User's Manual

M80 Hardware Design
M80_HD_V1.2 - 31 -
1) Use the EMERG_OFF pin only while failing to turn off the module by the command
“AT+QPOWD=1” and the PWRKEY pin. Please refer to Section 3.4.2.4.
3.3. Power supply
3.3.1. Feature of GSM power
The unit of GSM transmit in the wireless path is pulse string which is constructed by GSMK bit
string and we call it burst. The period of burst is 4.16ms and the last time of burst is 577us. The
burst current will reach 1.6A while idle current is as low as tens of milliampere. This sudden
change of current will produce large ripple of VBAT or pull the VBAT down to 3.3V, while the
module will shut down when VBAT drops to 3.3V. Due to these features, the power design for the
module is crucial.
The following figure is the VBAT voltage and current ripple at the maximum power transmitting
phase, the test condition is VBAT=4.0V, VBAT maximum output current =2A, C1=100µF
tantalum capacitor (ESR=0.7Ω) and C2=1µF.
Max:400mV
4.615ms
577us
IBAT
VBAT
Burst:1.6A
Figure 3: Ripple in supply voltage during transmitting burst
3.3.2. Minimize supply voltage drop
The power supply of the module is from a single voltage source of VBAT= 3.3V~4.6V. The GSM
transmitting burst can cause obvious voltage drop at the supply voltage thus the power supply
must be carefully designed and is capable of providing sufficient current up to 2A. For the VBAT
input, a bypass capacitor of about 100 µF with low ESR is recommended. Multi-layer ceramic
chip (MLCC) capacitor can provide the best combination of low ESR but small size may not be
economical. A lower cost choice could be a 100 µF tantalum capacitor with low ESR. A small
(0.1µF to 1µF) ceramic capacitor should be in parallel with the 100µF capacitor, which is