I need help for my project: I am using an adapter operating from 12V and I need 2.5A at 4.2V for my SIM808. In the datasheet it says it has a 2A peak. I used an LM2596S and WEBENCH® Designer LM2596(online calculator) and I designed this circuit:enter image description here

And now I need from the 4.2V an output of 3.3V for my ATmega328/P so I chose the LD1117 regulator and this is the circuit: enter image description here

Can someone tell me if this is OK: do I need to change anything or choose another regulator? I don't have the parts to test the circuit.

  • \$\begingroup\$ If you look at a datasheet for the 3.3 V LD1117 you will find it has a maximum dropout voltage of 1.1 V at 100 mA. This is more than required voltage drop of 0.9 V. Is there any possibility of running the ATmega328/P at a different voltage in its acceptable range of 1.8 V - 5.5 V? \$\endgroup\$ – Andrew Morton Dec 11 '16 at 17:21
  • \$\begingroup\$ Yes atmega328 can run at 5v but i need 3.3V for my accelometer and other sensors \$\endgroup\$ – suadss Dec 12 '16 at 16:10
  • \$\begingroup\$ I edited your question a little bit, which might bring it to the attention of people who are good at this sort of thing. Could you let us know the maximum current requirement of the 3.3V circuitry (the microcontroller and the sensors)? \$\endgroup\$ – Andrew Morton Dec 12 '16 at 18:30
  • \$\begingroup\$ Thank you Andrew, sorry for the late response i forgot about this, i made it i used MIC5203-3.3YM5 regulator . \$\endgroup\$ – suadss Apr 21 '17 at 16:06
  • \$\begingroup\$ That's a neat little regulator. Perhaps you could write a short answer to your question and explain your design decision in case anyone else has the same question in the future. Then you can come back in a couple of days and mark it as the accepted answer. \$\endgroup\$ – Andrew Morton Apr 21 '17 at 17:19

I used diffrent regulators, my input voltage is 9v the regulator that i used to power the MCU is MIC5203-3.3YM5 and the schematic i used is in the datasheet, to power the SIM i used mic29302 this regulator convertts my input of 9v to 4V. In the datasheet you have the schematic for this regulator.


you need to use LM2596-adj module which provide a variable voltage from 3-35V. And this circuit provide maximum of 3A current. you can easily adjust it to 4.2V using variable resistor,for 3.3V you can use AMS1117 or LF33CV.

enter image description here enter image description here


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