Powergating method and apparatus

The present invention relates, in general, to the field of powergating in integrated circuits, such as integrated circuit memories. More particularly, the present invention relates to a powergating method and apparatus in which the internal switched power supply voltage rails are fixed at a mid-point voltage during the standby mode of operation.

Various approaches to powergating have been used in the past wherein the internal or powergated internal power supply voltage rails are allowed to float during the low leakage standby mode. Since the floating voltage is not controlled, the internal power supply voltages can traverse the entire voltage range from the VCC power supply to the VSS power supply. Alternatively, other prior art approaches force the internal rail to the opposite power supply voltage during the standby mode. For example the internal VCC rail may be forced to a voltage at or near VSS, whereas the internal VSS rail may be forced to a voltage at or near VCC.

Both of these two powergating approaches can be present reliability problems due to excessive voltage across the gate oxides of the powergating switching transistors. For thin oxide devices, this excess voltage could cause breakdown and other reliability problems. Alternatively, these two powergating approaches could create manufacturability and cost concerns by requiring a thicker gate oxide where a thinner gate oxide would be preferred.

A typical prior art powergating circuit is shown in FIG. 1 . Powergating circuit 10 includes a P-channel transistor M 1 having a source coupled to the VCC power supply, a gate for receiving the PPG control signal for switching between the active and standby modes, and a drain for providing the switched PGVCC internal power supply voltage. Powergating circuit 10 also includes an N-channel transistor M 2 having a source coupled to the VSS power supply or ground, a gate for receiving the NPG control signal for switching between the active and standby modes, and a drain for providing the switched PGVSS internal power supply voltage. A power-controlled circuit 12 has its power supply terminals coupled to the internal power supplies provided by the drains of transistors M 1 and M 2 .

The reliability problem referred to above could be exacerbated if the PPG and NPG control signals are at boosted voltage levels as taught in co-pending U.S. patent application Ser. No. 10/325,524, entitled “Powergate Control Using Boosted and Negative Voltages”, filed on Dec. 19, 2002, which is hereby incorporated by reference. Referring to FIG. 2 , during standby, either PGVCC goes to or near VSS or PGVSS goes to or near VCC. The PGVCC voltage is shown to undesirably extend below a mid-point voltage 14 in FIG. 2 . Similarly, the PGVSS voltage is shown to undesirably extend above a mid-point voltage 14 in FIG. 2 . The PPG control signal may go to a voltage above VCC and the NPG control signal may go to a voltage below ground or VSS. In this case the voltage across the gate oxides of transistors M 1 or M 2 (specifically the gate-to-drain voltage) could become too large and cause breakdown.

What is desired is a simple powergating circuit for use in an integrated circuit that addresses the reliability problems posed by prior art powergating circuits due to excess voltage dropped across the powergating switching transistors.

A powergating circuit according to an embodiment of the present invention includes a P-channel transistor with a source coupled to VCC, a gate for receiving a first boosted or non-boosted powergating control signal, and a drain forming the internal switched VCC power supply. An N-channel transistor has a source coupled to VSS, a gate for receiving a second boosted or non-boosted powergating control signal, and a drain forming the internal switched VSS power supply. A controlled circuit such as a CMOS circuit, sense amplifier, or the like, has a first power node coupled to the internal VCC power supply and a second power node coupled to the internal VSS power supply. The powergating circuit of the present invention further includes a circuit for forcing the first and second internal power supply voltages to a mid-point reference voltage during the standby mode.

According to an embodiment of the present invention, the circuit for forcing the first and second internal power supply voltages to a mid-point reference voltage during the standby mode includes a first N-channel transistor having a current path coupled between the internal VCC power supply and a mid-point reference voltage source. A second N-channel transistor has a current path coupled between the internal VSS power supply and the mid-point reference voltage source. The gates of the first and second transistors are coupled together for receiving a gate control signal, which is desirably the first powergating control signal. Ideally, the mid-point reference voltage source is the bit line reference voltage source, which is readily available in integrated memory circuits.

According to an embodiment of the present invention, the internal powergated supplies are forced to a known voltage level between VCC and VSS, utilizing an already existing mid-point power supply, thus avoiding the excessive gate voltages and consequent reliability problems found in prior art powergating circuit designs.

In another embodiment of the present invention, a circuit is provided for driving the latch nodes on a sense amplifier, wherein the internal power supplies coupled to the driving circuit are switched to the mid-point reference level during the standby or low power mode.

The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a prior art powergating circuit;

FIG. 2 is a timing diagram illustrating various waveforms associated with the powergating circuit of FIG. 1 ;

FIG. 3 is a schematic diagram of a powergating circuit according to an embodiment of the present invention;

FIG. 4 is a timing diagram illustrating various waveforms associated with the powergating circuit of FIG. 3 ;

FIG. 5 is a schematic diagram showing a sense amplifier circuit and associated circuitry;

FIG. 6 is a schematic diagram showing a circuit for driving the latch nodes of a sense amplifier;

FIG. 7 is a schematic diagram showing a circuit for providing the internal switched VCC and VSS power supply voltages to the circuit of FIG. 6 ;

FIG. 8 is a schematic diagram showing a circuit for driving the latch nodes of a sense amplifier according to an embodiment of the present invention;

FIG. 9 is a schematic diagram showing a circuit for providing the internal switched VCC and VSS power supply voltages to the circuit of FIG. 8 , including a provision for supplying a fixed bit line reference voltage during a standby mode.

A powergating circuit 30 according to an embodiment of the present invention is shown in FIG. 3 . Powergating circuit 30 includes a P-channel transistor M 1 having a source coupled to the VCC power supply, a gate for receiving the PPG control signal for switching between the active and standby modes, and a drain for providing the switched PGVCC internal power supply voltage. Powergating circuit 30 also includes an N-channel transistor M 2 having a source coupled to the VSS power supply or ground, a gate for receiving the NPG control signal for switching between the active and standby modes, and a drain for providing the switched PGVSS internal power supply voltage. A power-controlled circuit 12 has its power supply terminals coupled to the internal power supplies providing by the drains of transistors M 1 and M 2 . Powergating circuit 30 further includes N-channel transistors M 3 and M 4 , which have serially-coupled current paths coupled between the PGVCC and PGVSS internal switched power supplies. The common source/drains of transistors M 3 and M 4 are coupled to a voltage ideally set halfway between the VCC and VSS power supplies. A readily available bit line reference voltage, BLREF, is used for the mid-point reference voltage. The gates of transistors M 3 and M 4 are coupled together to receive the PPG control signal.

Referring now to FIG. 4 , the PPG, NPG, PGVCC and PGVSS waveforms are shown for the circuit of FIG. 3 . During the standby mode, the PGVCC waveform is prevented from reaching the VSS voltage and is clamped at the mid-point voltage 14 , which is set to the BLREF voltage. BLREF is typically set to (VCC+VSS)/2. Similarly, the PGVSS voltage is prevented from reaching the VCC voltage, and is also clamped at the mid-point voltage 14 . In normal standby operation, the voltage across the gate oxides of transistors M 1 and M 2 is prevented from exceeding (VCC−BLREF) or (BLREF−VSS) and breakdown is prevented. These voltages across the gate oxide will increase slightly if the boosted control signals PPG and NPG are used, but an improvement in reliability will still be realized.

Most DRAMs today include many columns of sense amplifiers. Given the number of sense amplifiers on a typical DRAM integrated circuit, it is important to reduce the leakage power during low power or standby modes.

A single sense amplifier 50 and associated circuitry is shown in FIG. 5 . Sense amplifier 50 includes cross-coupled P-channel transistors M 2 and M 3 , cross-coupled N-channel transistors M 4 and M 8 , as well as P-channel latch transistor M 1 , and N-channel latch transistor M 5 . Transistor M 1 is coupled to the VCC power supply and receives an LPB latch signal. Transistor M 5 is coupled to the VSS power supply and receives an LNB latch signal. The latch signals are typically used to turn on transistors M 1 and M 5 , which allows transistors M 2 , M 3 , M 4 , and M 8 to resolve the data state present on the BL and BLB bit lines. Sense amplifier 50 includes transistors M 6 and M 9 for transferring the latched data to the DL and DLB data lines under control of the Y control signal. Transistors M 7 , M 10 , and M 11 are used to equalize the bit lines to the BLREF mid-point voltage level under control of the SH control signal.

One way to reduce the power of sense amplifier 50 is to take the LNB and LPB node below VSS and above VCC, respectively, so that the V GS voltages of transistors M 1 and M 5 become negative. The powergated circuit 60 for providing these voltages is shown in FIG. 6 . Circuit 60 includes a SENB control signal for switching the sense amplifier 50 between an active latching mode and a standby or power down mode through the previously described LPB and LNB signals. A first inverter path includes serially connected inverters M 1 /M 4 and M 2 /M 3 . Inverter M 1 /M 4 is coupled between PGVCC and VSS or ground. Inverter M 2 /M 3 is coupled between the PPDVCC power supply and the PGVSS internal power supply. The PPDVCC power supply has a value of VCC during normal operation, and a value of VCC+0.4 volts during the standby mode of operation. Transistors M 1 and M 4 are thin oxide transistors having a length of 0.07 μ. Transistors M 2 and M 3 are thicker oxide transistors having a length of 0.22 μ. A second inverter path includes serially connected inverters M 5 /M 6 , M 7 /M 8 and M 9 /M 10 . Inverter M 5 /M 6 is coupled between PGVCC and VSS or ground. Inverter M 7 /M 8 is coupled between VCC and PGVSS. Inverter M 9 /M 10 is coupled between PGVCC and the NPDVSS power supply. The NPDVSS power supply has a value of zero volts during normal operation, and a value of negative 0.4 volts during the standby mode of operation. Transistors M 5 , M 6 , M 7 and M 8 are thin oxide transistors having a length of 0.07 μ. Transistors M 9 and M 10 are thicker oxide transistors having a length of 0.22 μ. For a representative fabrication technology only according to an embodiment of the present invention, the gate oxide layer of the thicker transistors are about 35 Angstroms thick and the gate oxide layer of the thinner transistors are about 17 Angstroms thick.

Referring to FIG. 7 , circuits 72 and 74 are shown for providing the PGVCC and PGVSS internal switched power supply voltages. Circuit 72 is a P-channel transistor M 17 wherein the source and body contact are coupled to VCC, the gate receives the PPG powergating control signal, and the drain provides the switched PGVCC supply voltage. Circuit 74 is an N-channel transistor M 18 wherein the source and body contact are coupled to VSS, the gate receives the NPG powergating control signal, and the drain provides the switched PGVSS supply voltage. The transistors of circuits 72 and 74 can both be thin oxide transistors.

Referring back to FIG. 6 , when PPDVCC goes to VCC+0.4V to cause LPB to go to the boosted level, the V GS of transistor M 2 becomes too large to use a thin oxide P-type device and a thicker oxide device should be used. For the same reason, transistors M 3 , M 9 and M 10 should be thicker oxide devices as well. These thicker oxide devices require longer transistor gate lengths and are slower than the thinner oxide and devices.

An improved sense amplifier driving circuit 80 according to an embodiment of the present invention is shown in FIG. 8 , wherein the PGVCC and PGVSS voltages are clamped at the BLREF mid-point voltage during the standby or power down mode.

Circuit 80 includes a SENB control signal for switching the sense amplifier 50 between an active latching mode and a standby or power down mode through the LPB and LNB signals. A first inverter path includes serially connected inverters M 1 /M 4 and M 2 /M 3 . Inverter M 1 /M 4 is coupled between PGVCC and PGVSS. Inverter M 2 /M 3 is coupled between the PPDVCC power supply and PGVSS. Transistors M 1 , M 2 , M 3 and M 4 are now all thin oxide transistors having a length of 0.07 μ. A second inverter path includes serially connected inverters M 5 /M 6 , M 7 /M 8 and M 9 /M 10 . Inverter M 5 /M 6 is coupled between PGVCC and VSS or ground. Inverter M 7 /M 8 is coupled between PGVCC and PGVSS. Inverter M 9 /M 10 is coupled between PGVCC and the NPDVSS power supply. Transistors M 5 , M 6 , M 7 , M 8 , M 9 , and M 10 are now all thin oxide transistors having a length of 0.07 μ.

Referring to FIG. 9 , circuits 92 and 94 are shown for providing the PGVCC and PGVSS internal switched power supply voltages according to an embodiment of the present invention, wherein the power supply voltages are forced to the BLREF mid-point reference voltage during the standby mode.

Circuit 92 is a P-channel transistor M 17 wherein the source and body contact are coupled to VCC, the gate receives the PPG powergating control signal, and the drain provides the switched PGVCC supply voltage. In addition, circuit 92 includes N-channel transistor M 22 having a source/drain for receiving the BLREF reference voltage, a body contact coupled to ground or VSS, a gate for receiving the PPG control signal, and another source/drain coupled to PGVCC.

Circuit 94 is an N-channel transistor M 18 wherein the source and body contact are coupled to VSS, the gate receives the NPG powergating control signal, and the drain provides the switched PGVSS supply voltage. In addition, circuit 92 includes N-channel transistor M 23 having a source/drain for receiving the BLREF reference voltage, a body contact coupled to ground or VSS, a gate for receiving the PPG control signal, and another source/drain coupled to PGVSS.

The transistors of circuits 92 and 94 can all be thin oxide transistors under boosted or non-boosted operating conditions.

Referring back to FIG. 8 , when PPDVCC goes to VCC+0.4V to cause LPB to go to the boosted level, the V GS of transistor M 2 is not too large to use a thin oxide P-type device since the gate is clamped to the BLREF reference voltage. For the same reason, transistors M 3 , M 9 and M 10 can all be thinner oxide devices as well.

By changing the source of transistors M 4 and M 7 to the powergated supplies and then forcing PGVCC and PGVSS to BLREF during powerdown, nodes N 3 and N 2 go to BLREF instead of ground and VCC. This lowers the electric field on transistors M 2 , M 3 , M 9 and M 10 and allows these devices to be thin oxide devices. Circuit 80 can therefore be made smaller and faster than circuit 60 , since the minimum length for thin devices is smaller than the minimum length for medium or thick devices. Also the widths of thin oxide devices can also be made much smaller due to the higher drive capability. The only additional circuitry required is transistors M 22 and M 23 shown in FIG. 9 . The reduced drive on transistors M 10 and M 2 shown in FIG. 8 is acceptable since this takes place in the standby condition. It is also acceptable if the LNB and LPB nodes take a long time to get to the PPDVCC and NPDVSS levels. The zero volt VGS on transistors M 9 and M 3 are also acceptable since their body contacts are held to VCC and VSS or PPDVCC and NPDVSS and therefore a large back-gate effect helps to raise their threshold voltages.

While there have been described above the principles of the present invention in conjunction with specific components, circuitry and bias techniques, it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation to the scope of the invention. Particularly, it is recognized that the teachings of the foregoing disclosure will suggest other modifications to those persons skilled in the relevant art. Such modifications may involve other features which are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure herein also includes any novel feature or any novel combination of features disclosed either explicitly or implicitly or any generalization or modification thereof which would be apparent to persons skilled in the relevant art, whether or not such relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as confronted by the present invention. The applicants hereby reserve the right to formulate new claims to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.