TLC5916 LED Driver Current Monotonicity Hack

I’m using Texas Instruments TLC5916 constant-current LED driver chips for my my friend’s Totally Featureless Clock. An 8-bit configuration value sets the output current, with the external resistor defining the maximum value as described there.

The problem is that the current-versus-config-value curve has a non-monotonic discontinuity in the middle, where the Current Multiplier bit switches from 0 to 1. I don’t know in which alternate universe this design decision made sense, but right here and now…

It. Does. Not.

Why it’s a problem: the LED brightness tracks room illumination as seen by a CdS photoresistor, averaged over maybe half a minute. The brightness changes very slowly, so the jump when it goes from 0x7f to 0x80 is really eyecatching. At least to me, anyway.

Eyeballometric measurement of the curve shows the current at 0x80 pretty much matches the current at 0x60, soooo let’s just shove the second quarter of the curve (between 0x40 and 0x7f) downward until it meets the high-current value at 0x80.

This code does the trick:

if ((Brightness >= 0x40) && (Brightness <= 0x7f)) {
 Brightness = 0x40 + ((Brightness & 0x3f) >> 1);
}

Basically, that maps 0x7f to 0x5f. The output current for 0x5f is pretty close to the current for 0x80, making the step pretty much a non-issue.

You could, if you were fussy enough, work out the actual current mapping values from the data sheet equations and make the ends match up perfectly.