A Low-Power BL Path Design for NAND Flash Based on an Existing NAND Interface

Abstract

This paper is an extended version of a previously reported conference paper regarding a low-power design for NAND Flash. As the number of bits per NAND Flash die increases with cost scaling, the IO data path speed increases to minimize the page access time with a scaled CMOS in IOs. The power supply for IO buffers, namely, VDDQ, decreases from 3 V to 1.2 V, accordingly. In this paper, the way in which a reduction in VDDQ can contribute to power reduction in the BL path is discussed and validated. Conventionally, a BL voltage of about 0.5 V has been supplied from a supply voltage source (VDD) of 3 V. The BL path power can be reduced by a factor of VDDQ to VDD when the BL voltage is supplied by VDDQ. To maintain a sense margin at the sense amplifiers, the supply source for BLs is switched from VDDQ to VDD before sensing. As a result, power reduction and an equivalent sense margin can be realized at the same time. The overhead of implementing this operation is an increase in the BL access time of about 2% for switching the power supply from VDDQ to VDD and an increase in the die size of about 0.01% for adding the switching circuit, both of which are not significant in comparison to the significant power reduction in the BL path power of the NAND die of about 60%. The BL path is then designed in 180 nm CMOS to validate the design. When the cost for powering the SSD becomes quite significant, especially for data centers, an additional lower voltage supply, such as 0.8 V, dedicated to BL charging for read and program verifying operations may be the best option for future applications.