A DC-DC converter includes a chip including an error amplifier and a pulse width modulator having an input connected to an output of the error amplifier, and an inductor driven by said PWM in series with an output node of the converter, wherein a load current flows through the inductor. In a memory cell array, the unit cell includes a memory cell field effect transistor and a select field effect transistor. According to another embodiment, a method of retailing eyecup assemblies includes providing a retailer with eyecup assemblies differing from one another and providing the retailer with an eyeglasses frame. The propagation constant level of the region of delaying propagation constant is controlled by varying the voltage. The light-blocking layer adjoins to the second surface and has a plurality of striped light-absorbing units. A specified position on a transparent display for displaying a first image associated with the first object is determined.