App note from Richtek on their RT8487 compact buck-boost LED driver. Link here
RT8487 is a boundary mode constant current controller with internal high side driver, which can be used in buck and buck-boost configuration, to provide a constant output current to the (LED) load. It contains special circuitry for achieving high power factor and low input current THD, while minimizing external component count. The small SOT23-6 package keeps application footprint small, and makes RT8487 a cost effective solution for off-line LED drivers.