Design Context

I have a motor powered by an LDO, whose GND (GND_MT) is controlled by an N-channel MOSFET (IRLML6344) in a low-side switch configuration.
This MOSFET switches the ground of the entire motor power circuit (GND_MT), allowing it to activate only when the MOSFET channel is closed.

The MOSFET’s gate is driven by the Q̅ output of a D-type flip-flop (74LVC1G80GV), which toggles state when a button is pressed.

The circuit is powered by a Li-Ion battery. A USB-C connector on a second PCB provides 5V only for charging the battery, not for directly powering the system.

The Problem

When I touch the USB-C connector with the charger cable (even before fully plugging it in), the motor activates, as if the MOSFET were turning on by itself.

But it shouldn’t:

• The button hasn’t been pressed.
• The flip-flop logic shouldn’t change (but it does — seemingly triggered by this contact).
• The flip-flop supply remains stable and active throughout.

Things I’ve already tried without success:

• Added a 100kΩ resistor between GND_MT and BAT+ → no effect.
• Connected the USB-C shield to GND → no improvement.
• Placed various capacitors (100 pF to 10 nF) between GND and BAT+.
• Added a capacitor between the ENABLE signal (gate of the MOSFET) and GND → didn’t help.

What I’m looking for

I’m trying to find a simple solution that doesn’t require a full redesign. Ideally, something that stabilizes the behavior without permanently tying GND_MT to the system ground, since it must be controlled via the MOSFET.

I’ve attached the relevant part of the schematic showing the flip-flop, LDO, and MOSFET, as well as the USB-C charging section.

Any ideas or insight would be greatly appreciated!

 

The 74LVC1G08GV is a 2 input and gate, not a D-type flip-flop. You don't show the activation button in your diagrams.

 

To be rather blunt about this discussion, it seems that the Thread Starter (TS), has determined what the cause of the problem is and is therefore only showing us that portion of the circuit that they know holds the problem. So the portion of the circuit that actually contains the problem is a guess.

Unfortunately the problem is in a portion that we do not see, which means that it is only possible to provide random guesses.

 

It does not make much sense to have the motor supply negative floating and not connected to the circuit common. I'm not surprised that the circuit switches when the USB supply is connected.
Connect pin 3 of the FET to the motor negative and connect all the rest of the "GND" and "GND MT" terminals to circuit common.

 

If it appears that the FF is changing state when it should not be, then there certainly is a solution. But if KW has it right:" The 74LVC1G08GV is a 2 input and gate, not a D-type flip-flop."then the issue is quite different.
It certainly appears to me that the problem is in a portion of the circuit that we have not seen. That will include bioth the connections from the unseen button to what is intended to be a flipflop, and the connections from the FF to the motor control device and to the motor.. BECAUSE it is in the part that we do not see that I suspect the fault lies.
AND, That circuit shown in post #1, with IC3 connected that way, will not normally function as a flipflop.

 

The 74LVC1G80GV as shown in the schematic IS a single D-Type FF, unfortunately being misread as a 74LVC1G08GV 2-Input AND!

 

OK, so now I see that there is a transposing of the 0 and 8 leading to an incorrect description of the 74LVC1G08GV .
I am not at all familiar with that device, it would be quite useful for the TS to provide some information about it, as well as more information about the specific application circuit. What I see in post#1 is incomplete. In addition, it is certainly able to trigger on random glitches. Besides that, we are simply not shown the interconnection arrangements. There are a whole lot of things that can trigger that 74LVC1G08GV flipflop, and we are shown none of them.

 

I must apologize for misreading the part number od the 74LVC1G80GV. It is, indeed, a D-type flip-flop.
The rest of what I posted is still true though. Why would the TS switch the ground return of the whole motor power supply, leaving it floating when the FET switch is turned off. That is asking for intermittent problems! It would be so much simpler to just switch the negative terminal of the motor.

 

OK, one option is to move that capacitor to the other side ofthe series resistor, so that it directly shunts the FF input. That ought to solve the fast triggering problem.