Originally Posted by
rc-pilot-freak
I am building a PIC circuit that is driving a Mosfet. PIC is powered with 4 battery cells that swing from 5.4V to 4.6V (it goes lower but for me thats the lowest).
The PIC is regulated with LM2940CT-5.0V
I would like among other things that my PIC monitors his supply. If the voltage drops bellow 4.6V it should sound the buzzer (portb.2 = high).
On a Vref pin a put a LM336-2.5V reference diode:
5V-----R1(2.5k)-----o------- Vref input
I
I
LM336 diode
I
I
ground
So the Vref is 2.5V when the supply is over or exactly 5V (on PIC is always 5V).
Will I be able to read the correct voltage when it drops to 4.6V ?????? I don't think so. I can't change to 3.3V LDO since i need more than 4.5V to drive Logic Level Mosfet.
What can I do????
I have to ask first if weight is a factor. there are a zillion things you can do, unless weight is a huge factor. I will give some examples.
Code:
1. make a charge pump to supply a drive voltage for the MOSFET.
2. use analog device to monitor battery(an LM3914} or dozens of others
3. Use higher voltage power supply, more batteries
4 include a MOSFET driver IC. some of these I believe have a charge pump others like the irf4227 do not.
5. Use a low voltage micro isolated from Mosfet with opto coupler and run
MOSFET and trigger from unregulated supply (higher voltage).
Just some ideas, I hope they help.
JS
Lots of things
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