Interface circuit for mixed voltage I/O buffer to provide gate oxide protection

This application claims the benefit of U.S. Provisional Application No. 60/172,859, filed Dec. 20, 1999 and is hereby incorporated by reference in its entirety.

The present invention relates to input/output buffers generally and, more particularly, to an interface circuit for providing gate oxide over-voltage protection to a mixed voltage I/O buffer.

The trend in modern central processing units (CPUs) and microprocessors is to reduce the power supply operating voltage in order to reduce power consumption and increase chip density. Power supply reduction may impact other performance considerations. Due to design considerations, memory devices, such as dynamic random access memories (DRAMs), may operate at a different supply voltage than the CPU. Some devices also may be required to use more than one power supply voltage. For example, a CPU related device can respond to a signal at one voltage while other devices require another voltage. The signals can be generated by one circuit and received by another circuit.

One such configuration occurs with modern microprocessors that operate with a nominal power supply voltage of about 2.5V (or lower) while other circuits in the computer operate with a power supply voltage of about 3.3V. To facilitate communication between devices operating at different voltages, an input/output driver circuit is used.

Referring to FIG. 1, a diagram illustrating a conventional method of limiting an input voltage level to an input buffer is shown. In the conventional method, an NMOS transistor 10 is placed in series between a pad 12 and an input buffer 14 . The gate of the NMOS transistor 10 is connected to a supply voltage VCC. The use of the NMOS transistor 10 to limit voltage can degrade voltage levels at the input terminal of the input buffer.

The degraded voltage levels can result in increased static and dynamic currents in the input buffer 14 . Such a degration is especially true in an input buffer designed in low voltage technologies (e.g., 2.5V or less) since the threshold voltage does not scale proportionately with voltage. The voltage degradation will reduce the noise margin for the input buffer 14 . The NMOS transistor needs a thick gate oxide requiring dual gate oxide technology.

It would be desirable to provide an interface block between the PAD 12 and the input buffer 14 . Such an interface block would preferably take all the input voltage from 0V to >=Vcc+|vtp|(voltages >=Vcc+|Vtp| seen as an overvoltage condition) as seen at PAD 12 and output a voltage from 0v to Vcc. Such an interface should not draw undesired large (>1μA) currents from the PAD 12 or components (e.g., an input/output buffer).

The present invention concerns an apparatus comprising a first circuit and a second circuit. The first circuit may be configured to generate an output signal having a first voltage level and a first control signal in response to (i) an input signal having a second voltage level, (ii) an enable signal, and (iii) a plurality of node voltages. The second circuit may be configured to generate the plurality of node voltages in response to the first control signal. The first circuit may be configured to limit the first voltage level.

The objects, features and advantages of the present invention include providing an interface circuit for a mixed voltage I/O buffer that may (i) provide gate oxide protection, (ii) have no active voltage degradation, (iii) reduce static and dynamic currents, (iv) maintain a comparable noise margin in a low voltage circuit, (v) use different overvoltage detection thresholds for input and output buffer operation, (vi) provide early detection of an overvoltage condition at an input pad for an input buffer operation, (vii) provide sufficient safety margin for voltage level across an input buffer's gate oxide, (viii) provide early detection of an overvoltage condition that may compensate for the delay of overvoltage detection, (ix) provide improved speed of overvoltage detection, (x) provide fast removal of residual stored charge, (xi) reduce gate oxide stresses and improve circuit recovery from an over voltage condition, (xii) fit well for both input and output buffers, and/or (xiii) operate equally well for a number of voltage differences (e.g., 2.5V-3.3V, 1.8V-2.5V, etc.).

These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which:

FIG. 1 is a block diagram of a conventional method to limit a voltage presented to an input buffer;

FIG. 2 is a block diagram of a preferred embodiment of the present invention;

FIG. 3 is a schematic diagram of a logic block of FIG. 2;

FIG. 4 is a schematic diagram of another logic block of FIG. 2;

FIG. 5 is a schematic of a charge removal block of FIG. 4;

FIG. 6 is a graphical representation illustrating the desired operation of the present invention; and

FIG. 7 is a graphical representation illustrating an example operation of the present invention.

Referring to FIG. 2, a block diagram of a circuit 100 is shown in accordance with a preferred embodiment of the present invention. The circuit 100 may be implemented, in one example, as an interface circuit for a mixed voltage I/O buffer. The circuit 100 may provide gate oxide protection.

The circuit 100 generally comprises a logic circuit 102 , a logic circuit 104 and an input buffer 106 . The logic circuit 102 may be implemented, in one example, as an output buffer. The output buffer 102 and the input buffer 106 may be implemented in combination, in one example, as an input/output buffer. The logic circuit 102 may have an input 108 that may receive a signal (e.g., C 1 ), an input 110 that may receive a signal (e.g., DATA 13 IN), an output 112 that may connect to a node or present a signal (e.g., PAD), an input 114 that may receive a signal (e.g., OE), an input 116 that may receive-a signal (e.g., PMOS 13 CNT) and an input 118 that may receive a signal (e.g., FLOAT). The circuit 102 may also have an connection 120 that may present/receive a signal or node (e.g., IN) and an connection 122 that may present/receive a signal or node (e.g., CHARGE 13 NODE). In one example, the signal CHARGE 13 NODE and the signal IN may be common nodes connected between the logic circuit 102 and the logic circuit 104 .

The logic circuit 104 may have an output 124 that may present a signal or node IN 1 , an output 126 that may present the signal FLOAT and an output 128 that may present the signal PMOS 13 NT. The logic circuit 104 may also have a connection 130 that may present/receive the signal or node CHARGE 13 NODE and a connection 132 that may present/receive the signal or node IN. The 5 logic circuit 104 may also have an input 134 that may receive the signal OE and an input 136 that may receive the signal PAD.

The input buffer 106 may have a connection 138 that may receive the signal or node IN 1 and an input 140 that may receive a signal (e.g., EN). The input buffer 106 may also have an output 141 that may present a signal (e.g., OUT).

The signal EN may be an enable signal presented to the input buffer 106 . The signal OE may be an output enable signal (or control signal) presented to the logic circuit 102 and the logic circuit 104 . The signal PAD may be a pad voltage received from or presented to an external device (not shown). The signal DATA 13 IN may be a data input signal. The signal C 1 may be a control signal. The signal FLOAT, the signal IN, the signal PMOS 13 CNT and the signal CHARGE 13 NODE may be control voltages that may be used to control the operation of the circuit 100 .

Referring to FIG. 3, a more detailed diagram of the logic circuit 102 of FIG. 2 illustrating a preferred embodiment of the present invention is shown. The logic circuit 102 generally comprises a transistor PF 01 , a transistor N 01 , a transistor N 02 , a transistor PF 02 , a transistor PF 03 , a transistor N 03 , a transistor 150 , a transistor 152 , a transistor 153 , a transistor 154 , a transistor 155 , a transistor 156 , a transistor 158 and a transistor 160 . The transistor PF 01 , the transistor PF 02 and the transistor PF 03 may receive the signal FLOAT at a respective substrate node (connection). The transistors 152 and 154 may receive the signal OE. The transistors 156 and 160 may receive a digital complement of the signal OE (e.g., OEb). The transistors 150 , 155 and 158 may receive the signal DATA 13 IN. The transistor 153 may receive the signal C 1 . The signal PAD may be presented at a node formed by a connection of a source/drain of the transistor PF 01 and a source/drain of the transistor N 01 . The signal CHARGE 13 NODE may be presented to a gate of the transistor PF 03 . The signal IN may be presented to a source/drain of the transistor PF 02 . The signal PMOS 13 CNT may be presented to a gate of the transistor PF 02 .

Referring to FIG. 4, a more detailed diagram of the logic circuit 104 of FIG. 2 is shown. The logic circuit 104 generally comprises a voltage generation circuit 170 and a circuit 172 . The circuit 170 may be a floating n-well voltage generation circuit. The signal (or voltage) IN may be presented to the circuit 170 . The circuit 170 may be configured to generate the signal (or voltage) FLOAT in response to the signal IN. The signal FLOAT may be used to bias a number of n-wells (substrates) of a number of transistors.

The circuit 172 generally comprises a charge removal circuit 174 and a number of transistors and inverters. The charge removal circuit 174 may have an input 175 that may receive the signal or node IN. The charge removal circuit 174 may have an input 176 that may receive either (i) a signal or node (e.g., CHARGE 13 NODE 1 ) and/or (ii) a signal or node (e.g., CHARGE 13 NODE 2 ). The charge removal circuit 174 may have an output 178 that may present the signal or node CHARGE 13 NODE. The transistor P 1 and the transistor N 2 may receive the signal OE. The transistor PF 1 , the transistor PF 2 , the transistor PF 3 and the transistor PF 4 may have an n-well terminal that may receive the signal FLOAT.

Referring to FIG. 5, a schematic diagram of a preferred embodiment of a charge removal circuit 174 is shown. The charge removal circuit 174 generally comprises a transistor NC 1 , a transistor NC 2 , a transistor 180 , a transistor 182 , a transistor 184 , a transistor 186 , a transistor 188 , a transistor 190 and a transistor 192 . The transistor 180 may have an N-well terminal that may receive the signal FLOAT. Either the signal CHARGE 13 NODE 1 or the signal CHARGE 13 NODE 2 may be presented to a gate of the transistor NC 2 . The signal CHARGE 13 NODE may be presented at a drain of the transistor NC 2 . The signal CHARGE 13 NODE may represent the signal CHARGE 13 NODE 1 or the signal CHARGE 13 NODE 2 after a charge has been removed.

Referring to FIG. 6, a voltage vs. time diagram illustrating the desired operation of the circuit 100 is shown. The input PAD may be connected to the common system bus of a mixed voltage system. The input PAD is generally presented to the gates of transistors facing PAD. During normal operation a voltage 200 (e.g., VCC) is generally supplied to the input buffer 106 . During an overvoltage condition a voltage 202 (e.g., >=VCC+VTP) may be supplied to the input buffer 106 . The overvoltage condition when applied to the input buffer 106 may damage the gate oxide of the transistors of the input buffer 106 . The interface 100 may be implemented to control the overvoltage condition such that the voltage supplied is limited to VCC.

Referring to FIG. 7, a voltage vs. time diagram illustrating an example operation of the circuit 100 is shown. When an overvoltage condition threshold 204 is reached, the output of the circuit 100 is generally limited to VCC.

The present invention involves the design of interface circuitry that may prevent an overvoltage condition of the signal or node IN 1 of an input buffer that may be used in a mixed voltage environment system. The circuit 100 may ensure gate oxide protection for transistors that may receive the signal PAD. Such gate oxide protection may be achieved by avoiding a direct path from the signal PAD to the signal IN 1 . When an overvoltage SO a condition of the signal PAD exists, a correct logic level value may still be maintained on the signal IN 1 , which may ensure the correct operation of the input buffer 106 .

An overvoltage condition of the signal PAD (e.g., a voltage level >=3.3V) is generally sensed by the PMOS transistor PF 1 . The transistor PF 1 may have a floating n-well that generally starts tracking the voltage of the signal PAD during the overvoltage condition. The detection of the overvoltage condition of the signal PAD is generally used to present a control signal (e.g., CHARGE 13 NODE 2 ) to the gate of the PMOS transistor of pass transistor PT 2 . The control signal CHARGE 13 NODE 2 may be generated by a latch path formed by the inverters IL 1 , IL 2 , IB 1 and the pass transistor PT 1 . The control signal CHARGE 13 NODE 2 generally forces the gate of the PMOS portion of the pass transistor PT 2 to follow the voltage of the signal PAD. The signal CHARGE 13 NODE 2 may turn the pass transistor PT 2 “OFF”. A correct logical level of the signal IN 1 , in this condition, is generally maintained by the PMOS transistor PF 4 . The inverter IB 3 and the PMOS transistor P 3 may also contribute to maintaining a correct logic level of the signal An overvoltage detection threshold of Vcc−Vtn+|Vtp| or Vcc+|vtp| is generally provided by the transistors P 1 , N 1 , and N 2 , based on the output buffer signal OE. In a normal voltage condition of the signal PAD (e.g., 0 to VCC), the signal CHARGE 13 NODE 2 is generally maintained at a LOW state by the above latch path and the NMOS transistor N 5 .

Once the overvoltage condition of the signal PAD has ended, the charge removal circuit 174 may provide fast residual charge removal from nodes CHARGE 13 NODE 1 and CHARGE 13 NODE 2 . The charge removal circuit 174 may comprise two series connected NMOS transistors NC 1 , NC 2 , and a number of transistors 180 - 192 .

The present invention may be implemented as a method of overvoltage detection with control signal generation to provide gate oxide protection against over voltage at an input of an input buffer comprising the steps of (i) inputting the PAD voltage in to transistors, at or above overvoltage detection threshold, (ii) coupling the output to a latch in response to the above step and (iii) generation of a control signal.

The voltage PAD may arrive after going through an ESD protection structure. The method may allow the voltage PAD to pass through (e.g., inputting) the transistors PF 1 and N 3 during an overvoltage condition. The overvoltage condition may be detected by the transistors P 1 , N 1 , and N 2 , based on the control signal OE to I/O buffer 106 . A voltage rise of the signal CHARGE 13 NODE 1 may be detected by a latch formed by the inverters IL 1 , IL 2 , and the pass transistor PT 1 with inverter IB 1 . The latch may pull the signal CHARGE 13 NODE 1 to a HIGH state.

The step of generating control signals may include using the HIGH voltage of the signal CHARGE 13 NODE 1 to generate a control signal or voltage (e.g., NMOS 13 CNT) which may control the NMOS transistor N 5 . The control voltage NMOS 13 CNT may help generate an appropriate voltage at the gate of the PMOS portion of the pass transistor PT 2 , via the PMOS transistor PF 3 . The HIGH voltage of the signal CHARGE 13 NODE 1 may be presented to the inverters IB 1 , IB 3 , IL 1 , and IL 2 to generate the signals PMOS 13 CNT and NMOS 13 CNT.

The present invention may be used to generate a control signal to provide gate oxide protection, without any voltage degradation and change in a logical voltage level of the signal IN 1 . The gate oxide protection may be provided by cutting off the direct path from the signal PAD to the transistors of the input buffer 106 in the case of an over voltage condition on the signal PAD. Such a cut off may include turning OFF the pass transistor PT 2 with the use of the control signal NMOS 13 CNT and the transistors N 4 , N 5 , and PF 3 .

The voltage of the signal or node IN 1 may be held HIGH in the cut off condition to ensure proper operation of the input buffer. Maintaining the signal IN 1 HIGH may include applying the signal PMOS 13 CNT to a gate of the transistor PF 4 , which generally pulls the voltage of the signal IN 1 to a HIGH level.

The various signals are generally “on” (e.g., a digital HIGH, or 1) or “off” (e.g., a digital LOW, or 0). However, the particular polarities of the on (e.g., asserted) and off (e.g., de-asserted) states of the signals may be adjusted (e.g., reversed) accordingly to meet the design criteria of a particular implementation.

The present invention may provide fast charge removal from the nodes charged in overvoltage condition by the transistors NC 1 and NC 2 and the inverter IR 1 (e.g., formed by transistors 188 and 190 ). Such charge removal may remove the residual charge from the nodes charged, which may reduce the stress on the gate oxide and improve the circuit recovery from an overvoltage condition.

The present invention may be implemented in a mixed voltage compatible I/O buffer, with gate oxide protection and reduced high voltage related hazards. Such an architecture may be implemented without active voltage degradation to protect gate oxide. The architecture may reduce static and dynamic current levels and may maintain a comparable noise margin for a low voltage circuit.

The circuit 100 may implement a number of different overvoltage detection thresholds for input and output buffer operation. Implementing different detection thresholds may provide early detection of an overvoltage condition of the signal PAD for input buffer operation. A sufficient safety margin may also be provided for voltage levels across gate oxide and may compensate for the delay of the overvoltage detection circuit. Different detection thresholds may also improve the speed of the overvoltage detection.

Fast removal of residual stored charge may further reduce gate oxide stresses and may improve circuit recovery from an overvoltage condition. The circuit 100 may fit well for both an input buffer and output buffer implementation. The circuit 100 may remove the hazards related with operation in a mixed voltage system (e.g., gate oxide protection and high leakage current paths). The circuit may be implemented with interfaces operating at various voltages, such as 2.5V-3.3V. The circuit 100 may also be implemented using lower supply interfaces (e.g., 1.8V-2.5V) or lower.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.