When controlling a group of LEDs for lighting applications, the relationship between LED brightness and electrical power is a key technical indicator that determines the quality of visible light. In low-power applications, it may not be possible to illuminate the entire set of LEDs to achieve large brightness due to the limited current source. In order to achieve excellent brightness when using a low power supply, the power consumption of each LED must be effectively managed. This requires a power management technique that provides power to only one of a set of LEDs at a given time. The time interval is also managed to achieve the desired illumination intensity without visually detecting that each set of LEDs is alternately lit.
To determine the number of LEDs that illuminate during a given time interval, the available power and luminous intensity of the application must be determined. The relationship between the luminous intensity and the forward current characteristics in the LED data sheet must be carefully examined to select the LED that meets the desired brightness level. After determining the number of LEDs required to achieve the desired brightness level, the LED can be calculated as follows. The ratio of total current to available current to determine the number of LEDs that can be powered during a given time interval:
The number of LEDs = total current required by the LED / total available power supply
In addition, the frequency at which a group of LEDs are turned on/off must be adjusted. The lighting time should be sufficient to cause all of the LEDs to illuminate, and the extinguishing time is limited by the time it takes for the set of LEDs to begin to become significantly darker. The off time limits the number of other LED groups that can be controlled in the application when a group of LEDs is off. Therefore, the extinction time limits the number of LED groups that can be managed by the time interval.
The low-cost implementation of this technology requires a clock source, a digital monoflop for controlling several sets of LEDs, and an OR gate that can detect the start condition with a simple On/Off switch.
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