I am confused, looking for some clarification....
I have a multiplexed LED array that uses constant current shift registers on the cathodes, and PNP transistors on the anodes.
The microcontroller power supply is independent from the LED power supply. Previously I was using a 5V microcontroller and a supply voltage of 4.2V for the blue LED's. This worked OK.
I changed to a different microcontroller that only runs on 3.3V, and of course I can't turn off the PNP transistor because I can only put
3.3V to the base through the microcontroller.I thought I can change to P-channel MOSFETS with a logic drive. I bought some Diodes Inc DMG6968U (P-channel enhancement mode mosfet). On my board, the drain is connected to the LED's, the source is connected to the 4.2V supply... the problem is that the MOSFETs never turn off, so any column I try to light up, it will be lit up for all the rows, not just the one row.
I haven't used MOSFETS before, so I am sure there is something really basic I am missing. I thought a logic level drive MOSFET would let me control the 4.2V LED voltage with the 3.3V microcontroller voltage.
I thought of adding a "pull up" resistor between the gate and source, but then I'm putting 4.2V into the microcontroller, and the datasheet says it can only handle Vcc+0.5V, which would be 3.8V.
What is is that I am missing and what is the best solution?
Thanks!
CJ