I’d stipulate the same, but have been dealing with this LED stuff for the past few months myself, and here’s what I’ve found:
Kind of. You are absolutely correct that directly controlling the brightness via PWM will not work – you’ll get flicker, or lines in the image that correspond to the PWM duty cycle as soon as it drops below 100%. But depending on your LED driver, this can be overcome.
Using an LED driver chip like the ones in the driver we’re using, allows you to dim the LEDs without flicker. The driver is constant current to the LEDs, but the amount of current is controlled by either PWM or analog voltage.
Currently (haha, see what I did there?) the Sparkfun Femtobuck driver we’re using as a stopgap passes the PWM pulses through to the output, which is no good - with a duty cycle below 100%, you get lines in the resulting image. However, that driver can also dim the LEDs by changing an analog voltage on the input signal on a scale of 0-100%, between 0V and 2.5V (instead of using PWM). So you’re still getting constant current to the LEDs, which you want, but you’re not getting the PWM pulse that causes the flicker.
But PWM is great - very easy to quantify brightness from the software side, so it’s preferable if you can do it. The good news is that PWM can be converted to an analog voltage with a DAC chip. Basically, PWM in → Smooth analog voltage out. Modify the PWM duty cycle and you get a corresponding change in voltage. Connect that analog voltage to a driver like the Femtobuck, and you should have smooth dimming.
We’re having someone design this for us now - essentially it’s a clone of the Femtobuck driver with slightly higher allowable voltage, but with an integrated DAC so you can use either straight PWM to the DAC or use a jumper to bypass it, to send analog voltage directly in (assuming you have a device that can output voltage)
If it works, I’ll make it available. Should have the design in the next day or two, and then I can order parts and be testing it in the scanner next week.
I think you should reconsider controlling the brightness with voltage and instead use a constant current driver for each color channel. Using a driver that’s designed to power LEDs with constant current should gets you more consistent lighting across a series. See the first answer here.