Good afternoon all…
I have a proven circuit in a small temperature transmitter.
If you are looking at the schematic – ignore Q2 for the moment and assume the battery –ve is connected to the circuit 0v.
It is primarily supplied by a 18650 Li battery – with internal over/under protection.
This battery can be charged by a standard 5v wall wart using the included CN3063 charger chip.
The MCU is supplied by a 3.3v LDO regulator.
The principle of operation is simple:
When 5v is supplied Q1 FET will turn off, preventing current flow back to the battery.
The 5v supplies the LDO via diode D2.
The CN3063 chip starts charging the battery.
When the 5v is removed, Q1 turns on and allows the battery to supply the LDO.
The 4u7 cap is a requirement of the charger chip.
That’s the background – yes, it works, proven in the field.
Now my question:
These devices are used in sub-zero temperatures.
The staff have been instructed not to plug them into the charger until 24 hours after the device has come out of the freezer.
As this cannot be guaranteed, I want to introduce an extra level of protection for the Li battery.
This is where the FET Q2 comes in…
Can I put Q2 in circuit here?
Further information:
The charger chip does not have an enable input.
The MCU receives the following feedback information:
The CN3063 chip has two open collector indicators – Charged and Charging.
The MCU can determine when 5v is applied by using these.
There is a DS3231 RTC in the circuit (not shown) that can also provide the on board temperature.
The entire circuit is potted in epoxy so an assumption can be made that the battery is at roughly the same temperature as the DS3231.
Once the temperature of the overall circuit raises to a safe level the MCU can activate CHARGE_ENABLE.
Datasheet for Q2 also attached.
Appreciate your feedback.