DC-lighting and pulse-lighting are available as methods for lighting LEDs. For example, pulse-lighting is utilized as an LED-based backlight of a liquid crystal display. In general, in the case of an LED-based backlight, the LEDs are pulse-lighted by means of pulse width modulation (PWM) at a specific frequency (for example, 10 KHz), and the brightness is adjusted by means of the ratio (duty ratio) between on-time and off-time within each cycle. In this case, during each cycle, an LED drive circuit is used to carry out switching operations such that many LEDs arranged two-dimensionally are turned on (lighted) at the same time at the beginning of the pulse-lighting, and then they are all turned off (turned off).
There is a problem that a high-frequency LED drive current that flows when the PWM-based switching operation is carried out to turn on/off the many LEDs at the same time becomes a source of electromagnetic interference or EMI. A standard EMI value is set at a defined value that must be satisfied in a final product, that is, it must be lower than the standard value. Since EMI as electromagnetic waves emitted from an applicable product affects the product itself and its surroundings (it may appear in the form of false operation of a system circuit inside a product, a screen disturbance, or radio noise), a component such as a shield or a filter is added to the source as a measure to reduce the emission of electromagnetic waves when a measured value exceeds the standard value. However, the conventional measure is disadvantageous in that it incurs high component and development costs.
USPTO Backgrounds
USPTO Backgrounds