I have a simple circuit; an MCU, a FET and a motor. The MCU drives a low side TN0702 FET which completes the motor circuit. Quite simple, so not sure if understanding the problem requires a schematic, but I can add one if needed. The motor draws 315mA, and the FET is rated up to 500mA.
The motor is powered by 2 C batteries and the circuit works when the MCU is powered by 3.3V (i.e. MCU has it's own power source). However, when I try to use the motor's power source (2.5V-3.0V) for the MCU, the MCU doesn't deliver enough voltage to turn the FET on (even though VGS(th) is 0.5V-1.0V). As the battery depletes, the problem worsens; at 3.0V the FET does not perform too well, but at 2.5V, the FET is barely on and because the FET isn't getting enough voltage at the gate; it overheats and fries.
So, perhaps one solution is a gate driver, right? But, where do I get the required voltage from? Is it that I'm just using the wrong FET?
I found that a power FET such as the IRLB8721 drives the motor faster than the TN0702 (comparing both with 5V at the gate), but I want to use the TN0702 as it has a lower VGS(on). The IRLB8721 has no problem (when the gate voltage is high) as it has a higher VGS(on); 1.35V-2.35V. That said, I want to keep the circuit compact, and a TO-220 power FET seems like overkill for 350mA.
Perhaps the solution here could be a gate driver (maybe using the TN0702 as a gate driver for a TO-92 FET that can handle more current), so where is the higher voltage going to come from? Even though the TN0702 VGS(th) is between 0.5V and 1V, it seems to only turn the motor on if the voltage at the gate is above 4V. I'm a bit confused by this. (Not sure if this makes sense; I was experimenting with a new circuit so perhaps the FET was damaged)
Maybe I figured out a possible cause to the VGS(th) not behaving how I expect: if I'm interpreting the datasheet correctly, and based on my limited understanding of transistors, if the motor current is very high, the gate on resistance increases. So, a relatively high current will take more voltage to turn the gate on. But is this understanding correct? Is this really the case for the IRLB8721 too which can handle very high currents of 62A?
Assuming we want to avoid changing the battery configuration, ultimately I believe that I need a higher voltage source for the FET gate, and one that is higher than the motor power source can provide. Is the solution perhaps to a) add a voltage booster circuit just for the FET gate, b) a voltage boost circuit for the MCU, c) a charge pump for the FET gate, or d) a better solution?
Because I'm hacking/modifying an existing product, I'd like to avoid modifying the battery compartment (but I can if it's the best way to solve the problem). The batteries provide the MCU with 2.5V, which turns out is just enough to power it (though surprisingly, it seems to still work at 1.8V which is well below the datasheet minimum). These voltages are too low to drive the FETs that I have.
Edit: Ok, so it turns out that this is the real problem...
One more thing, and possibly the main issue... if VIN at the MCU is 2.5V, when the motor turns on there's a relatively large voltage drop to 1V for about 500ms due to the inrush current, which recovers to 2.24V while the motor is running. Surprisingly the MCU stays on (perhaps there's a cap on the dev board that helps), but this also means that the output that drives the FET gate drops to [just below] 1V, which is far too little to turn the FET on (and great for destroying the FET). The MCU is an ESP8266 which draws 70mA with WiFi on and 20mA with WiFi off.