1. Field of the Invention
The present invention relates to a semiconductor memory device and, more particularly, to a semiconductor memory device including a power supply circuit.
2. Description of the Background Art
In recent years, in order to realize cost reduction lower electric power consumption and higher speeds, miniaturization of transistors utilized in semiconductor memory devices has been progressed. In order to ensure the reliability of miniaturized transistors, it is necessary to lower the power supply potential which is fed from outside.
Semiconductor memory devices are generally equipped with an internal power supply circuit. For example, in the case of a DRAM, the internal power supply circuit serves to reduce a high power supply potential supplied from the outside to a lower potential, in order to supply a potential which is harmless to miniaturized transistors and capacitors in memory cells.
However, if the power supply potential supplied from the outside is reduced, this produces a problem that a sufficient driving potential for operations can not be supplied. Particularly, when the voltage generated between the gate and source of a MOS transistor constituting a sense amplifier in the memory array is not sufficiently higher than the threshold voltage of the MOS transistor, the driving capability of the MOS transistor is degraded, thereby increasing the time for amplifying micro-signals generated on bit line pairs.
A method for overcoming this problem is a sense amplifier driving method which is generally called a “over-drive sensing method”.
Conventionally, the power supply for the high-side potential of sense amplifiers for detecting and amplifying small potential differences read into bit line pairs has been a potential from the internal power supply circuit, namely a potential generated by reducing the power supply potential supplied from the outside. On the contrary, with the “over-drive sensing method”, the high-potential side power supply line of the sense amplifier is supplied with a power supply potential from the outside, instead of a reduced potential from the internal power supply circuit, for a constant time interval just before and after the activation of the sense amplifiers, thereby increasing the operation power supply for the sense amplifiers and enabling the sense amplifiers to amplify micro-signals at high speeds.
As a concrete example of the over-driving sensing method, for example, Japanese Laid-Open Patent Publication No. 2001-216779 discloses an internal power supply potential generating circuit and the control method thereof for semiconductor devices. The internal power supply potential generating circuit includes an over-drive driving circuit and two types of step-down regulators having different output potentials. Further, in the reading operation, the over-drive driving circuit is operated at first and then the first step-down regulator is operated, and on standby the second step-down regulator which outputs a lower potential than that of the first step-down regulator is operated, in order to reduce the current consumption.
Conventional over-drive sensing methods supply an external power supply potential to the sense amplifiers for a constant time interval, regardless of the amplitude of the external power supply potential. With this configuration, even if the external power supply potential is equal to or higher than the maximum value of specification of product or equal to or lower than the minimum value of specification of product, the external power supply potential is supplied to the sense amplifiers for a constant time interval. Therefore, if the external power supply potential approaches or exceeds the maximum value of specification of product, excessive voltages are applied to memory cells through the sense amplifiers.
On the other hand, if the external power supply potential approaches or decreases below the minimum value of specification of product, there has been generated a problem that the effect of high-speed operations of the sense amplifiers with over-drive method can not be obtained.