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The imbalance in the currents of strings of LEDs will cause fast degradation or even failure in some LEDs. Such current imbalance should be avoided. To balance the common average current, a small duty cycle may necessitate a large LED turn-on current amplitude, which may cause temporal overheat and low luminous efficacy of the LEDs. This paper presents a current balancing method based on pulse-width modulation of a common bus voltage to each LED string to achieve the intended average current. An optimal feedback control scheme is proposed to maximize the duty cycles and minimize the bus voltage. As a result, at least one of the LED strings is operating at unity duty cycle. The analysis, implementation, and verification are detailed in this paper.