Illuminating electronic device and illumination method

Abstract

A display and keyboard illumination of battery-operated portable electronic devices using LEDs are provided. A flyback converter is modified into a dedicated converter suitable for using two LEDs and their diode-like operation is utilized. Firstly, a specified amount of energy of the battery voltage (V,) is stored in the coil (L) of a primary circuit until, controlled by a comparator (K). The LEDs are connected in parallel with the coil, in direction such that they substantially prevent the current leaking through them. Secondly, the coil is disconnected from the battery by a switch (SW). A comparator (K) detects when the coil current reaches a specified threshold level and, by using a control circuit (SWC), then causes the switch (SW) to become non-conductive. The self-induction of the coil builds up a voltage exceeding the forward bias of the LEDs, which (thirdly) keeps on passing the current and illuminating. Fourthly, the control circuit (SWC) receives a rising edge of a switching frequency signal and causes the switch (SW) to become conductive again. Thus, the supply power of the LEDs is substantially independent of variations in battery voltage and the illumination is substantially stable.