I built a small prototype as shown in my diagram with the Si926 n-FET, 250 ohm pull up resistor. In my prototype board I have only 4 channels connected (only two shown). Only one LED is ever turned on at a time. When I set 3 outputs off (ground) and only 1 on, the FET turns on and I am able to supply current close to my max of 100mA for specific LEDs. I can do this for any of 4 channels, one at a time with the other channels off. However, I had a pcb made with 32 channels, 32 FETS and 32 LEDs (only 16 pull ups) and 2 decoder chips. Now my circuit behaves oddly. If I turn on only one channel and others, as described, then slowly turn up the current source, the current supply drops out at 1ma. Drop out, is when the current source detects an open circuit or extremely high impedance and subsequently disables its output.
NOTE: I am not using the PWM of max6964. The brightness of the LEDS is controlled by the current source. The constant current source is part of a design restriction. It is supplied by a constant current supply that is controlled elsewhere. It is a bench top current source. One source for all LEDs.
Is it possible that having 32 of these output circuits is somehow effecting (lowering) the FET gate voltage with the decoder chip output low (ground) and thus causing the FET to not switch on?
Do I need a different pull up resistor? I wanted to keep it small so the voltage drop would be in the range of Vgs 1 to 2.5v.
Would changing the FET to Toshiba SSM6N43FU with Vgs 0.35v to 1V do the trick? Toshiba SSM6N43FU