17.8 A 90.5%-Efficiency 28.7µ VRMS-Noise Bipolar-Output High-Step-Up SC DC-DC Converter with Energy-Recycled Regulation and Post-Filtering for ±15V TFT-Based LAE Sensors

Abstract

The applications of large-area electronics (LAEs) based on thin-film transistors (TFTs) are rapidly expanding from displays to sensors. For the TFT gate drivers, high-voltage bipolar supply rails (approximately ± 15V) are required; so far, they have been typically generated from a battery $(V_{BAT})$ by employing switched-capacitor converters (SCCs) [1]. Since a high SNR is crucial for TFT-based sensors such as an under-display fingerprint sensor [2], the noise and ripple of the SCC output, which are prone to be coupled to the readout AFE, should be minimized. As a straightforward method, an LDO can be utilized as a post-regulator in series with the SCC. However, the relatively large dropout voltage $(V_{DO})$ of the LDO significantly degrades the efficiency [3]. In contrast, small VDO reduces LDO loop-gain due to the pass-transistor working in the triode region, resulting in decreased supply-ripple rejection (PSR). From the perspective of SCC, owing to its fixed voltage conversion ratio (VCR), the VDO cannot be finely regulated over a wide variation of VBAT. For fine regulation, the complexity (cost) overhead or the power loss will increase in the SC circuit. In this work, an energy-recycled optimal VDO control (EROC) technique in the SC bipolar step-up stage is proposed for higher efficiency. Also, load-current-reused (LCR) post-regulator is presented to achieve high PSR while extremely minimizing the power loss at the pass-transistor.