Novel Low Power Drivers for Liquid-Crystal Displays Johan Stiens, Fr´ ed´ eric Devisch, Roger Vounckx, Maarten Kuijk LAMI, Laboratory of Micro and Opto Electronics, Electronics Department University of Brussels (VUB) Pleinlaan 2, 1050 Brussels, BELGIUM Phone: +32-2-629 23 97 Fax: +32-2-629 28 83 E-mail: jstiens@vub.ac.be WWW: http://etro.vub.ac.be/lami/jstiens/welcome.html Abstract — In this paper we focus on a novel generic method to decrease the power dissipation of the large number of output buffers (hundreds to thousands) that drive the large set of pixels, capac- itor loads. By simply adding one or more appropri- ate surface mount inductors to the driver IC, RLC resonant circuits can be periodically constructed to charge and discharge the pixels. When the RLC resonant circuit is interrupted after half an oscilla- tion period, a maximum amount of energy can be recycled, hence minimizing the dynamic power dis- sipation. After the RLC oscillation is interrupted, the pixels are snapped to their required gray level. We will discuss how the principle of RLC reso- nant circuits can be implemented for both passive and active matrix LCD’s. The implemented cir- cuits also reduce quiescent current, therefore di- minishing the static power. A prototype driver for a 24 × 200 passive LCD has been successfully im- plemented in 2.4mm CMOS technology: in a first experiment energy recycling efficiency of 15% have been demonstrated. Keywords — Energy recycling, output drivers, low power, liquid crystal display I. Introduction The demand for Liquid Crystal Displays (LCD) continues to exceed supply. LCD’s are implemented as screens on almost all types of digital devices, from portable electronic products such as watches, mo- bile telephones, lab tops, notebooks to desktop de- vices such personal computers, and projection dis- plays. The size of the display area has steadily grown and the general performance of LCD has steadily im- proved in the last years. This evolution holds the lime- light on the important issue of the power dissipation of the growing LCD’s. Especially in the development of portable electronic devices, the design driving force is the lowering of the power dissipation of every sin- gle component in order to increase the lifetime of the battery-operating devices. Amongst the different fac- tors contributing to the power dissipation of a LCD, are the background illumination, the pixel driving and the DC electronics. The background illumination can be completely eliminated in reflective mode applica- tions where natural incident light can be used. The remaining electronic power dissipation can be roughly divided in a 70/30 ratio between the pixel driving and the DC power. The most popular and most widely spread LCD are based on Twisted Nematics, Super Twisted Nematics and Cholesterics. Displays fabri- cated with these kinds of LCD-materials can only op- erate in combination with polarizers and analyzers. These polarizing devices induce optical losses such that more driving power is needed for the back light il- lumination or higher levels of natural incident light are required. More recently much effort has been spent in the development of P olymer D ispersed LCD [1]. From an optical point of view these PDLCD’s are attractive, as polarizers are no longer needed. However, the driv- ing voltages for the PDLCD pixels are higher, which partially counter-balances the optical advantages. In this paper we propose a novel method to dras- tically lower the power dissipated when driving the pixels, even at extended voltage levels, such that even- tually the LCD consumes less energy. The method is generally applicable to the rows as well as the columns of passive as well as active matrix displays. Cur- rent methods for driving the electrical signals onto LCD’s have been proposed to have already significant issues with power dissipation and image quality. Er- hart [2] implemented a capacitively based energy re- covery method for AMLCD displays. At the begin- ning of each row time, the column busses are shorted ISBN: 90-73461-18-9 473 c STW, 1999 10 19-01:071