User's Manual

Smart Machine Smart Decision
SIM5320_Hardware Design_V1.01 2011-11-10
19
Figure 7: VBAT_RF voltage drop during burst emission (GSM/GPRS)
3.1.1 Power Supply Pin
Two VBAT_RF and two VBAT_BB pins are dedicated to connect the supply voltage.
Table 4: Pin description
Pin type Pin name Min Typ Max Unit
VBAT_RF 3.3 3.8 4.2 V
POWER
VBAT_BB 3.3 3.8 4.2 V
Note: 1.Though the VBAT_RF and VBAT_BB are supplied by the same voltage level, they are different pins. VBAT_RF
is for RF section and VBAT_BB is for baseband system.
2. When the module is power off, users must pay attention to the issue about current leakage. Refer to Chapter 3.10.2
Note2.
3.1.2 Design Guide
Mostly, user connects the VBAT_RF and VBAT_BB pins with one power supply. Make sure that the input
voltage at the VBAT_BB pin will never drop below 3.3V even during a transmit burst when the current
consumption rises up to 2A. If the power voltage drops below 3.3V, the module may be shut down
automatically. Using a large tantalum capacitor (above 100uF) will be the best way to reduce the voltage
drops. If the power current cannot support up to 2A, users must introduce larger capacitor (typical 1000uF)
to storage electric power, especially GPRS multiple time slots emission.
For the consideration of RF performance and system stability, another large capacitor (above 100uF)
should be located at the VBAT_RF pin and some multi-layer ceramic chip (MLCC) capacitors (0.1uF)
need to be used for EMC because of their low ESR in high frequencies. Note that capacitors should be put
beside VBAT_RF pins as close as possible. Also User should minimize the PCB trace impedance from the
power supply to the VBAT pins through widening the trace to 80 mil or more on the board. The following
figure is the recommended circuit.