SDG #190 Testing the high current LED driver PCB (AL8843 & LM3409) and is XHP70 really 4000 lumens?

This is a build and bench validation episode rather than a straight product review: a custom high current LED driver board pairing an AL8843 3A buck regulator with an LM3409 hysteretic buck controller, intended to feed a Cree XHP70.2 emitter in a high output light engine.

The interesting engineering content is in how the two converters are used together and where the practical limits show up. The AL8843 is a convenient part on paper, but its current rating only holds up under specific thermal and duty conditions, so the loaded tests matter far more than the datasheet headline number. Load testing on the assembled board reveals how the regulation behaves as current climbs, how much heat the inductor and switch node dissipate, and whether the design can actually sustain the drive current the emitter needs rather than just survive a brief pulse.

A genuinely useful takeaway for anyone laying out their own switching supply is the point about capacitor ripple current rating. It is easy to pick input and output caps purely on capacitance and voltage, then find they run hot and age quickly because the RMS ripple in a hard driven buck exceeds what the part is specified for. That is the kind of detail that separates a board that works on the bench from one that survives in a torch or automotive fixture.

The closing section tackles the marketing question directly: whether an XHP70.2 genuinely delivers 4000 lumens. As with most LED figures, the answer depends on drive current, junction temperature and how honestly the flux bin is being quoted, and the measured reality is more modest than the headline claim once thermal droop is factored in.