A New SRAM-Embedded Pixel Circuit That Modulates Accurately Gray Level for PWM-Driven Micro-LED Displays

A new analog pulse width modulation (A-PWM) pixel circuit is proposed for micro-light-emitting diode (micro-LED) displays, which improves the accuracy of modulating gray level by efficient switching of its in-pixel static random access memory (SRAM). Varied gray levels are realized successfully with the integral of a constant current over the corresponding designated duty for target gray. In practice, however, the imperfection of the current’s falling due to unavoidable switching delays results in the nonlinear inaccuracy of displayed gray levels, undermining displaying quality. To remedy the problem, the proposed in-pixel SRAM is able to conduct faster switching than conventional pixel circuits for precise displaying of grays. The simulated results show the falling time is significantly reduced to $0.8~\mu \text{s}$ from averaged $287~\mu \text{s}$ of that without SRAM, obtained a near-constant gray output of 6.63 nA*s/V. Based on experiments, the measured results show the DNL is improved from +0.05LSB-to-+0.78LSB to 0.09LSB-to-+0.14LSB, leading to the INL is reduced from the maximum +20.5LSB to +4.16LSB. In other words, the effectiveness offered by the proposed in-pixel SRAM in modulating accurately gray level is verified.