I want to switch power of 12V laser sensor from 3V3 STM32H753VI GPIO. I will power on or off for long time (It's not PWM here) I am not comfortable with Mosfet and Optocoupler so I need help to be sure I can route this to my board. I want to go high side because the sensor laser set 12V to output when I cut the ground. I need to be isolated because of isolated power supply.
Laser power is connected to CN1 and draw about 100mA and can work between 10V to 14V.
The STM32H753VI work at 3V3 and GPIO will be set as open drain.
I want to use EL3H7-G (C) Optocoupler and STR2P3LLH6 Mosfet because of small footprint.
When GPIO is shorted to ground current flow to EL3H7-G led, I get forward voltage from sheet (page 3) and curve (page 4)
GPIO could not handle more than 20mA to the GPIO so I will set If=10mA
I will go with R1=200ohms
OPTOCOUPLER Ic CURRENT
I want to When I look into EL3H7-G datasheet (page 3) I can see optocoupler transistor can be saturated with If=10mA and Ic=1ma so Vce(sat) will be at 0.2V max . (On the curve I can see I can go to 6ma into saturated region)
I will go with R2=10kOhms
Vgs is -R2 voltage
From STR2P3LLH6 datasheet (page 3) I can see Vgs(th)=-2.5V
Vgs (-11.8V) is more lower than Vgs(th) (-2.5V) so Mosfet will turn On.
I can see on STR2P3LLH6 datasheet (page 2) Vgs max is +/-20V
so I will probably not destroy the gate with -11.8V
On STR2P3LLH6 datasheet (page 5) I can see the Rdson is about 48mOhms for 100mA
I can now compute Mosfet dissipated power
P (480µW) is really lower than Ptot (0.35W)
- Do you think Mosfet will fully On or Off as I want ?
- Do you think I will destroy the Mosfet, Optocoupler or STM32 ?
- Do you think optocoupler and Mosfet are about a good choice ?
- Do you think It's good to add small value R3 between collector and gate ?
Thanks in advance for your help
I implement this design but got an Always On Problem here
My problem Always On Problem came from my PCB and not from schematic and method here, you can use it without problem, as soon as you respect you MOSFET footprint :)