I have a scenario similar to Tuning a constant-current LED driver, per channel:
also considering use of two TLC5928 but with
- If <= 20 mA per channel
- 28 channels have Vf ~ 8V < 9.6V
- 4 channels have Vf ~ 4V < 4.8 V
- Vled driven by regulating 12V down to 10V (0.4V headroom for turn-on region of CC driver) for most channels, 5.2V for four channels
- Rref = 2.52 k
In the other question, it's suggested that dimming be done by either:
- moving to a TLC5948A that has built-in PWM dimming; or
- PWM Vled [by using] a high-side switch circuit which (depending on the rest of your circuit) could be as simple as a P-channel MOSFET
I'll do one of those if I have to, but I'm somewhat uncomfortable with the idea of PWM dimming on this board as it will also have a 2.4 GHz RF section (even though it's under a can). For that reason I want the broad design to reduce EMI where that's easy. Could I, instead of PWM dimming, fake a higher reference resistor by inserting an NPN pair in series with the minimum resistances? Something like
Would probably need a matched pair like the BCM847BS. Linearity is not very important so long as the left and right drivers are matched enough for the human eye; the brightness ramp can have an arbitrary compensation curve in software.
DAC ideally non-switched, maybe an R2R ladder since resolution and linearity aren't very important. Any DAC output and transistor drive changes would be trivially slow, on the order of a couple seconds. I'm on the fence about the regulators being SMPS or linear and am now leaning toward linear, because it won't produce switching noise, and there's lots of room in the heat-sinking and power budgets.
Is this approach practical? Will NPN saturation get in the way of maximum brightness? The BCM847 claims gain matching of 10% and Vbe matching of 2mV. Would the worst-case mismatch be visible to the eye?