Scalability of Quasi-Hysteretic FSM-Based Digitally Controlled Single-Inductor Dual-String Buck LED Driver to Multiple Strings

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There has been growing interest in single-inductor multiple-output (SIMO) dc-dc converters due to its reduced cost and smaller form factor in comparison with using multiple single-output converters. An application for such a SIMO-based switching converter is to drive multiple LED strings in a multichannel LED display. This paper proposes a quasi-hysteretic finite-state-machine-based digitally controlled single-inductor dual-output buck switching LED driver operating in discontinuous conduction mode (DCM) and extends it to drive multiple outputs. Based on the time-multiplexing control scheme in DCM, a theoretical upper limit of the total number of outputs in a SIMO buck switching LED driver for various backlight LED current values can be derived analytically. The advantages of the proposed SIMO LED driver include reducing the controller design complexity by eliminating loop compensation, driving more LED strings without limited by the maximum LED current rating, performing digital dimming with no additional switches required, and optimization of local bus voltage to compensate for variability of LED forward voltage VF in each individual LED string with smaller power loss. Loosely binned LEDs with larger VF variation can, therefore, be used for reduced LED costs.