Why parallel FET LED dimming?
In last week´s post I didn´t explain in as much detail as I wanted to about why I´m building a complex, two-stage power supply and then making all the effort to put MOSFETs in parallel to each of the four LEDs. It is complex, and it means putting a bunch of circuitry on the small MCPCB where the LED will be mounted. I could have put four 2A sync buck LED drivers, each running from the two-cell LiON battery stack – even the green LED which claims to have a Vf of 5.0V at 25ºC would still work even if the batteries were almost drained because the UVLO is set to 6.0V. So why?
The rise and fall times of the LED current in the LEDs is the answer. I need them to be very fast. The faster the rise and fall times as LED current shifts from a given LED to its parallel FET and back again, the less of the dimming cycle is taken up with the LED current in an unknown state. (Meaning, somewhere between zero and 2A.) For the best color blending, that time should be minimized. That way, the greatest percentage of the dimming cycle has the current in a known state, including at the minimum and maximum dimming duty cycles.
Normally, anything that PWM dims below about 200 Hz will produce a strobing effect, and the human eye will see it blink, especially if the light source is moving. That might be cool for the illuminated sword, but I would keep it as an effect that can be turned on and off. Then, as you get close to 1000 Hz, several components in the power supply start to vibrate as the currents shift, making an irritating hum. It´s mainly the power inductors but also any large ceramic capacitors, especially multi-layer (MLC) capacitors. Hopefully the sound effects would mask the buzzing, but again, if you´re going to do something, do it right the first time. The only choice is then to go beyond 20 kHz. If you have pets and/or you´re an animal lover, you want to go to 30 kHz to save your dog´s and cat´s ears, too. If I used four buck LED drivers and then their logic PWM pins then the rise and fall times would be very slow, and at 30 kHz I would have barely any useable dimming duty cycle at all.