This paper presents an asynchronous boost-type light-emitting diode (LED) driver, integrating an ambient light sensor (ALS), based on a constant off-time control technique. The LED driver automatically regulates the LED current according to the surrounding illumination as sensed by the on-chip ALS. In backlighting applications, the brightness of the light source is adjusted inversely; therefore, we enabled the selection of two modes of operation: illumination and backlighting. The proposed design is capable of reducing power dissipation for both illumination and backlighting applications. For verification, a prototype was fabricated using the TSMC 0.25-μm 60-V bipolar-CMOS-DMOS process with a chip size of 1900μm × 1330μm.