Abstract:
A circuit receives an input signal characterized by a first pair of rail voltages and generates in response thereto an output signal characterized by a second pair of rail voltages. The circuit comprises first and second transistors coupled in series between a high reference voltage and a low reference voltage. The input signal drives a control lead of the second transistor. The logical inverse of the input signal drives a control lead of a third transistor, which couples a charge source to the control lead of the first transistor in response thereto in order to turn off the first transistor. The charge source can be either a voltage source or a charged capacitive node. Of importance, the third transistor does not have to overcome contention with other transistors to turn off said first transistor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims the benefit of U.S. patent application Ser. No. 12/392,021, filed Feb. 24, 2009, entitled “High Boosting-Ratio/Low-Switching-Delay Level Shifter,” naming Jason Su as inventor, the contents of which is hereby incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    This application pertains to circuits and methods for shifting voltage levels of a signal. 
         [0003]    It is known in the art to have a first portion of an electronic system use a first set of rail (e.g., power supply) voltages to communicate digital signals and a second portion of the system use a second set of rail voltages to communicate digital signals. Such a system typically comprises level shifter circuits to shift the voltage levels of signals from the first set of rail voltages to the second set of rail voltages so that the first portion of the system can communicate with the second portion of the system.  FIG. 1A  illustrates a prior art level shifter circuit  1  for receiving input signals IN 1  and IN 1 ′ having a first pair of rail voltage values and generating therefrom output signals OUT 1  and OUT 1 ′ having a second pair of rail voltages. (As used herein, signal IN 1 ′ is the logical inverse of signal IN 1  and signal OUT 1 ′ is the logical inverse of signal OUT 1 . The apostrophe indicates a logical inverse.) 
         [0004]    Signals IN 1  and IN 1 ′ are characterized by rail voltages VSS and VDDL (0 and 0.7 volts, for example), and signals OUT 1  and OUT  1 ′ are characterized by rail voltages VSS and VDDH (0 and 1.2 volts, for example). When signal IN 1  is high, transistor T 1  is on, signal OUT 1 ′ is low, transistor T 4  is on, and signal OUT 1  is high. When signal IN 1  is low, the opposite is true. 
         [0005]      FIG. 1B  illustrates a second prior art circuit  2  for receiving signals IN 1  and IN 1 ′ and converting them to output signals OUT 1  and OUT 1 ′. There are several problems with circuits  1  and  2 . 
         [0006]    Regarding circuit  1 , signal IN 1  transitions from VSS to VDDL and signal IN 1 ′ transitions from VDDL to VSS. Prior to this transition, transistors T 1  and T 4  are off, transistors T 2  and T 3  are on, signal OUT  1 ′ is high (i.e. at voltage VDDH), and signal OUT 1  is low. At the time of transition, signal IN 1  turns on transistor T 1 , pulling signal OUT 1 ′ low and thereby turning on transistor T 4 . Eventually, the voltage at OUT 1  rises to voltage VDDH and turns off transistor T 3 . Unfortunately, there is a time window during which transistors T 2  and T 4  are both on, which results in contention. The contention slows the completion of switching and draws excess current and therefore consumes excess power. A similar window occurs in transistors T 1  and T 3  when signal IN 1  transitions from voltage VDDL to voltage VSS and signal IN 1 ′ transitions from voltage VSS to voltage VDDL. 
         [0007]    If the high voltage of signal OUT 1  (i.e. VDDH) is much larger than the high voltage of signals IN 1  and IN 1 ′ (voltage VDDL), transistor T 1  becomes weaker than transistor T 3 . At some point, transistor T 1  will be unable to resolve the contention, and will fail to pull signal IN 1 ′ sufficiently low. This mechanism sets operation limits of circuits  1  and  2 . Thus, one important feature of a successful level shifter design is to accommodate a high VDDH to VDDL ratio. Another important feature of a level shifter design is to minimize contention between the pull-up PMOS transistors T 3  and T 4  and the pull-down NMOS transistors T 1  and T 2 , and cause the level shifter circuit to latch as quickly as possible. Thus, it would be desirable to pull signal OUT 1  high to turn off transistor T 3  more quickly to minimize switching delay and switching current. It is also desirable to minimize DC current. 
         [0008]    Circuit  2  also exhibits problems. For example, when signal IN 1  is high, both transistors T 5  and T 7  are on, thereby causing circuit  2  to draw excessive current and consume excessive power. 
       SUMMARY 
       [0009]    A circuit constructed in accordance with an embodiment of the invention converts an input signal characterized by a first pair of rail voltages to an output signal characterized by a second pair of rail voltages. 
         [0010]    The present invention includes a circuit for converting an input signal characterized by a first pair of rail voltages to an output signal characterized by a second pair of rail voltages. In one embodiment, the circuit comprises an input lead for receiving said input signal, first and second transistors coupled in series between a first voltage supply lead and a second voltage supply lead, and third and fourth transistors coupled in series between said first voltage supply lead and said second voltage supply lead. A control lead of said third transistor may be coupled to a first node between said first and second transistors, and a control lead of said first transistor may be coupled to a second node between said third and fourth transistors. The second transistor turn on in response to said input signal transitioning from a first level to a second level, and the fourth transistor turns on in response to said input signal transitioning from the second level to the first level. The circuit further includes a fifth transistor for coupling the control lead of said third transistor to a first charge source to turn off the third transistor when the input signal transitions from said second level to said first level, and a sixth transistor for coupling the control lead of said first transistor to a second charge source to turn off the first transistor when the input signal transitions from said first level to said second level. 
         [0011]    In one embodiment, the circuit further comprises an inverter having an input coupled to the input lead and further coupled to a control lead of the second transistor. An output of the inverter is coupled to a control lead of the fourth transistor. 
         [0012]    In one embodiment, the fifth transistor has a first terminal coupled to the control lead of the third transistor, a second terminal coupled to the first charge source, and a control lead coupled to the input signal. Furthermore, in one embodiment, the sixth transistor has a first terminal coupled to the control lead of the first transistor, a second terminal coupled to the second charge source, and a control lead coupled to an inverse of the input signal. 
         [0013]    In one embodiment, the first and second charge sources each comprise capacitances. 
         [0014]    In one embodiment, the first charge source comprises an MOS transistor having a gate coupled to the second terminal of the fifth transistor and a source terminal and a drain terminal coupled to a first voltage, and wherein the second charge source comprises an MOS transistor having a gate coupled to the second terminal of the sixth transistor and a source terminal and a drain terminal coupled to a second voltage. 
         [0015]    In one embodiment, the fifth transistor has a first terminal coupled to the control lead of the third transistor, a second terminal coupled to a rail voltage, and a control lead coupled to the input signal, and wherein the sixth transistor has a first terminal coupled to the control lead of the first transistor, a second terminal coupled to the rail voltage, and a control lead coupled to an inverse of the input signal. 
         [0016]    In one embodiment, the fifth transistor has a first terminal coupled to the control lead of the third transistor, a second terminal coupled to a rail voltage, and a control lead coupled to an inverse of the input signal, and wherein the sixth transistor has a first terminal coupled to the control lead of the first transistor, a second terminal coupled to the rail voltage, and a control lead coupled to the input signal. 
         [0017]    In one embodiment, the fifth transistor couples a first capacitive node to the control lead of said third transistor, and the sixth transistor couples a second capacitive node to the control lead of said first transistor, said first capacitive node being charged through said fifth and first transistors when the fifth and first transistors are on, said second capacitive node being charged through said sixth and third transistors when said sixth and third transistors are on. 
         [0018]    In one embodiment, the fifth transistor couples said first voltage supply lead to the control lead of said third transistor, and the sixth transistor couples the said first voltage supply lead to the control lead of said first transistor. 
         [0019]    In one embodiment, the circuit further comprises a first inverter for receiving a first voltage at said first node and providing a first output signal at a first output lead, a second inverter for receiving a second voltage at said second node and providing a second output signal at a second output lead, and a third inverter for receiving said input signal, said third inverter controlling said fourth transistor, said input signal controlling said second transistor. 
         [0020]    In one embodiment, the first and third transistors have a first conductivity type and the second and fourth transistors have a second conductivity type. 
         [0021]    In one embodiment, the fifth and sixth transistors have said first conductivity type. 
         [0022]    In one embodiment, the fifth and sixth transistors have said second conductivity type. 
         [0023]    In one embodiment, the first pair of rail voltages comprise a first low voltage and a first high voltage and the second pair of rail voltages comprise a second low voltage and a second high voltage, wherein the first low voltage of said first pair of rail voltages equals the second low voltage of the second pair of rail voltages, the first high voltage of said first pair of rail voltages is less than the second high voltage of the second pair of rail voltages, wherein the first voltage supply lead receives the second high voltage of the second pair of rail voltages, and the second voltage supply lead receives the second low voltage of the second pair of rail voltages. 
         [0024]    In another embodiment, the present invention includes a system comprising the level shift circuits described herein, where system further comprising first circuitry using signals characterized by said first pair of rail voltages and second circuitry using signals characterized by said second pair of rail voltages, wherein said circuit permits said first circuitry to communicate a signal to said second circuitry. 
         [0025]    In another embodiment, the present invention includes a method of level shifting a signal comprising receiving an input signal, the input signal transitioning from a first voltage to a second voltage, on a transition of the input signal from the first voltage to the second voltage, activating a second transistor, wherein the second transistor is coupled in series with a first transistor between a first high voltage rail and a first low voltage rail, deactivating a fourth transistor, wherein the fourth transistor is coupled in series with a third transistor between the first high voltage rail and the first low voltage rail, wherein a control lead of the third transistor is coupled to a first node between said first and second transistors and a control lead of the first transistor is coupled to a second node between the third and fourth transistors, and activating a sixth transistor, the sixth transistor coupling the control lead of the first transistor to a second charge source, and in accordance therewith, turning off said first transistor, and on a transition of the input signal from the second voltage to the first voltage, activating the fourth transistor, deactivating the second transistor, and activating a fifth transistor, the fifth transistor coupling the control lead of the third transistor to a first charge source, and in accordance therewith, turning off said third transistor. 
         [0026]    In one embodiment, the first charge source comprises a first capacitance, wherein a voltage on the first charge source is stored on the first capacitance when said input signal is at said first voltage, and wherein the second charge source comprises a second capacitance, wherein a voltage on the second charge source is stored on the second capacitance when said input signal is at said second voltage. 
         [0027]    In one embodiment, the first and second charge sources are the first high voltage rail or a second high voltage rail less than said first high voltage rail. 
         [0028]    In one embodiment, the sixth transistor is activated by the input signal and wherein the fifth transistor is activated by an inverse of the input signal. 
         [0029]    In one embodiment, the fifth transistor is activated by the input signal and wherein the sixth transistor is activated by an inverse of the input signal. 
         [0030]    In one embodiment, the second voltage is equal to a second high voltage rail, and wherein first high voltage rail is greater than the second high voltage rail. 
         [0031]    The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIGS. 1A and 1B  illustrate first and second prior art level shifter circuits. 
           [0033]      FIG. 2  illustrates a circuit constructed in accordance with a first embodiment of the invention using P channel pull-up transistors, N channel pull-down transistors, and P channel transistors for coupling charge sources to the control leads of the P channel pull-up transistors. 
           [0034]      FIG. 3  is a waveform diagram illustrating signals in the circuit of  FIG. 2 . 
           [0035]      FIG. 4  illustrates a circuit constructed in accordance with another embodiment of the invention using P channel pull-up transistors, N channel pull-down transistors, and N channel transistors for coupling a voltage supply terminal to the control leads of the P channel pull-up transistors. 
           [0036]      FIG. 5  illustrates a circuit constructed in accordance with another embodiment of the invention using P channel pull-up transistors, N channel pull-down transistors, and P channel transistors for coupling a voltage supply terminal to the control leads of the P channel pull-up transistors. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIG. 2  schematically illustrates a circuit  100  which receives an input signal IN 1  characterized by rail voltages VSS and VDDL and generates therefrom an output signal OUT 1  (logically equal to signal IN 1 ) characterized by rail voltages VSS and VDDH and an output signal OUT 1 ′ (the logical inverse of signal OUT 1 ) also characterized by rail voltages VSS and VDDH. In one embodiment, VSS is ground, VDDL may be in the range of 1.1-0.9 volts with a typical value of 0.8 volts, and VDDH may be in the range of 1.2-0.0 volts with a typical value of 1.2 volts, for example. When a signal is at voltage VSS, it is typically considered a logical zero or low. When a signal is at voltage VDDL or VDDH, it is typically considered a logical one or high. 
         [0038]    Of importance, the DC power consumption of circuit  100  is limited by virtue of the fact that there are no DC paths between VDDH and VSS or between VDDL and VSS that simultaneously conduct. Also, the timing of the change of state of circuit  100  does not depend upon a pull-up transistor overcoming a pull-down transistor or vice versa. 
       State of Circuit  100  Prior to Time TIME 1   
       [0039]    Referring to  FIG. 3 , signal IN 1  may transition from a logical 0 to a logical 1 (i.e. from voltage VSS to VDDL) at time TIME 1  (see  FIG. 3 ). Prior to time TIME 1 , transistor T 11  is off and inverter  102  pulls node N 10  to voltage VDDL, thereby turning on transistor T 12 , which in turn pulls node N 11  to voltage VSS, which turns on transistor T 13  and pulls node N 12  to VDDH. Since node N 12  is at voltage VDDH, inverter  104  pulls output signal OUT 1  to voltage VSS. Also, since node N 11  is at voltage VSS, inverter  106  pulls output signal OUT 1 ′ to voltage VDDH. (As used in  FIG. 3 , VN 11 , VN 12 , VN 13  and VN 14  refer to the voltages at nodes N 11 , N 12 , N 13  and N 14 .) 
         [0040]    Since node N 12  is at voltage VDDH, transistor T 14  is off, and since signal IN 1  is low, transistor T 15  is on. Transistor T 17  is configured to function as a capacitor (e.g., the source and drain are coupled together to form a single terminal). Accordingly, the gate of transistor T 17  charges to voltage VDDH via transistors T 15  and T 13 . Since node N 10  is at voltage VDDL and node N 11  is at voltage VSS, transistor T 16  is off. Transistor T 18  is likewise configured to function as a capacitor, but its charge state remains unchanged. For reasons explained below, the gate of transistor T 18  is charged to a voltage VDDL+VT, where VT is the threshold of transistor T 16 , which is around 0.4V typically. 
       State of Circuit  100  at Time TIME 1   
       [0041]    At time TIME 1 , signal IN 1  goes to voltage VDDL, thereby turning on transistor T 11 , and causing inverter  102  to pull node N 10  to voltage VSS, which in turn turns off transistor T 12  and turns on transistor T 16 . When transistor T 16  turns on, charge from the gate of transistor T 18  flows to node N 11 , pulling node N 11  high, and turning off transistor T 13  (see portion  300  of waveform VN 11 ). Because transistor T 13  is shut off by the surge of charge from the gate of transistor T 18 , the contention between transistors T 11  and T 13  is minimized. 
         [0042]    Because transistor T 11  turns on and transistor T 13  turns off, node N 12  falls to voltage VSS and transistor T 14  turns on, thereby pulling node N 11  high (to VDDH). See portion  301  of waveform VN 11 . Since node N 12  is at voltage VSS, inverter  104  raises output signal OUT 1  to voltage VDDH. Because node N 11  is at voltage VDDH, inverter  106  causes output signal OUT 1 ′ to go to voltage VSS. 
         [0043]    As mentioned above, charge stored at the gate of transistor T 18  pulls node N 11  high when transistor T 16  turns on. For reasons explained below, prior to transistor T 16  turning on, node N 14  is charged to a voltage equal to VDDL+VT, where VT is the threshold voltage of transistor T 16 . As soon as transistor T 16  turns on, charge from the gate of transistor T 18  flows and charges node N 11  to a voltage VN 11  as follows: 
         [0000]        VN 11=( VDDL+VT ) CT 18/( CT 18+ CN 11) 
         [0000]    where CT 18  is the capacitance at the node comprising the gate of transistor T 18  and the source of transistor T 16 , and CN 11  is the capacitance at node N 11  when transistors T 16  and T 12  are off Voltage VN 11  turns off transistor T 13 . Thereafter, the voltage at node N 11  is brought all the way to VDDH as transistor T 14  turns on. This, in turn, causes the capacitance at the gate of transistor T 18  to be charged to voltage VDDH. 
         [0044]    Also as mentioned above, because transistor T 11  turns on and transistor T 13  is off, node N 12  is pulled low (to voltage VSS), thereby turning on transistor T 14 , which turns on and pulls node N 11  high (to voltage VDDH). Because node N 12  is low, inverter  104  brings signal OUT 1  to voltage VDDH, and because node N 11  is at voltage VDDH, inverter  106  brings signal OUT 1 ′ to voltage VSS. 
         [0045]    As mentioned above, prior to time TIME 1 , voltage VN 13  at the gate of transistor T 17  is pulled to VDDH. After time TIME 1 , since signal IN 1  is at VDDL, charge flows off of the gate of transistor T 17  through transistor T 15  until the voltage at the gate of transistor T 17  equals VDDL+VT. At this point  302 , transistor T 15  turns off and the charge at the gate of transistor T 17  remains at VDDL+VT. A similar mechanism is responsible for pulling the voltage at the gate of transistor T 18  to VDDL+VT prior to time TIME 1 . 
         [0046]      FIG. 4  illustrates an alternative embodiment of the present invention. Circuit  400  of  FIG. 4  is similar to circuit  100  of  FIG. 2 . In this embodiment, instead of using PMOS transistors T 15  and T 16 , circuit  400  employs NMOS transistors T 45  and T 46  coupled between nodes N 12  and N 11  and VDDL, respectively. Additionally, the drain of transistor T 45  is coupled to receive voltage VDDH and the drain of transistor T 46  is coupled to receive voltage VDDH, rather than using capacitors to stored the charge. Additionally, signal IN 1  drives the gate of transistor T 46 , and inverter  102  drives the gate of transistor T 45 . The drive signals for T 46  and T 45  are changed because opposite polarity devices are used in this example. 
         [0047]      FIG. 5  illustrates an alternative embodiment of the present invention. Circuit  500  of  FIG. 5  is similar to circuit  100  of  FIG. 2 . However, in this embodiment, instead of being coupled to capacitance set by transistor T 17 , the source of transistor T 15  is coupled to receive voltage VDDH; and instead of being coupled to a likewise capacitance set by transistor T 18 , the source of transistor T 16  is coupled to receive voltage VDDH. 
         [0048]    Circuit  100  exhibits better delay (i.e. faster switching speed), lower power consumption, and can perform with a greater VDDH/VDDL ratio than typical prior art level shifters. Advantageously, PMOS transistors T 15  and T 16  provide fast response time. 
         [0049]    Circuit  400  has the advantage of a complete shut-off for logic 0 at nodes N 11  and N 12 . In contrast, in circuit  500 , if VDDH exceeds VDDL by an amount exceeding the threshold voltage of transistors T 15  and T 16 , there will be a DC path through transistors T 15  and T 11  when signal IN 1  is high, and between transistors T 16  and T 12  when signal IN 1  is low. 
         [0050]    While the invention has been described with respect to specific embodiments, those skilled in the art will appreciate that changes can be made in form and detail without departing form the spirit and scope of the invention. For example, instead of using a MOS transistor gate to serve as part of a capacitive node as in circuit  400 , a metal layer (or pair of metal layers) can be used to form the capacitive node. Also, different electronic elements can also be added to the circuit of the present invention. Also, instead of having the circuit convert first rail voltages VSS and VDDL to rail voltages VSS and VDDH, in some embodiments, the circuit converts voltages VSSH and VDD to VSSL and VDD. (VSSL is a lower voltage than VSSH.) Accordingly, all such changes come within the invention.