"tantalum polymer cap, Scope, L effects? You lost me Ta caps from VDD to where? No scope is available, Rail noise, what is a rail? Better yet can you indicate which rail(s) you are talking about the circuit drawing,
Tant polymer instead of a electrolytic or a regular tantalum capacitor. For this reason (its ESR very low) :
Place cap from Vdd to ground.
"Scopoe L effects", using a scope to look for excessive transients that could damage other
components, specifically the MOSFET or the Vdetector....
Rail is the supply buss, like Vdd of Vcc as standard nomenclature.
Rail under discussion is power supply connection, eg. "rail" to Vdetector/motor.
"poor ground system so the V detector is affected by load dump into ground." You lost me. What do you mean a poor ground system? Can you show me a spot in the circuit diagram of a poor ground system? And what is meant by load dump to ground? What load? What ground?
Motor current is supplied by buss and fed thru MOSFET into ground, thats load dump.
Load is the motor.
Ground is your ground, as shown on your schematic labeled GND.
A poor ground system is where, typically, an analog components power is connected and that ground
then goes to other components for ground and loads, like motor. Daisey chained. Versus analog
components ground trace goes all the way, in a separate trace to board edge where ground meets
the board. Same for motor ground. That way the analog component does not see the IR drop due to
motor current until they meet.
Read about the different grounding techniques such as PCB ground plane, ground plane vias, decoupling and connector grounding.
Your LIC has fairly high ESR, so a low ESR cap to ground might help in parallel to LIC, physically close to
motor leads.
Advise you get/rent/borrow a scope if you are designing electronic circuits with mixed signal in them.
Post a pic of your prototype so we can see how its wired/routed.
R2 turns on the mosfet by charging its gate C when the Vdetector output is off.
I'm sorry guys I don't buy the problem is tht the motor is pulling down the source.
How do you know that w/o observing with a scope ?
Regards, Dana.