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

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

The closing section addresses 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 quoted, and the measured reality is more modest than the headline claim once thermal droop is taken into account.