Abstract:
A pass transistor signal level translator between a first voltage level and a higher second voltage level having a bias circuit for the pass transistor including a first switching circuit coupled to the first voltage level for providing a bias voltage that is less than the first voltage level. A second switching circuit is coupled to the second voltage level for providing a pulse at substantially the second voltage to the bias voltage. A voltage clamping circuit is coupled between the bias voltage and a reference voltage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of application Ser. No. 60/683,884 filed on May 23, 2005 which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This application relates to a biasing circuit for a signal translator circuit and more specifically to a biasing circuit of a bidirectional voltage level translator circuit for open-drain applications.  
       BACKGROUND OF THE INVENTION  
       [0003]     Electronic devices are being designed to use lower and lower power supply voltages in order to reduce the power drain for the devices and the heat generated therein. This is especially helpful in portable, battery operated devices such as cellular telephones, PDAs, MP3 Players, for example because they are both battery powered and highly compact, so that heat dissipation is a problem. In addition, the lower voltage supplies allow higher speed operation, because the voltage swing of the signal is smaller and will therefore take place in less time than when a higher supply voltage is used. Therefore, even circuits that are not intended for highly compact and battery powered mobile applications are using lower and lower voltage supplies in order to obtain the speed benefits as well as the reduced power supply requirements and lower heat dissipation, which allows for reduced ventilation requirements. In this continuum of redesigns at lower voltages, it is common to have some circuits for a particular system available at the lowest voltage whereas other components may not yet have been redesigned and require higher voltage supplies in order to operate. If these two devices are utilized in a common system, there needs to be a circuit interfacing the signal flow between them in order that the different power supply voltages, and therefore signal swings, can be accommodated. A signal level translator circuit meets this need. Typically, these circuits are bidirectional in nature so that the signal flow can occur from either device to the other device. Although devices having a pin for selection of the direction in which the signal flow is to occur are known, it is preferable to eliminate this pin, and therefore the cost associate therewith, and design the circuit to automatically detect the direction in which the signal flow will occur. Circuits that perform this function are known in the art.  
         [0004]     A signal translation circuit for I 2 C logic, for example, which utilizes open drain circuits on either end of the signal translator generally use a directionless pass gate transistor signal translator. The principle for operation of this type of circuit is shown in  FIG. 1  generally as  100 . Terminal A is connected to a circuit operating at a first signal voltage and terminal B is connected to a circuit operating at a second, higher signal voltage. Each of the terminals is pulled up to its respective voltage VCCA or VCCB by a respective pull-up resistor  102 ,  114 . The terminals are connected together via a pass transistor  110  which is a NMOS transistor having a gate bias  108 . As will be seen in later described circuits, the gate bias voltage is commonly coupled to the chip enable signal. A one-shot circuit  106  has one input connected to the terminal A side of the pass transistor  110  and a second input connected to the B side of the pass transistor  110 . If the signal on one terminal transitions from a high level to a low level, the other terminal is pulled down to that voltage via the pass transistor  110 , which is biased in its ON state. However, due to the body effect, the pass transistor is not very effective in pulling up the other terminal to its respective voltage supply. The pull up resistors  102 ,  114  provides sufficient drive to maintain the voltage on terminal A or terminal B, respectively once the transition is made. The one-shot circuit detects the transition and operates one of the pull-up PMOS transistors  104 , 112  in order to pull the respective terminal up to its respective voltage supply. In view of the fact that one of the terminals will already be at its respective voltage supply level, both transistors  104 , 112  can be turned on at once, as it will have no effect on the terminal that is already at its respective voltage supply.  
         [0005]     The gate of the pass transistor should be biased to a voltage which is approximately one volt above the lower of the two VCC supply voltages. If the bias voltage is too high, there will be undesirable feed-through current from the high voltage terminal to the low voltage terminal. If the voltage is too low, the ON resistance of the pass transistor will be too high. A known bias circuit is shown is  FIG. 2  generally as  200 . As discussed above, the bias supply is connected to the chip enable line and is active low. This chip enable signal is inverted through a pair of inverters INV 1  and INV 2  and applied to the gates of a PMOS transistor P 1  and NMOS transistor N 1  coupled between the voltage supply VCCB and ground. The drains of the two transistors are connected via resistor R 1 . Once PMOS P 1  is turned on by way of ENABLE node being driven to a low state, it provides a pull-up current to the bias line BIAS for the pass transistor through resistor R 1 . The transition is also applied to PMOS transistor P 8  and NMOS transistor N 4  which have the gates coupled together and coupled to the enable input. The output on line s 1  is coupled to the base of PNP transistor Q 1  having its emitter coupled to the bias line and its collector coupled to ground. With enable low, the base of Q 1  is biased to VCCA through P 8  and Q 1  clamps node BIAS to one VBE above VCCA. The inclusion of a bipolar transistor Q 1  requires that this circuit be implemented in a BiCMOS process. In addition, power is dissipated through resistor R 1  and the enable time is longer or else a higher current through resistor R 1  is required.  
         [0006]     A second known biasing circuit is shown in  FIG. 3  generally as  300 . In this circuit, a NMOS transistor N 1  is coupled to the chip enabled signal via a pair of inverters INV 1  and INV 2 . It switches a current source having a current I to the BIAS line. The transitions are provided by a PNP transistor having its emitter coupled to the BIAS line and collector coupled to ground and a base coupled to a reference voltage which is equal to the input voltage reference VCCA plus an offset voltage. In addition to requiring a BiCMOS process, it requires complex bias generators that have a long delay time or a higher current.  
         [0007]     Previous implementation of the bias circuits such as the ones shown in  FIGS. 2 and 3  have a high static current on the order of 200 uA and have relatively long enable times on the order of two microseconds for the pass gate to be biased on.  
         [0008]     Accordingly, there is a need for a signal translator circuit that can be fabricated by an all CMOS process and which can have a faster enable time and a lower static current requirement.  
       SUMMARY OF THE INVENTION  
       [0009]     This and other objects and features are provided in accordance with one aspect of the invention by a pass transistor signal level translator between a first voltage level and a higher second voltage level having a bias circuit for the pass transistor comprising a first switching circuit coupled to the first voltage level for providing a bias voltage that is less than the first voltage level. A second switching circuit is coupled to the second voltage level for providing a pulse at substantially the second voltage to the bias voltage. A voltage clamping circuit is coupled between the bias voltage and a reference voltage.  
         [0010]     Another aspect of the invention includes a pass transistor signal level translator comprising a first terminal related to a first voltage level. A second terminal is related to a second voltage level, the second voltage level exceeding the first voltage level. A pass transistor is coupled between the first and second terminals. A bias circuit controls the pass transistor comprising a first circuit providing a voltage less than the first voltage level coupled to a gate of the pass transistor. A pulse generator generates a pulse at the second voltage level coupled to the gate of the pass transistor. A voltage clamping circuit is coupled between the gate of the pass transistor and a reference voltage.  
         [0011]     A further aspect of the invention includes a method of biasing a pass transistor in a pass transistor signal level translator circuit coupled between a first terminal related to a first voltage and a second terminal related to a second voltage greater than the first voltage comprising providing a first switched voltage related to the first voltage. Providing a second switched voltage related to the second voltage. Providing a steady state voltage related to the second voltage; and clamping the bias voltage at one V T  above the first voltage.  
     
    
     BREIF DESCPRITION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic block diagram showing a basic design for a pass transistor signal translator circuit;  
         [0013]      FIG. 2  illustrates a known biasing circuit;  
         [0014]      FIG. 3  illustrates a second known biasing circuit;  
         [0015]      FIG. 4  illustrates a biasing circuit in accordance with the present invention; and  
         [0016]      FIG. 5  illustrates a trigger and one-shot circuit suitable for use with the pass gate signal translation circuit having the biasing circuit illustrated in  FIG. 4 .  
     
    
     DETAILED DESCRIPTION  
       [0017]      FIG. 4  shows an embodiment of the present invention generally as  400 . The biasing circuit  400  is coupled to the active low enable signal and provides a biasing signal BIAS to the gate of a N channel pass transistor, as shown in  FIG. 1 . The chip enable signal is coupled to the input of an inverter INV 1  the output of which is coupled to the input of the inverter INV 2  and to one input of a two input NAND gate NAND. The output of the inverter INV 2  is coupled to the gates of a PMOS transistor P 1  and an NMOS transistor N 1  coupled between the power supply VCCB and ground. The source of transistor P 1  is connected to VCCB and the source of transistor N 1  is connected to ground. The drains of both transistors are coupled together via resistor R 1 . The junction of the drain of transistor N 1  and resistor R 1  is coupled via inverter INV 3  to the second input of the NAND gate. The output of the NAND gate is coupled to the gate of a boost NMOS transistor N 2 , the drain of which is connected to VCCB and the source of which is connected to the output BIAS. Connected between VCCB and the output BIAS are five long channel PMOS transistors P 2 , P 3 , P 4 , P 5  and P 6 , connected in series. The gates of these transistors are tied together and connected to the chip enable signal. Also connected to the chip enable signal is the gate of an NMOS transistor N 3  which has its drain connected to the output BIAS and its source connected to ground. A PMOS transistor P 7  has its source and back gate connected to the output BIAS and its drain connected to ground. The gate of this transistor is connected to the lower voltage supply level VCCA. Connected between the chip enable signal and the output BIAS is a switching circuit having a PMOS transistor P 8  connected between VCCA and a NMOS transistor N 4 . The source of transistor of P 8  is connected VCCA and the source of transistor N 4  is connected to ground. The drains of the two transistors are connected together via a Schottky diode D 1 . The enable signal is coupled to the gates of the two transistors, which are connected together and the output of the circuit between the cathode of the diode D 1  and the drain of transistor N 4  is connected to the output BIAS.  
         [0018]     In operation, the five long channel PMOS transistors are biased ON to provide the steady state biasing signal for the NMOS pass transistor such as the transistor  110  shown in  FIG. 1 , for example. When the chip enable signal is active, transistor P 8  is biased on provide a voltage which is one Schottky diode drop less than the lower voltage VCCA to the bias level output. This brings the voltage on the gate of the pass transistor up to a voltage close to VCCA very quickly. This can raise the BIAS voltage to approximately 90% of its final value for example. The circuit also provides a one-shot comprising the inverters INV 1 , INV 2  and INV 3  the transistors P 1  and N 1 , resistor R 1  and the NAND gate to drive transistor N 2  which bypasses the PMOS transitors P 2 , P 3 , P 4 , P 5  and P 6  to connect the higher supply voltage VCCB to node BIAS through NCH N 2 . This futher pulls node BIAS up from one Schottky diode drop below VCCA up to an intermediate voltage above VCCA, but below VCCB—the pullup action of NMOS N 2  being limited by the body effect. However, as stated earlier, it is desirable to have the voltage only one volt or one V T  above the voltage level VCCA in order to avoid large current flows from the supply VCCB to VCCA, which is highly undesirable. Therefore, this higher voltage is only applied for a short period of time controlled by the duration of the one-shot signal and the voltage is clamped by transistor P 7  to provide a voltage which is approximately one V T  above the voltage VCCA at the output. By providing the voltage VCCB for a short period of time only, a substantial current flow is only through the limiting transistor P 7  for this short period of time. Then only the smaller current provided by the transistors P 2 -P 6  is required to be handled by the clamp transistor P 7 .  
         [0019]     The circuit shown in  FIG. 4  has a maximum static current on the order of five uA and an enable time on the order of twenty nanoseconds, compared with the two hundred uA static current required by the known circuits and a two microsecond enable time for those circuits. Furthermore, the multi stage process generating the bias voltage minimizes overshoot or undershoot at the bias output while also providing enough drive to charge quickly up the gate capacitance of pass transistor and any bypass capacitances that are placed at the gate of the bypass transistor to reduce noise at the gate.  
         [0020]     A one-shot circuit suitable for use with a pass transistor signal level translator as shown in  FIG. 1  can be the detection circuit and one-shot circuit with feedback described in the U.S. patent application Ser. No. 11/382,995 entitled “ACCELERATOR OUTPUT STAGE THAT ADJUSTS DRIVE DURATION TO LOADING” (TI39817) filed May 12, 2006 and U.S. patent application Ser. No. 60/683,934 filed on May 23, 2005 and incorporated herein by reference in their entirety; And by U.S. patent application Ser. No. 11/239,194 entitled “CIRCUIT FOR DETECTING TRANSITIONS ON EITHER OF TWO SIGNAL LINES REFERENCED AT DIFFERENT POWER SUPPLY LEVELS” (TI39818)) filed on Sep. 29, 2005 and incorporated herein by reference in its entirety. A modification of these circuit s suitable for utilization in a pass gate voltage translator is shown in  FIG. 5  generally as  500 . As can be seen in  FIG. 5 , signals from both port A and port B are coupled to the pull-up trigger circuit the output of which fires a one-shot circuit to provide drive to the upper output transistor UOP for both the A port and the B port. Signals from the A port and the B port are applied through inverters INV 4 , INV 5 , INV 6  and INV 7  to the input of a NAND gate  4  the output of which is applied to one input of the NAND gate for the A port and for the B port output transistors and to a transistor P 5  which shorts out the transistor R 1  in the one-shot circuit to reset it. The signal from the A port passes through a signal translator VCCA to VCCB both in the feedback circuit and in the pull-up trigger circuit so that the circuits can operate off of the VCCB supply.  
         [0021]     While the invention has been particularly shown and described with reference in preferred embodiments thereof it is well understood by those skilled in the art that various changes and modifications can be made in the invention without departing from the spirit and scope of the invention as defined by the appended claims.