I have been working for a small arduino based project which involves a single cell 18650 LiPo battery charger plus an LDO for 3.3V regulation.
First of all let me clear it that, I have come up with this idea based on the automatic voltage selection on Arduino Uno board and my knowledge on MosFETs are somewhat at a beginner level. So I need advice/recommendation/criticism/guidance on what I am trying to achieve.
I have attached the schematic.
My project will run on a single cell 18650 LiPo battery and also can be powered through a micro USB charger.
For charging the battery I have used MCP73831, as 3.3V LDO I have used MCP1825 IC and I need to achieve the following features:
While the USB is plugged in, the battery should be charging through the MCP73831 and 3.3V should be coming through the MCP1825.
Here VIN represents the 5V coming from VBUS line of the USB connector. This 5V should be supplied to the input of MCP73831 and also to IN-pin of MCP1825.
While USB is unplugged, there will be no input to the charger. So, the MCP1825 should be powered from the battery, which will eventually supply regulated 3.3V to other circuitry. There should also be a protection from the back powering of the battery voltage to the input of charger IC
So, I have used FDN340P as its a P-Channel MosFET. Connections are: Drain to Vbat, Source to IN-pin of MCP1825, Gate to VIN-pin of MCP73831. The diode D1 is between VIN and IN pin to prohibit the back powering.
So far i have learned, my understanding on this is, as Q1 is a P channel mosfet, when USB is plugged in, the gate will have a large positive voltage. So the Fet will switch off and the Vbat is isolated from the LDO.
When its unplugged from USB, the Gate will be pulled low through the 10k R3 resistor causing the mosfet to switch on. Therefore, IN-pin of MCP1825 will be powered from Vbat.
Diode D1 will protect the battery from back powering the charger IC.
Please, let me know if this configuration is right or if i need any modification, how should i do it.
N.B: FDN340P, MCP73831, MCP1825 all are SMD parts and can’t test them on breadboard. So, I have to go for a SMD PCB to test this circuit, therefore, before I am going to try that, I want to make sure as much as i can that I m doing it right. I hope I have explained my situation.