Low-noise TTL-compatible CMOS off-chip driver circuit

Abstract

Low-noise TTL-compatible off-chip driver (OCD) circuits are very important, especially for low-power electronics, but scaled-down CMOS technology requires a lower operating voltage of 3.3 V, while most applications require 5 V. The dual power-supply requirement makes the design of OCD challenging, first because pull-up devices, especially p-MOS devices, must be able to handle an off-chip voltage of 5.6 V, which is higher than an on-chip VDD of 2.8 V, and second because pull-down devices should be able to discharge a capacitive load of 5.6 V while operating at a minimum on-chip VDD of 2.8 V. This extreme difference in operating voltage makes the circuits susceptible to ringing and performance degradation due to hot-electron effects. In this paper, we describe a low-noise OCD which has been successfully used in IBM second-generation 4Mb low-power DRAM (LPDRAM) and in other products. For pull-ups, two stacked p-MOS devices with floating n-wells are used, but they are operated in different modes depending on the supply voltage. The pull-down devices are basically composed of two stages, one of which is in the diode configuration with its gate and drain shorted together during the pull-down. Detailed circuit designs to achieve low noise while meeting the performance requirements are described.