I'm trying to switch a high-current, 12V load (large parallel LED strip, with the anode connected to
LOAD-) using a PWM chip. My approach right now is to use a Power MOSFET to switch the load; and due to the high capacitance of the gate and higher gate threshold, I'm using a second MOSFET to drive the first MOSFET.
This is the schematic:
5V is the output of a linear voltage regulator rated for 800 mA, which I don't think is an issue in this circuit.
The PWM Driver (a WS2801) is pull-down, so both FETs have a pull-up resistor to return the gate voltage to 5V.
I'm seeing that Q1 behaves as expected: when there's no signal applied, Q1 is pulled up high by R8, and conducts. Q1 conducting pulls Q2 low, so Q2 stays "off" in this state.
When a PWM signal is applied, Q1 turns on and off per the signal. However, the Q2 never fully conducts. Upon further inspection, the gate of Q2 never goes above 700 mV or so. It definitely pulses between 0 V(ground) and ~700 mV in a way that matches the PWM signal, so Q1 is definitely conducting, but for some reason the gate of Q2 is never pulled up high to 5V.
I feel like I'm missing something fairly obvious here, but can't figure it out -- why isn't Q2 switching as I expect it to?