for VBAT on PCB must be able to support the safe transmission of

2 A current and ensure no obvious loop voltage decrease. Therefore, the loop line width of VBAT

is required 2 mm and the ground level should be as complete as possible.

Separation

The module works in burst mode that generates voltage drops on power supply. Furthermore,

this results in a 217Hz 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 is better

to use separated power supplies. The module shall be supplied by an independent power, like a

DC/DC or LDO. SeeFigure 2-6.

DC/DC or LDO should output rated peak current larger than 2 A.

The inductor used in Reference Design (b), should be a power inductor and have a very low

resistance. The value of 10 H, with average current ability greater than 1.2A and low DC

resistance, is recommended.