Low Power Sense Amplifier

The present invention relates generally to sense amplifiers, and specifically, to low power sense amplifiers for CMOS memories.

Most CMOS memories use a differential type sense amplifier as illustrated in FIG. 1. Advantages of the differential type sense amplifier are its high speed and its sensitivity to the small voltage changes between the bit lines. However, a major drawback of this type of amplifier is its high current consumption. This makes the use of a differential type sense amplifier undesirable in a CMOS memory, since a main advantage of CMOS technology is its low power comsumption. This problem is exacerbated for large word memories where each bit in a word requires a sense amplifier.

Some CMOS memories use cross-coupled sense amplifier latches as illustrated in FIG. 2 which do not use current in some situations. However, these cross-coupled sense amplifier latches are considerably slower than the differential type sense amplifier.

Thus, an object of the present invention is to provide a sense amplifier for use in a memory device which has low power consumption, yet is not significantly slower than the high power differential type sense amplifier.

Another object of the present invention is to provide a sense amplifier which is CMOS compatible.

A further object of the present invention is to provide a sense amplifier which consumes relatively little power, but has isolation between the data and bit lines.

This object and other objects of the present invention are attained by providing a sense amplifier with bit line sensors for sensing the voltage levels of a pair of bit lines, and a latch connected by data lines to the bit line sensors for latching the sensed voltage levels. Also provided is a means for preventing the formation of a conductor path to ground simultaneously through both the bit line sensors and the latch when the sensed voltage levels are latched. In preferred embodiments, means for isolating the bit lines from the data lines is provided.

By preventing formation of a conductor path to ground simultaneously through both the bit line sensing means and the latch means, current is not drawn in a fixed state. This reduces the operating power substantially. In providing bit line isolation, some speed is gained so that the overall speed of a memory device using the present invention is not significantly slower than one using a differential sense amplifier. In other words, the present invention sacrifices some speed in order to reduce operating power. However, the present invention is still faster than cross-coupled sense amplifier latches since the matrix cell devices attached to the bit lines must work against the cross-coupled sense amplifier latches.

Other objects, advantages, and novel features of the present invention will be become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

FIG. 1 is an example of a prior art differential type sense amplifier;

FIG. 2 is an example of a prior art cross-coupled sense amplifier latch; and

FIG. 3 represents a schematic and logic diagram of a low power sense amplifier incorporating the principles of the present invention.

FIG. 1 shows a prior art device using a differential type sense amplifier. As can be seen from this schematic, current is able to flow to the ground from the supply voltage when the amplifier is in a fixed state. The current paths include the current source 68. Also, devices 56 and 58 provide a path to ground when a column is selected. This is due to the bit lines being charged high between read operations, so that devices 58 turn on partially and draw current from devices 56.

When V.sub.REF is at a specified voltage, devices 58 are on. This allows 64 to conduct current drawn from the p-channel devices 60. Both sides of the amplifier are equalized by device 62. Thus, when the sense amplifier is enabled through device 66, a current path to ground is provided through devices 60, 64, and 66. Also, if the data is such that node 59 is high during a read, then that side of the differential amplifier turns on and provides a current path by pulling the gates of devices 60 low. As noted earlier, while providing speed, the differential-type sense amplifier as described above has high power consumption, making it less desirable in CMOS applications.

FIG. 2 shows a prior art device using a cross-coupled sense amplifier latch. While this circuit does not allow the supply voltage to flow current to ground when in a fixed state due to the bit line isolation devices 70 or 72 which are in the path to the data lines, it is too slow since the matrix cell devices attached to the bit lines work against the latch when data is switched. Since the matrix cell devices are very small, they cannot provide enough current to quickly switch the cross-coupled latch. Thus, although low in power consumption, this arrangement is too slow.

Referring to FIG. 3, a preferred embodiment of the present invention shows a logic diagram of a low power sense amplifier.

In a memory matrix, a number of pairs of bit lines are shown representing columns O through N. Only the sense amplifier of bit lines for column "O" will be described, since the remaining columns are similar. The bit lines 10, 12 for column "O" are connected to the drain of N-channel devices 14 and 16. In turn, the sources of devices 14 and 16 are connected respectively to the drains of N-channel devices 18 and 20, the sources of which are connected to ground. The column "O" select line is connected to the gates of devices 14, 16 and connected through an inverter 22 to the gates of devices 18, 20.

A third pair of gate devices 24, 26 have their gates connected between the sources of devices 14, 16 and the drains of devices 18, 20. The drains of devices 24, 26 are connected to Data and Data lines, while their sources are connected to ground. Devices 24, 26, also shown as N-channel devices, provide bit line to data line isolation.

Prior to selecting a column, the column select line is low. The inverter 22 pulls the gates of devices 18, 20 to a logic high turning them on, and this in turn pulls the gates of devices 24, 26 to a logic low. In this way, the gates of devices 24, 26 are discharged prior to the sensing of the matrix location between each column select and the devices 24, 26 are held off. This prevents the storing of charge on the gates of devices 24, 26, one of which was previously pulled to a logic high by its appropriate bit line. Otherwise, the stored charge would have to be discharged through one of the bit lines, thereby slowing down its selection. By this arrangement, the bit lines are discharged and isolated from the data lines.

When column select is high, inverter 22 pulls the gates of devices 18, 20 to a logic low, turning them off. This releases the gates of devices 24, 26 and allows the bit lines to pull the gates of devices 24, 26 to their respective voltage levels, so that one of the devices 24, 26 will turn on while the other will remain off.

Connected to the Data and Data lines in a sense amplifier latch 28. This sense amplifier latch has two P-channel devices 30, 32, the drains which are connected respectively to the Data and Data lines and their sources of which are connected to a logic high voltage. Connected to the gates of the P-channel devices 30, 32 is logic circuitry 33 comprising a separate branch for each device 30, 32. Each branch has a NAND gate 34, 36, connected to either the Data and Data line, respectively, in series with an inverter 38, 40 which is to the gate of the respective device 30, 32. Both NAND gates 34, 36 also receive a second input, namely a sense amplifier enable and precharge input (SAEPI) 42.

The logical circuitry 33 formed by NAND gates 34, 36 and inverters 38, 40 provide a logical low at the gates of the P-channel devices 30, 32 whenever either the sense amplifier enable and precharge input is a logic low or either of the Data or Data lines are a logic low. Alternative embodiments of the logic circuity 33 are contemplated, in addition to the embodiment illustrated in FIG. 1.

Prior to the column selection, SAEPI is a logic low. This holds the gates of the P-channel devices 30, 32 low via logic circuitry 33, allowing them to conduct pulling both the Data and Data lines up to the supply voltage. Just before a column is selected, SAEPI is set to a logic high. This turns off devices 30, 32 via logic circuity 33 and briefly allows the Data and Data lines to float at the supplied voltage. Once a column is selected, inverter 22 pulls devices 18 and 20 low, to release the gates of devices 24, 26. Either Data or Data line is pulled low, and the sense amplifier latch 28 turns on the P-channel device 30 or 32 which is attached to the line which has remained high. This keeps either the Data or Data line high and the other low, so that the data stored in the matrix cell is now on the Data line.

As an example, assume bit line 10 is high, and bit line 12 is low. When column "O" is selected, turning on device 14, 16, device 24 will turn on and device 26 will remain off. This pulls the Data line low from the high voltage at which it had been floating. The Data line remains at a high. Since the Data line is low, the voltage at the gate of device 32 is a logic low irrespective of the SAEPI input and NAND gate 36 so that device 32 is conducting. This keeps the Data line at a logic high. The high input from the SAEPI and Data line to NAND gate 34 assures a logic high at device 30, thereby turning the P-channel device off, so that the Data line will not be pulled up to a high and will remain discharged or logic low. In this way, the information on the Data and the Data lines will remain latched. The data stored in the matrix cell is now on the Data line and is available at a data output 44.

As can be gathered from the above description, when the sense amplifier is in a fixed state, either device 24 or series device 30 will be off while the other is conducting, and either device 26 or series device 28 will be conducting while the other is off. This assures that there is no conductive path from the supply voltage to ground, and thereby prevents draw of current in a fixed state.

The Data and Data lines are shown in FIG. 3 connected to an output enable 46. The embodiment illustrated in FIG. 3 of the output enable 46 shows inverters 48, 50 connected to the Data and Data lines respectively. The outputs of the inverters 48, 50 are connected to the inputs of a NOR gate 52 which provides an output enable signal.

When both the Data and Data lines are at a logic high, the output enable is at a high also. When a column is selected and either the Data or Data line is pulled low, the output enable is also pulled low. The output enable can thus be used to automatically tristate the output buffer when data is not selected.

As shown in FIG. 3, the data output 54 has three inverters. These serve to invert the signal on the Data line, which is the bit line signal that had been inverted by device 24. Thus, the true bit line signal is available at the data output due to the triple inversion.

The arrangement shown in FIG. 3 shows that a single sense amplifier latch can be used to latch data from a plurality of sense amplifiers, although only the latching of the data from one amplifier has been described above.

From the preceding description of the preferred embodiments, it is evident that the objects of the invention are attained and although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. The spirit and scope of this invention are to be limited only by the terms of the appended claims.