Dual-channel lights have a brighter minimum level for a few reasons.
On a 1x7135 driver…
- … it emits no light while the chip is powered up but sent a signal of zero. It’s off unless you’ve got a leaky, damaged chip. The lowest level is then determined by PWM and rise time. It uses 8-bit PWM, so there are 256 different levels, and the rise time is roughly half of a PWM clock cycle… so the brightness of each level is approximately 350mA * (N - 0.5) / 255. The brightest level for that chip is about 130 lumens. So moon on a 1x7135 driver is about 0.25 lm: 130 lm * 0.5 / 255.
On a linear (or dual linear) regulator…
- … simply powering up the regulator causes it to emit some light, even when sent a control value of zero. This base level varies quite a bit from one light to another, and sometimes even with temperature or voltage. The value is typically about 0.3 lm +/- 0.4 lm. So… 0 to ~0.7 lm. It’s not guaranteed to light up at “zero”, but most lights do.
- … and that’s per channel. So on a single-channel light, the minimum is typically about 0.0 to 0.7 lm, while on a dual-channel light, the “zero” value would be about 0.0 to 1.4 lm… very approximately.
- Then add PWM on top of that. The lowest level on a single-channel light is 0/16383, and on a dual-channel light it’s 2/16383. This is multiplied by the total output for that channel, which on a 9A light can be like 2000 lm. This works out to about 0 lm extra on a single-channel light, or 0.24 lm on a dual-channel light. 2000 lm * 2/16383 = 0.24 lm… plus whatever the “zero” baseline is.
- On dual-channel lights, the PWM value per channel is used to determine whether to turn that channel’s regulator on or not. With a value of 0, the regulator is off, while a value of 1 or higher turns it on. This makes it possible to turn one set of LEDs off at the extreme ends of the tint ramp. This raises the minimum brightness slightly, from ~0.3 lm per channel to ~0.4 lm per channel.
- The lowest total PWM value on dual-channel lights is 2, to make it possible to get a moon mode using the middle tint. It’s divided, so each channel gets 1/16383. Otherwise, moon could only be one tint or the other.
I hope this makes sense. On average, the bottom level for a good FET+1 driver is about 0.25 lm, while the current Noctigon linear drivers are often about 0.3 lm, and a dual-linear is about 0.8 lm… ish.
Measuring a single-channel KR4 vs a dual-channel D4S, I’m seeing ~0.28 lm on one and ~0.67 lm on the other. Most of that is a matter of having two slightly-leaky regulators vs one, but some is also that it’s using 2/16383 vs 0/16383 for its lowest PWM value.
To get significantly better moon modes, it would need a second regulator for each channel, with a much lower limit. Like, instead of a 9A chip having N steps from 0 to 2000 lm, or a 350mA chip having N steps from 0 to 130 lm, it could have a 20mA chip with N steps from 0 to 8 lm… Much more precise control. But this is difficult or impossible to fit onto small drivers, especially dual-channel drivers, because there is no physical space left on the circuit board.