1. Field of the Invention
The invention relates to the field of sensing circuits in memories, particularly in MOS static memories.
2. Prior Art
In static metal-oxide-silicon (MOS) memories a sensing node which is selectively coupled to column lines in the array is charged through a pull-up transistor. When the column lines in the array are coupled to a selected cell the charge on the sensing node may discharge depending upon the binary state stored in the cell. A sense amplifier coupled to the sense node senses the drop in potential or the discharge of the node if it occurs, thereby producing an output signal representative of the binary state of the cell.
In such prior art circuits it is apparent that if the pull-up transistor has a low conductive resistance, it will charge the sense node more quickly, thereby allowing more frequent reading. However, a low conductive resistance of the pull-up transistor results in slower discharging of the sense node since the pull-up transistor will resist the discharge. During discharge it is desirable then to have a pull-up transistor having a high impedance. In general, a compromise is necessary to obtain both acceptable charging and discharging times.
The problem of selecting a compromise conductive resistance becomes more difficult where a plurality of column lines are coupled to a single sense node or where the memory cells are current limited. The larger capacitance associated with the plurality of memory lines require that the resistance of the pull-up transistor be low to compensate for this added capacitance, whereas the higher impedence, current limited cells, require a higher resistance pull-up transistor to achieve rapid discharge of the sense node.
As will be seen, the present invention provides a feedback circuit which provides an alternate current path for charging the sense node. When the potential on this node drops below a predetermined level the alternate current path prevents the node from further dropping in potential and, moreover, since there is an alternate charging path the pull-up transistor may have a higher impedance.