I need to drive 50 (standard 5mm) LEDs, controlling the brightness of each LED individually, down to only 0.25mA and up to 25mA.
I started using TI's TLC5947 to drive these LEDs. This chip, as many of these CC LED drivers, use an external resistor for current reference. Using the built-in grayscale PWM, you can get a good brightness range. To extend even more this range, I change the current reference between the max and min on the datasheet. However, this chip can only go down to 2mA (practically 1mA) before the outputs get unstable. I need to go down to 0.25mA on each LED.
For a single LED it would not be a big problem, probably I just would implement a constant current sink using an op-amp. But when you have multiple LEDs, the problem is the number of control lines required, the cost of the circuit, and the complexity to keep to a minimum the difference between each "channel" of the circuit.
I need a way to implement this without having to replicate 50x an op-amp based circuit, to keep the complexity and cost realistic.