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SIM868 powers down after few seconds

I designed a circuit with SIM868 module, and i followed the recommendation from datasheet to unsure suffusion current.

The main power supply is getting from VCC:12V source converted to 5V by an DC-DC step down converter that can output up to 3A, after that the voltage is reduced by tow diodes that also have output current up to 3A.

The following is the power supply circuit with battery charger:

Power supply circuit

And this is the module circuit diagram

Module circuit

I wanted to test it and a problem occured. When I turn on the module, it automatically powers off after a few seconds.and sometimes if i remove SIM card or detach antenna, module stays on, So, I assume it powers down after it connects to network. and that can be because of power lack.

For that i added and external power source 3.7V that can output also more than 2A that is recommended by datasheet connected directly to module VDD pins, but the same problem occurs.

Did anyone have a problem like this and knows how to solve?

Updates:

I checked with the oscilloscope and I found that when the module tries to connect to the GSM network the voltage drops from 3.7 V to 2.75 V, even if I directly connected the power supply to the VDD pins of the module.

enter image description here

Oscilloscope configuration:

  • 500mV / division

  • 20us / division

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