User's Manual

Smart Machine Smart Decision
SIM868_Hardware_Design_V1.00 21 2016-06-20
If there is a high drop-out between the input and the desired output (VBAT), a DC-DC power supply will be
preferable because of its better efficiency especially with the 2A peak current in burst mode of the module. The
following figure is the reference circuit.
Figure 7: Reference circuit of the DC-DC power supply
The single 3.7V Li-ion cell battery can be connected to SIM868 VBAT/GPS_VBAT pins directly. But the Ni-Cd
or Ni-MH battery must be used carefully, since their maximum voltage can rise over the absolute maximum
voltage of the module and damage it.
When battery is used, the total impedance between battery and VBAT/GPS_VBAT pins should be less than
150mΩ.
The following figure shows the VBAT voltage drop at the maximum power transmit phase, and the test condition
is as following:
VBAT=4.0V,
A VBAT bypass capacitor C
A
=100µF tantalum capacitor (ESR=0.7Ω),
Another VBAT bypass capacitor C
B
=1uF~10uF.
577us 4.615ms
Burst:2A
I
VBAT
VBAT
Max:350mV
Figure 8: VBAT voltage drop during transmit burst
4.1.1. Power Supply Pin
Pin34 and Pin35 are GPS_VBAT and VBAT input; Pin36 and Pin37 are GND of power supply. VDD_EXT
output 2.8V when module is in normal operation mode.
When designing the power supply in customers’ application, pay special attention to power losses. Ensure that
the input voltage never drops below 3.0V even when current consumption rises to 2A in the transmit burst. If the
power voltage drops below 3.0V, the module may be shut down automatically. The PCB traces from the VBAT
pins to the power supply must be wide enough (at least 60mil) to decrease voltage drops in the transmit burst.
The power IC and the bypass capacitor should be placed to the module as close as possible.
VBAT
MIN:3.0V
Figure 9: The minimal VBAT voltage requirement at VBAT drop