I am working on a design for a 16-channel LED driver. At the moment the basic design is based on three main components:
- NCP781BMN050TAG Voltage regulator
- PCA9685 I2C PWM Driver
- CSD19534 MOSFET
The circuit needs to initially drive a 110 V LED light with 250 mA LED strings, but should to be adjustable to drive sets of other LEDs e.g Ledengin LZ7
simulate this circuit – Schematic created using CircuitLab
Without further components I think this design should work in principle with M1 getting an RDS(on) of ~30 mΩ (other similar MOSFETs had a larger RDS(on)). The RDS(on) can be reduced further if I can supply a higher voltage (14.1 mΩ at 6 V and 12.6 mΩ at 10 V).
(This combination of R2, M1, Q1, R2, and D1..n are repeated for each channel though R1 will be varied dependent on different D1..n on each channel.)
The PCA9685 can supply a limited current of 10 mA per channel and if I'm reading it right 25 mA total, and it can operate PWM at up to 1526 Hz.
Simulation at 1526 Hz PWM (assuming I did it right) shows if I have R2 = 10 kΩ I get an inrush current of 500 μA which should be no issue.
If I reduce R2 to 1 kΩ I get 5 mA which is likely too large to operate more than 4 channels at once, and 100 Ω gives 50 mA which is too large for a single channel and the PCA9685.
How do I determine the best value for R2?
Would the inefficiency resulting from a large R2 make it worth putting in a gate driver e.g a MAX620EWN+ (would need 4) or a cheaper alternative?
Or is there another cost-effective way to make this more efficient?
Simulation model and output (the New macro are a PLW7070GA Spice model)