Abstract:
The present invention proposed a slew-rate control circuitry without the use of external components such as amplifiers. Therefore slew-rate control circuitry of the present invention not only provides an IC with build-in slew-rate control, but also reduces number of transistors used externally which will increase gate-oxide reliability of the IC. The slew-rate control circuitry of the present invention is primarily comprised by an output buffer and feedback circuitry, the output buffer mainly consisted four transistors and depends on output of the IC, these four transistors will interact with each other to control the slew-rate of IC output. Additional feedback circuitry and gate-tracking circuitry are also disclosed to enhance the performance of the slew-rate control circuitry.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to an slew-rate control circuitry, and particularly to a slew-rate control circuitry consists of output buffer and feedback. 
         [0003]    2. Description of the Related Art 
         [0004]    For a good design of an integrate circuit (IC), output driver is one of the essential component which will determine IC&#39;s overall performance especially on the field of data and telecommunication system since they are often required to comply with specified interface. As an example, the output driver for RS232 serial interface standard must support the driving of a resistive load of about 3-7 k ohms with a voltage swing greater than ±5 volts and the driving of a capacitive load of about 2500 pF at a frequency of 250 Kbps since the load for RS232 is ohmic-capative type. Therefore it is very important to have precise control of the slew rate of the output driver so that the edge transitions of the output signal can be maintained within acceptable range and additionally ensure an accurate control of the output waveform as the load connected to the output driver varies which will often affect data transmission rate. For an ideal design of the RS232 output buffer, the slew rate must be kept under 30 volts/usec in order to minimize undesirable high frequency components of the output signal which may cause high electromagnetic interference (EMI) due to high output edge switching rates. 
         [0005]    Since the output buffers for IC has so many constraints due to the above-mentioned requirements, traditional design of output buffers is being limited by DC operating characteristics which affect the sizing of output transistor. As a result, high current peaks will occur with the simultaneous switching of output buffers and the inductive power supply noise will create high voltage drop.  FIG. 1  is a circuit for a traditional output buffer with slew-rate control, the pre-driver  2  is connected to a P-channel Metal Oxide Semiconductor (PMOS)  4  and a N-channel Metal Oxide Semiconductor (NMOS)  6 . The pre-driver  2  further connected to a supply voltage VDD of 2.5 volts in one end and VSS of 0 volts at another end. The outcome of this circuit provides a switching voltage with push-pull configuration to the I/O pad  8 . The output voltage from I/O pad  8  is then connected to a resistor  10  for ESD protection, a input inverter  14  and another NMOS  12  for CDM ESD protection before going to the internal circuit. The disadvantage with such design is that when the input signal has greater voltage than the VDD, leakage current is likely to occur at the output of PMOS  4  and parasitic drain-to-well junction diode section  16  due to the large biased voltage. In addition, there will also be gate-oxide reliability concern at the output of NMOS  6 , gate-grounded NMOS  12  and input inverter  14  since they are exposed outside of the IC. 
         [0006]    Based upon the foregoing, the aim of the present invention is to provide a slew-rate control circuit with better output buffer performance and gate-tracking ability without the requirement of using external component. The output buffer of the slew-rate control circuit not only provides an IC with build-in slew-rate control and better gate-oxide reliability, but also reduces the number of MOS used externally which decreases the cost of manufacturing. 
       BRIEF SUMMARY 
       [0007]    Based upon the foregoing, the present invention is proposing a slew-rate control circuitry with output buffer and gate-tracking ability without the requirement of using external component such as amplifiers. Therefore the output buffer of the slew-rate control circuit not only provides an IC with build-in slew-rate control, but also reduces number of transistors used externally which will increase gate-oxide reliability of the IC. 
         [0008]    The output buffer of slew-rate control circuitry of the present invention comprises a first P-channel MOS transistor switch having its source connected to a supply voltage and its gate receives an output signal from said input controlled logic unit and a second P-channel MOS transistor switch having its source connected to drain of said first PMOS switch, its gate connected to a ground and its drain connected to an output. A first N-channel MOS transistor switch having its drain connected to a reference voltage and its gate receives an output signal from said input controlled logic unit and a second N-channel MOS transistor switch having its drain connected to source of said first N-channel MOS transistor switch, its gate connected to a ground and its drain connected to said output. Therefore depends on output of the IC, the four transistors will interact with each other to control the slew-rate of the output buffer. Additional feedback circuitry and gate-tracking circuitry are also disclosed in accordance with the present invention to enhance the performance of the output buffer and slew-rate control. 
         [0009]    Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout, and in which: 
           [0011]      FIG. 1  is a schematic diagram of a conventional output buffer with slew-rate control; 
           [0012]      FIG. 2  is a schematic diagram of output buffer circuit of the present invention; 
           [0013]      FIG. 3  is a schematic diagram to a preferred embodiment of the present invention; 
           [0014]      FIG. 4  is a schematic diagram to a preferred embodiment of the present invention; 
           [0015]      FIG. 5  is a schematic diagram to a preferred embodiment of the present invention; and 
           [0016]      FIG. 6  is a chart representing the relationship between slew-rate and load for the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The slew-rate control circuitry of the present invention primarily comprises an output buffer which receives signal from at least one input and provides at least one output, and at least one feed back circuitry. Referring to  FIG. 2  for the output buffer in accordance with the present invention which comprises two P-channel MOS transistors (PMOS  18  and  20 ) and two N-channel MOS transistors (NMOS  22  and  24 ). PMOS  18  has its source connected to a supply voltage V+ and its drain connected to the source of PMOS  20 , the gate of PMOS  18  in addition receives an input signal from an input controlled logic. The source of NMOS  24  is connected to a reference voltage V− and the drain of NMOS  24  is connected to the source of NMOS  22 , the gate of NMOS  24  also receives an input signal from the input controlled logic like PMOS  18 . Both gates for PMOS  20  and NMOS  22  are connected to a ground and the source of PMOS  20  is connected to the drain of NMOS  22  to provide a transmission output TxOUT  26 . 
         [0018]    In order to provide more stable slew rate control, the circuitry for the present invention requires at least one feedback circuitry. As shown in  FIG. 3 , at least one feedback capacitor can be added between the output  26  and the input controlled logic  36 . In this embodiment of the present invention, a feedback capacitor  34  is added to output  26  in one end, the other end of feedback capacitor  34  is connected to feedback MOS device  30 . Feedback MOS device  30  is simply comprised by a PMOS and a NMOS. The drain of PMOS is connected to the source of NMOS to provide an output to the feedback capacitor  34 . The source of PMOS of Feedback MOS device  30  and the drain of NMOS of feedback MOS device  30  is connected the ground. With the configuration mentioned above, the charging and discharging for feedback capacitor  34  can be well controlled by the feedback MOS device  30 . To provide better efficiency for the feedback, a designer can add more set of feedback MOS device to the IC. For example, another feedback capacitor  32  can be added between feedback capacitor  34  and the input controlled logic  36 . Same as the circuitry mentioned above, another feedback MOS device  28  is required to control the current stored in feedback capacitor  32 . 
         [0019]    In order to avoid the feedback MOS device mentioned above from gate-oxide overstress which typically occurs during shutdown mode, the feedback circuitry can further be modified into a gate-tracking circuitry by placing few MOS between the feedback capacitor and the input controlled logic as shown in  FIG. 4  to increase the gate-oxide reliability. In  FIG. 4 , gate-tracking circuitry  38  which comprises a PMOS and a NMOS is placed between the feedback capacitor  34  and the feedback capacitor  32 . The source of PMOS of gate-tracking circuitry  38  is connected to a supply voltage V+ and its gate. The drain of NMOS of gate-tracking circuitry  38  is connected to its gate and a reference voltage V−. The drain of PMOS and the source of NMOS for the gate-tracking circuitry  38  are then connected to the capacitor  34  to bias the feedback point of capacitor  34  to V+/V−. Structure of gate-tracking circuitry  40  for capacitor  32  is the same as the gate-tracking circuitry  38  for capacitor  34 , the source of PMOS of gate-tracking circuitry  40  is connected to a supply voltage V+ and its gate. The drain of NMOS of gate-tracking circuitry  40  is connected to its gate and a reference voltage V−. The drain of PMOS and the source of NMOS for the gate-tracking circuitry  40  are then connected to the capacitor  32  to bias the feedback point of capacitor  32  to V+/V−. As a result, the possibility of having gate-oxide overstress can be eliminated. 
         [0020]    Most IC requires at least two power sources such as supply voltage and reference voltage. However, in order for an IC to effectively shut off the power supply during shutdown (tri-state) mode, it is a benefit to have third power supply and extra switches to halt the power supply during the shutdown mode as shown in  FIG. 5 . In  FIG. 5 , four more switches are being added into the gate-tracking circuitry, two PMOS  42  and  44  are connected in parallel, both have their source connected to the drain of PMOS  18  (source of PMOS  20 ) of the output buffer. The gate for both PMOS  42  and  44  is connected to a supply voltage V+ and the drain of PMOS  42  is connected to the gate of PMOS  18  where the drain of PMOS  44  is connected to gate of feedback PMOS. Two NMOS  46  and  48  are connected in parallels, both have their drain connected to the drain of NMOS  22  (source of NMOS  24 ) of the output buffer. The gate for both NMOS  46  and  48  is connected to a reference voltage V− and the source of NMOS  48  is connected to the gate of NMOS  24  where the source of NMOS  46  is connected to gate of feedback NMOS. With these MOS switches ( 42 ,  44 ,  46 ,  48 ), they can effectively turn-off the feedback MOS device during the shutdown mode. 
         [0021]    The gate-tracking circuitry further includes two gate controlled logic and a MOS shutdown (tri-state) circuitry  50  is being implemented which consists series connection of PMOS and NMOS transistors. In shutdown circuitry  50  of the present invention, by looking down from top to bottom, a first NMOS has its gate connected to the supply voltage V+ and its source connected to the drain of a first PMOS and the drain of PMOS  42 . The drain of the first NMOS is connected to the source of the first PMOS and the point of their connection also received an input signal from the input controlled logic  36 . The gate of the first PMOS is connected to a gate controlled logic  52 . A second NMOS has its gate connected to the supply voltage V+ and its source connected to the drain of a second PMOS and the drain of PMOS  44 . The drain of the second NMOS is connected to the source of the second PMOS and the point of their connection also received an input signal from the input controlled logic  36 . The gate of the second PMOS is also connected to the gate controlled logic  52 . A third PMOS has its gate connected to the reference voltage V− and its drain connected to the source of a third PMOS and the source of NMOS  46 . The source of the third PMOS is connected to the drain of the third NMOS and the point of their connection also received an input signal from the input controlled logic  36 . The gate of the third PMOS is connected to a gate controlled logic  54 . A fourth PMOS has its gate connected to the reference voltage V− and its drain connected to the source of a fourth PMOS and the source of NMOS  48 . The source of the fourth PMOS is connected to the drain of the fourth NMOS and the point of their connection also received an input signal from the input controlled logic  36 . The gate of the third PMOS is connected to the gate controlled logic  54 . With such circuit configuration, the shutdown circuitry can block the ± voltage into the input controlled logic  36  during shutdown mode for the output buffer.  FIG. 6  is a chart representing the relationship between slew-rate and capacitive load for the present invention where the solid line represents positive slew-rate and the broken line represents negative slew-rate. It can be easily observed from the chart, the slew-rate can be controlled to lower than 30V/us with loading capacitance from 0 pF to 5000 pF. 
         [0022]    The above description is given by way of example, and not limitation. Given the above disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention disclosed herein, including configurations ways of the recessed portions and materials and/or designs of the attaching structures. Further, the various features of the embodiments disclosed herein can be used alone, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the scope of the claims is not to be limited by the illustrated embodiments.