# 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:

And this is the module circuit diagram

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?

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.

Oscilloscope configuration:

• 500mV / division

• 20us / division

It is stated in the datasheet that the module will shut down in a few seconds if the supply voltage drops below 3V.

The use of two diodes in series for dropping the voltage is not optimal since, their voltage drop is dependent on the current flowing through them. Considering their smallest voltage drop (2 x 200mV = 400mV), would would supply too much voltage (4.6V) to the module. Considering their highest voltage drop you have two scenarios:

1. With data mode GSM850 there's a current consumption of about 378mA, which causes a voltage drop of 2x300mV = 600mV. Hence, the supply voltage of the module is about 4.4V.
2. During Tx burst there's a 2A peak current. You would have to measure how much the voltage drop is with the aid of an osci. I reckon that is the problem which is causing your module to reset.

You can try adding more capacitance to the VCC pin of your module in order to prevent this voltage drop. However, the problem with the imprecise supply voltage still remains. If you are using the 5V only to bias the IC charger, you could also modify the feedback of the IC MP1484 so that the output voltage is 4.4V, so that you wouldn't need to add the two diodes.

• No, I did not use an SD card in this design, and the EXT_PWR is only used to detect whether the external power is there or not, it is connected to the microcontroller May 28 '20 at 13:02
• How are you supplying the SD card? May 28 '20 at 13:17
• I'm not actually using the SD card, it is not connected May 28 '20 at 14:33
• Ok. I will edit my answer. Apparently you have a problem with your two diodes, which are causing the supply voltage to drop below 3V. I will update my answer. May 28 '20 at 15:00
• @Hakraf My answer was updated. May 28 '20 at 15:21