I have a switching power supply that operates at 4 V output. I want to have my GSM always powered when my primary voltage is removed. I use a simple Li-Ion battery 4.2 V and I want to use it as backup battery.
What is the best way to make a switch over? I'm looking for something robust. I don't want something that it could not work in some circumstances.
If my primary voltage rail was much above battery's maximum voltage I could use a simple comparator.
I was thinking that I could increase my primary voltage to 4.3 V and use a comparator but I do worry about GSM operation because it has maximum voltage 4.4 V.
Photo as a reference: