Abstract:
An improved voltage tolerant protection circuit for input buffer comprising a transmission gate circuit receiving input from the pad for passing the input signal to the input of the input buffer, a control signal generator electrically coupled between the transmission gate circuit and the pad to provide a control signal for operating the transmission gate circuit, and an N-Well generation circuit electrically coupled between the pad and the transmission gate circuit, and also electrically coupled to the control signal generator for generating a bias signal for the transmission gate circuit and the control signal generator. Thus, the present invention provides a voltage tolerant protection circuit that prevents electrical stress on transistors, minimizes power supply consumption and transfers signals without any change in amplitude.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims priority from prior Indian Patent Application No. 1009/Del/2004, filed on Oct. 21, 2004, which is based on and claims priority on Indian Provisional Patent Application No. 1009/Del/2004, filed on Jun. 2, 2004, the entire disclosure of each the two-above referenced applications is hereby incorporated by reference in their entirety.  
       FIELD OF THE INVENTION  
       [0002]     The present invention generally relates to a voltage tolerant input protection circuit, and more particularly to a voltage tolerant protection circuit for input buffer.  
       BACKGROUND OF THE INVENTION  
       [0003]     The significance of voltage protection circuit for input buffer is discussed with reference to a differential receiver circuit. Herein, the voltage protection circuit is a circuit between the I/O pad of an integrated circuit and a differential receiver, the protection circuit producing a signal that is used as an attenuation free input signal for the differential receiver circuit. For a differential receiver operating on 3.3V technology having common mode input range equal to 0.8V to 2.5V and having differential input sensitivity equal to 200 mV, it is essential that both the inputs of the differential receiver are free of any amplitude attenuation in common mode input range. To make the differential receiver five-volt tolerant, the input voltage must be equal to the supply voltage. Stated differently, the input voltage must exceed and cross the supply voltage.  
         [0004]     Conventionally, NMOS transistors are used in voltage tolerant protection circuits, wherein the gate of the NMOS transistor is connected to the supply voltage. The source is connected to the Pad and the drain is connected to the Input Buffer. If the voltage at Pad is less than or equal to VDD−Vt (NMOS Threshold), the signal at the input buffer follows the Pad voltage without any amplitude attenuation. When Pad voltage is higher than the NMOS threshold then the signal at the input buffer is attenuated at NMOS threshold. For minimum allowed supply voltage, the signal at the input buffer goes beyond NMOS threshold and for large common mode input range the value of NMOS threshold may lie between the common mode input range, thus resulting in signal degradation. Further, delay is introduced on the rising edge of the signal at the input buffer for high frequency operation of the input buffer.  
         [0005]      FIG. 1  illustrates a prior art voltage protection circuit as per US Patent Application Publication No. 2004/0007712 A1, which is hereby incorporated by reference in its entirety. Here, NMOS transistors are used for protection. As per the given circuit, VOUT follows the Pad voltage from 0V to VDD−vt (PMOS threshold), and supply voltage (VDD) is outputted at VOUT whenever input voltage crosses the PMOS threshold. There is static consumption on the supply voltage through transistors  224  &amp;  226  when Pad voltage is less than VDD−vt (PMOS threshold) and it is undesirable to have a direct path between power supply and ground in normal operating condition. When Pad voltage is greater than VDD−vt (PMOS threshold), and 3.3V transistors are used in the protection circuit, there can be electrical stress on PMOS  234 . Electrical stress on the transistors in the protection circuit is undesirable and often results in the output signal attenuation.  
         [0006]     Accordingly what is needed is a method and system to overcome the problems encountered in the prior art voltage protection circuits and to provide a voltage tolerant protection circuit for an input buffer that prevents stress on transistors, minimizes power supply consumption and transfers signals without any change in the amplitude.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides an improved voltage tolerant protection circuit for input buffer. The protection circuit according to the present invention consumes little or zero power on pad in normal operating conditions, and minimum power consumption when pad is operating at a higher voltage. Further, the protection circuit provides an attenuation free signal up to the supply voltage for the input buffer and mitigates stress on the transistors in the protection circuit.  
         [0008]     The present invention provides an improved voltage tolerant protection circuit for input buffer comprising a transmission gate circuit receiving input from the pad. A control signal generator connected between the transmission gate circuit and the input pad provides a control signal for operating the transmission gate circuit. An N-Well generation circuit connected between the pad and the transmission gate circuit, and also connected to the control signal generator generates a bias signal for the transmission gate circuit and the control signal generator.  
         [0009]     The transmission gate circuit comprises a first transistor receiving the control signal from the control signal generator and the bias signal from the N-Well generation circuit to transfer pad voltage from threshold voltage to a supply voltage. A second transistor is connected to the first transistor. The second transistor receives a supply voltage and a ground, as control signals to form a closed path in response to the pad voltage being higher than the supply voltage. A third transistor is connected to the first and second transistors for providing an output equal to supply voltage when the pad voltage crosses the supply voltage. The first transistor is a PMOS transistor and the second and third transistors are NMOS transistors.  
         [0010]     The control signal generator comprises a first PMOS transistor connected to pad. The first PMOS transistor receives control signals from the supply voltage and the N-Well generation block for avoiding power consumption on the input pad. A second NMOS transistor is connected to the first PMOS transistor. The second NMOS transistor receives control signals from the supply voltage and the ground for transferring a voltage potential in response to the pad voltage being less than the supply voltage.  
         [0011]     A third NMOS transistor is connected to the second NMOS transistor. The third NMOS transistor receives control signals for transferring a voltage potential to the source of the second NMOS transistor in response to the pad voltage being less than or equal to PMOS threshold and to form an open circuit path when pad voltage is greater than the PMOS threshold.  
         [0012]     A fourth PMOS transistor is connected to the input pad. The fourth PMOS transistor receives the bias signal and supply voltage for providing a closed path for conduction when pad voltage is greater than the PMOS threshold.  
         [0013]     A fifth NMOS transistor is connected to the fourth PMOS transistor. The fifth PMOS transistor receives control signals from the supply voltage and the ground to provide controlled voltage response when the pad voltage is greater than the supply voltage.  
         [0014]     A sixth NMOS transistor is connected to the fifth NMOS transistor. The sixth NMOS transistor provides a controlled potential at the source of the fifth NMOS transistor.  
         [0015]     A seventh NMOS transistor is connected to the sixth NMOS transistor. The seventh NMOS transistor provides a controlled closed circuit path.  
         [0016]     An eighth NMOS transistor and a ninth NMOS transistor are connected to the sixth NMOS transistor for outputting a NMOS threshold potential in response to the pad voltage being less than or equal to the supply voltage.  
         [0017]     A tenth PMOS transistor and an eleventh NMOS transistor are connected to the sixth and seventh NMOS transistors and to the eighth NMOS transistor. The tenth PMOS transistor and eleventh NMOS transistor both provides a true value (i.e. without change in amplitude) of the supply voltage to the third NMOS transistor.  
         [0018]     The sixth and seventh NMOS transistors are connected in series.  
         [0019]     The drain terminals of the eighth and ninth NMOS transistors are connected to their respective gate terminals, and source of the eighth NMOS transistor is connected to drain of the ninth NMOS transistor.  
         [0020]     The tenth PMOS transistor and the eleventh NMOS transistor are connected to each other to form an inverter circuit.  
         [0021]     The present invention also provides a method for protecting an input buffer circuit comprising:  
         [0022]     transferring a signal from a input pad to the input buffer through a transmission gate circuit;  
         [0023]     providing a control signal to the transmission gate circuit by a control signal generator, the control signal being zero volt in response to a voltage on the input pad being less than or equal to supply voltage, and being of same value as the voltage on input pad when the voltage on the voltage on the input pad is higher than the supply voltage, thereby avoiding electrical stress on the transistors; and  
         [0024]     providing a bias signal for the control signal generator and the transmission gate circuit, the bias signal being equal to supply voltage when the voltage on the pad input is less than the supply voltage and equal to threshold potential in response to the voltage on the pad voltage being greater than supply voltage.  
         [0025]     The foregoing and other features and advantages of the present invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0027]      FIG. 1  is a prior art voltage protection circuit diagram.  
         [0028]      FIG. 2  is the block diagram of a voltage protection circuit for an input buffer in accordance with an embodiment of the present invention.  
         [0029]      FIG. 3  is the circuit diagram of transmission gate of the voltage protection circuit of  FIG. 2 , in accordance with an embodiment of the present invention.  
         [0030]      FIG. 4  is the circuit diagram of control signal generation of the voltage protection circuit of  FIG. 2 , in accordance an embodiment of the present invention.  
         [0031]      FIG. 5  is the circuit diagram of a conventional NWELL (bias for PMOS transistors) generation block of the voltage protection circuit of  FIG. 2 , in accordance with an embodiment of the present invention.  
         [0032]      FIG. 6  is the graph of the DC sweep of the protection circuit in accordance an embodiment of the present invention.  
         [0033]      FIG. 7  is the graph of the transient simulation of the circuit in accordance an embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     It should be understood that these embodiments are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in the plural and vice versa with no loss of generality.  
         [0035]      FIG. 2  shows the block diagram of the improved voltage tolerant protection circuit for input buffer, which comprises a Transmission gate ( 11 ), for transferring signal from PAD to the Input Buffer ( 14 ). A Control Signal Generator ( 12 ), to generate control signal PMOSCTRL for the Transmission gate ( 11 ), so as to enable the Transmission gate ( 11 ) to transfer the voltage from PAD to the Input buffer ( 14 ). An NWELL Generator ( 13 ) to provide bias voltage to the Transmission gate ( 11 ) and the Control Signal Generator ( 12 ), for minimizing power dissipation.  
         [0036]      FIG. 3  defines the circuit diagram of the Transmission Gate ( 11 ). Here, PAD is the input signal and VOUT is the output signal of the transmission gate. VOUT is the input signal for the Input buffer. The Transmission gate circuit is required to operate for the following input/output parameters.  
         [0000]     i. When Pad voltage≦VDD, VOUT=Pad voltage  
         [0000]     ii. When Pad voltage&gt;VDD, VOUT≦VDD  
         [0037]     For obtaining the above stated output parameters, the drain of NMOS M 1  is connected to PAD, source is connected to VOUT and gate is connected to the Supply voltage (VDD). As a result of this connection, the voltage VOUT follows the PAD voltage up to the NMOS threshold (VDD−Vtn).  
         [0038]     PMOS M 0  is required to transfer the Pad voltage from NMOS threshold to the supply voltage. To achieve this, the source terminal of transistor M 0  is connected to the Pad, drain terminal is connected to VOUT, and gate of M 0  receives the signal PMOSCTRL from the Control Signal Generator. When the Pad voltage is less than or equal to the supply voltage, the value of PMOSCTRL signal is zero volt and true Pad voltage is transferred to VOUT, through NMOS M 1  and PMOS M 0 . Further, PMOSCTRL signal is equal to Pad voltage when Pad voltage is greater than the supply voltage. Consequently, the PMOS is switched off to avoid the transference of Pad voltage higher than supply voltage. PMOS M 0  receives signal NWELL at its bias from the NWELL generation block. NWELL potential is equal to supply voltage when Pad voltage is less than the supply voltage and NWELL potential is VPAD−Vt (Threshold Voltage) when Pad voltage is higher than the supply voltage. When Pad voltage is higher than the supply voltage, there should not be any risk of consumption from Pad to bulk of transistor PMOS M 0 . Further, when Pad voltage is higher than the supply voltage, PMOS M 0  is switched off and only NMOS M 1  is operational. In this case VDD−Vtn (NMOS Threshold) appears as output voltage on VOUT, which is not the true supply voltage VDD and can thereby cause power consumption at the Input buffer. To overcome these constraints and get true value of supply voltage on VOUT, the drain of NMOS M 24  is connected to VOUT, source is connected to the supply voltage and gate is connected to Pad, the Pad voltage being higher than the supply voltage. Whenever Pad voltage is greater than VDD+Vtn (NMOS Threshold), supply voltage is outputted at VOUT. Size of the transistor NMOS M 24  is such that the voltage VOUT is equal to the supply voltage as soon as the PAD voltage crosses VDD.  
         [0039]      FIG. 4  is a circuit level diagram of the Control Signal Generator of  FIG. 2  that receives the input signal from Pad. PMOSCTRL signal is generated from this block as a result. The Control Signal Generator is required to operate for the following input/output parameters. 
        i. PMOSCTRL=0, when Pad voltage is less than or equal to VDD+Vt (PMOS Threshold).     ii. PMOSCTRL=Pad Voltage, when Pad voltage is higher than VDD+Vt (PMOS Threshold), here the PMOSCTRL signal follows the Pad voltage.          
         [0042]     For obtaining the above stated parameters, the source terminal of PMOS M 9  is connected to PAD, drain receives the PMOSCTRL signal, gate is connected to the supply voltage (VDD) and bias is connected to signal NWELL, to avoid power dissipation at the bulk of PMOS M 9 .  
         [0043]     Drain of NMOS M 10  is connected to PMOSCTRL, source of NMOS M 10  is connected to the drain of NMOS M 11  and gate of NMOS M 10  is connected to VDD. As a result, stress on M 10  and M 11  is prevented when PMOSCTRL follows the Pad voltage, and zero volt signal is transferred when Pad voltage is less than VDD. Thus, the drain of M 11  does not exceed NMOS Threshold and drain to source voltage of transistor M 10  is VPad−VDD, so that the transistors are not stressed up to the Pad voltage.  
         [0044]     Source of NMOS M 11  is connected to ground GND, drain of NMOS M 11  is connected to source of NMOS M 10  and gate is connected to the signal NMOSOFF. The value of NMOSOFF signal is equal to supply voltage when Pad voltage is less than or equal to the supply voltage and it is equal to zero when Pad voltage is greater than the supply voltage.  
         [0045]     As per the above description of M 9 , M 11  and M 11 , the three transistors operate for the following input/output parameters: 
        i. When PAD voltage is less than or equal to VDD+Vt (PMOS Threshold), then the value of NMOSOFF is VDD so that NMOS M 11  transfers zero volt to the source of M 10 . As gate of NMOS M 10  is connected to VDD, it transfers zero volt to PMOSCTRL. As a result the gate to source voltage of PMOS M 9  is positive, hence PMOS M 9  is switched off, thereby outputting a zero potential at PMOSCTRL,     ii. When PAD voltage is greater than VDD+Vt (PMOS Threshold), a zero value of voltage is outputted at NMOSOFF, therefore the gate to source voltage of NMOS M 11  is zero, thus resulting in a switched off NMOS M 11 . As a result the gate to source voltage of PMOS M 9  is negative and the Pad voltage is transferred to PMOSCTRL, thus PMOSCTRL follows the Pad voltage.        
 
         [0048]     It is desirable to have the value of NMOSOFF signal equal to VDD when Pad voltage is less than or equal to the supply voltage, and equal to zero when Pad voltage is greater than VDD.  
         [0049]     To achieve the above stated objective, PMOS M 15  &amp; M 16 , NMOS M 19 , M 20 , M 21 , M 22 , M 23  &amp; M 25  are used. Source of PMOS M 15  is connected to PAD, gate of PMOS M 15  is connected to VDD and drain of PMOS M 15  is connected to drain of NMOS M 19 . PMOS M 15  is on when pad voltage is greater than VDD+Vt (PMOS Threshold). Width of PMOS M 15  should be kept high to transfer Pad voltage at the drain of PMOS M 15  as soon as pad voltage crosses VDD.  
         [0050]     Gate of NMOS M 19  is connected to VDD, source of NMOS M 19  is connected to the drain of NMOS M 20 , NMOS M 21  and to the gate of PMOS M 16  and NMOS M 23 . NMOS M 19  is used to avoid any stress on MOS M 16 , M 20 , M 21 , M 22 , M 23  &amp; M 25 . In any case source of NMOS M 19  does not exceed VDD−Vt (NMOS Threshold).  
         [0051]     NMOS M 20  and M 25  are connected in series. Drain of NMOS M 20  is connected to source of M 19 , source of NMOS M 25  is connected to ground GND and gates of both NMOS M 20  &amp; M 25  are connected to signal NMOSOFF. NMOS M 20  &amp; M 25  should be long channel transistors for good switching at the drain of NMOS M 20 . When Pad voltage is higher than VDD, gates of M 16  &amp; M 23  should be close to VDD−Vt (NMOS Threshold), thus the current through these M 20  and M 25  should be very less.  
         [0052]     Drain of NMOS M 21  and M 22  are connected to their gate, wherein both the transistors operate like diodes. Source of NMOS M 21  is connected to drain of NMOS M 22 , source of NMOS M 22  is connected to ground GND. NMOS M 21  and M 22  are used to provide 2*Vt (NMOS Threshold) to the gate of M 16  and M 23 , when Pad voltage is less than or equal to VDD. NMOS M 21  &amp; M 22  are long channel transistors for reducing power dissipation on Pad, the Pad voltage being greater than VDD.  
         [0053]     PMOS M 16  and NMOS M 23  are connected together to form an inverter. Switching threshold for this inverter should be greater than 2*Vt (NMOS Threshold) and less than VDD−Vt (NMOS Threshold) for obtaining the desired value of NMOSOFF.  
         [0054]     As per the above description of M 15 , M 16 , M 20 , M 21 , M 22 , M 23  &amp; M 25 , the circuit operates for the following input/output parameters.  
         [0055]     When Pad voltage is less than or equal to VDD+Vt (PMOS Threshold), Vgs (Gate to source voltage) of PMOS M 15  is positive, consequently M 15  is switched off. A potential 2*Vt (NMOS Threshold) is established at the gate of M 16  and M 23 , due to NMOS M 21  &amp; M 22 . The switching threshold of the inverter formed by M 16  and M 23  causes NMOSOFF approach towards VDD, thereby causing NMOS M 20  &amp; M 25  to be switched on, thus the potential at the gates of M 16  and M 23  is zero volts. Thereby, NMOSOFF approaches true value of VDD.  
         [0056]     When PAD voltage is greater than VDD+Vt (PMOS Threshold). Vgs of PMOS M 15  is negative so the PMOS M 15  is switched on. As gate of NMOS M 19  is connected to VDD, gate of M 16  &amp; M 23  achieve a voltage level of VDD−Vt (NMOS Threshold). As a result of the switching threshold of the inverter formed by M 16  and M 23 , NMOSOFF become ZERO and NMOS M 20  &amp; M 25  are switched off.  
         [0057]      FIG. 5  illustrates the circuit diagram of a conventional NWELL generator. PAD is the input signal for the NWELL generator and it generates bias signals for PMOS transistors in the protection circuit.  
         [0058]     As described earlier, bias voltage for each PMOS transistors in the protection circuit is desirable at VDD when Pad voltage is less than VDD and at VPAD−Vt (Threshold voltage) when Pad voltage is greater than VDD.  
         [0059]     To achieve this, the source of PMOS M 2  is connected to VDD, drain &amp; bulk of PMOS M 2  is connected to NWELL, and gate of PMOS M 2  is connected to PAD. Source of PMOS M 4  is connected to PAD, gate of PMOS M 4  is connected to VDD, bulk of PMOS M 4  is connected to NWELL and drain of PMOS M 4  is connected to gate of PMOS M 3  and to the drain of NMOS M 12 . Source of PMOS M 3  is connected to VDD, drain &amp; bulk of PMOS M 3  is connected to NWELL. Gate of NMOS M 12  is connected to VDD, source is connected to drain and gate of NMOS M 14 . This NMOS is used to avoid stress on M 14  and M 5 . NMOS M 14  and M 5  are drain-gate connected transistors for providing 2*Vt (NMOS Threshold) on the gate of PMOS M 3  when Pad voltage is less than VDD+Vt (PMOS Threshold).  
         [0060]     As per the above stated description, the transistors M 2 , M 3 , M 4 , M 5 , M 12  &amp; M 14 , operate to perform the following functions:  
         [0061]     When Pad voltage is less than or equal to VDD+Vt (PMOS Threshold), Vgs of PMOS M 4  is greater than PMOS threshold, thus PMOS M 4  is switched off. Due to drain gate configuration of NMOS M 14  and M 5 , gate of M 3  is at 2*Vt (NMOS Threshold). As a result the gate voltage (Vg) of PMOS M 3  is less than the threshold voltage, hence VDD is outputted at NWELL. For Pad voltage range 0 to VDD−Vt (PMOS Threshold) PMOS M 2  is switched on.  
         [0062]     When Pad voltage is greater than VDD+Vt (PMOS Threshold), Vgs of PMOS M 4  is less than PMOS threshold, thus PMOS M 4  is switched on and gate of PMOS M 3  and M 2  are at same potential as on PAD, thereby resulting in switching off PMOS M 2  and M 3 . As source of PMOS M 4  is connected to PAD and bulk is connected to NWELL, there is one diode formed between source and bulk. If source voltage is higher the bulk voltage plus threshold voltage, diode conducts and NWELL become VPAD−Vt (Threshold voltage).  
         [0063]      FIG. 6  shows the effect of using protection circuit for the Input buffer in dc-sweep. Here, X-Axis of this graph is the PAD voltage while the Y-Axis is the voltage to the Input Buffer. The protection circuit is simulated for three supply voltage levels 3.0V, 3.3V and 3.6V. As shown in the figure, the PAD voltage is varied from 0V to 5.6 Volt. VOUT follows the PAD voltage up to VDD+Vt (PMOS Threshold, ˜0.35V in this simulation), and the value of VOUT is either VDD−Vt (NMOS threshold) or VDD. Simulation results show that the Input buffer is protected from the higher PAD voltage and there can be full swing from 0V to supply voltage (VDD) at VOUT. Thus, it is concluded from the simulation results that attenuation free signal is obtained for the Input buffer.  
         [0064]      FIG. 7  shows the transient simulation results. Here, the circuit is simulated for the supply voltage levels of 3.0V, 3.3V and 3.6V. Pulse of amplitude 5.6V is applied on the PAD for generating a pulse of amplitude equal to supply voltage (VDD) at VOUT, thus providing the required voltage level to the Input Buffer.  
         [0065]     Although a specific embodiment of the invention has been disclosed, it will be understood by those having skill in the art that changes can be made to this specific embodiment without departing from the spirit and scope of the invention. The scope of the invention is not to be restricted, therefore, to the specific embodiment, and it is intended that the appended claims cover any and all such applications, modifications, and embodiments within the scope of the present invention.