SDG #265 Easy Constant Current LED Driver with the AL5809

The AL5809 is a deceptively simple part: a two terminal linear constant current regulator that sits in series with an LED string and holds the current steady without inductors, switching noise or a pile of support components. This build looks at what that simplicity actually buys you in a real circuit, and where the compromises show up.

The key advantage over a plain series resistor is that the LED current stops tracking supply voltage. With a resistor, a few volts of variation on the rail translates directly into visible brightness change and a risk of overdriving the LED; the AL5809 absorbs that headroom instead. That makes it attractive for indicator lighting, panel backlights and any design where the input rail is loosely regulated or comes straight from a battery.

The trade off is the same one that applies to every linear regulator: the excess voltage is dissipated as heat inside a small package, so headroom and thermal design matter more than the two pin count suggests. Choosing a variant with the right fixed current rating keeps that dissipation sane, and stacking devices or adjusting the string voltage gives some control over where the power goes.

Dimming is the more interesting part of the exercise. Because the device regulates current rather than voltage, brightness control needs a different approach to the usual resistor-and-pot method, and the results here show how predictable the response is across the range. Also covered: PCB rework using Metcal PTZ tweezers to swap parts on the test board, which is a useful demonstration of desoldering small two terminal packages cleanly.