Research On Charge Pump Used In Flash Memory

Flash memory has only a single-voltage power supply, and the voltage is far low from the high-voltage requirement to achieve writing and erasing. So charge pump(CP), as high-voltage generation circuit, is an important part of flash memory system. Recently, flash memory has been widely used in portable electronic products, such as Smartphone, tablet and other mobile electronic devices. To meet these applications’ requirements, some specs for charge pump such as transferring efficiency, output voltage stability and chip area have become increasingly crucial.There are two methods to improve the performance of charge pump: optimization of parameters and circuit structure. Firstly, dynamic and static analysis of charge pump operation has been given in detail, and it also expounds low power-consumption design method. Then, three charge pump topologies – cross coupled CP, CTS CP and four-phase CP – are analyzed in detail, and based on this analysis, new charge pump topology is proposed. Proposed charge pump combines cross coupled CP topology and four-phase CP topology and achieves better transferring efficiency and current driving capability.The proposed circuit uses low power-consumption method to get better performance and uses cross-coupled structure to control gate bias of the associated transistor. New four phase waveform eliminates power consumption of associated capacitor in overlap time. Cross-coupled output stage has no threshold drop, which improves output efficiency. Lastly, a charge pump system is designed in TSMC 0.18 um process including bandgap reference circuit, comparator and voltage divider, and related simulation results are also given in the end. The efficiency of proposed charge pump can reach to 64%, and have maximum load current 510 uA with 13.5V output voltage.