SDG #171 Potential circuit issues killing the AL8805 LED Drivers

This episode is a fault finding post mortem on the AL8805 buck LED driver used in the high output microscope ring light project, and it doubles as a primer on why cheap hysteretic LED drivers fail in the field. Rather than blaming the silicon outright, the investigation works through the surrounding circuit conditions that can push the part outside its safe operating area.

The main suspects are inductor selection and saturation behaviour, which directly set peak switch current and can drive the internal MOSFET well beyond its rating during transients; switch node ringing and voltage overshoot that can exceed the absolute maximum input rating even when the nominal supply looks safe; and the interaction between input capacitance, supply lead inductance and hot plug events, which is a classic cause of failure in small integrated switchers. Thermal derating also receives attention, since the AL8805 in a small package has limited dissipation headroom once it is pushed toward its current ceiling with several LEDs in series.

Layout recurs as a theme, particularly the loop area on the high side of the converter and the placement of the current sense resistor and feedback path. Poor routing turns an otherwise adequate design into one that fails intermittently, which matches the pattern of drivers that work on the bench and then fail later.

For anyone building their own LED lighting, the practical conclusions are to specify inductors on saturation current rather than inductance value alone, add input protection or damping if the supply is long or switched, and leave real margin on both current and voltage instead of designing to the datasheet limits.