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SIM800_Hardware Design_V1.09 21 2016-06-30
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. The following figure is the reference circuit. FB101 is very important,
customer can get better EMI feature with appropriate filtering bead.
Vin
Vout
FB
U101
1
2
3
4
5
LM2596-ADJ
+
100uH
MBR360
L101
C101
+
C102
D102 C103
R102
R101
FB101
330uF
VBAT
2.2K
1K
100uF
1uF
C104
100nF
DC INPUT
PWR_CTRL
GND
On/Off
c
Figure 7: Reference circuit of the DC-DC power supply
The single 3.7V Li-ion cell battery can be connected to SIM800 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 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
=1µF. (See C
A
and C
B
in figure 5)
577us 4.615ms
I
VBAT
VBAT
Max:350mV
Burst:2A
Figure 8: VBAT voltage drop during transmit burst
4.1.1. Power Supply Pin
The VBAT pins are used for power input, and pin 62,63,64,65 should be connected to the power GND. VRTC
pin is power supply of the RTC circuit in the module. VDD_EXT will output 2.8V when module powered up.
When designing the power supply in user’s application, pay special attention to power losses. Ensure that the
input voltage never drop below 3.4V even when current consumption rises to 2A in the transmit burst. If the
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