Ghosting in Multiplexed LED Matrix: Analysis and Deghosting Circuit Optimization

Ghosting is an undesired phenomenon in multiplexed LED matrix, where LEDs that should remain OFF emit a faint residual light due to transient current paths occurring during the switching sequence. This effect can degrade image quality, contrast, and visual accuracy, particularly in automotive backlighting applications driven by LED driver integrated circuits. The objective of this thesis is to investigate the physical mechanisms responsible for ghosting and caterpillar effects in multiplexed LED matrix, to evaluate the effectiveness of the high-side de-ghosting feature already integrated into the investigated LED driver, and to enhance the overall de-ghosting feature with a dedicated low-side de-ghosting circuit. The work begins with a theoretical analysis of the mechanisms responsible for ghosting and of the main techniques proposed in the literature to mitigate it. Existing de-ghosting strategies are also reviewed and compared with the solutions adopted by commercial LED drivers. An extensive experimental characterization was then carried out using dedicated evaluation boards to investigate high-side ghosting, low-side ghosting, ghosting caused by capacitors connected in parallel with the LEDs, and caterpillar effects associated with both open-circuit and short-circuit faults. The measurements confirmed the effectiveness of the integrated high-side de-ghosting feature when its operating parameters are properly selected and provided a detailed understanding of the influence of the main operating conditions on the observed phenomena. Based on the acquired understanding of the low-side ghosting mechanism, a dedicated external low-side de-ghosting circuit was designed and validated through circuit simulations. The influence of its design parameters was investigated, and the proposed solution was verified under both normal operating conditions and fault scenarios. Simulation results demonstrated that the circuit effectively suppresses low-side ghosting without compromising the existing high-side de-ghosting functionality or the mitigation of caterpillar effects. Overall, the objectives of the thesis were successfully achieved. The proposed methodology provides a comprehensive understanding of ghosting and caterpillar phenomena in multiplexed LED matrix and demonstrates an effective low-side de-ghosting solution that complements the existing high-side de-ghosting feature, contributing to the development of more reliable and higher-quality automotive backlighting and rear-lighting systems.