Abstract:
A CMOS output buffer uses feedback from a ground node to reduce ground bounce by utilizing a tolerable ground bounce limit, making it less sensitive to operating conditions and processing parameters. An input to the NMOS device of the output buffer is provided by the output of a control element which receives a first input from a pre-driver and a second input (i.e., the feedback) from the ground node.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of integrated circuits, and, more particularly, to a CMOS buffer.  
         BACKGROUND OF THE INVENTION  
         [0002]    In integrated circuits, output buffers are used for interfacing core logic with external devices. One prominent problem in output buffers is “ground bounce.” More particularly, one basic property of an inductor is that the change of current therethrough produces a voltage across the inductor, which is directly proportional to the rate of change of current through the inductor. This may be represented as:  
           
         dV=LdI/dT,  
       
           [0003]    where dV is the voltage generated, L is the inductance, and dI/dT is the rate of change of the current.  
           [0004]    Thus, it may be said that the voltage across the inductor bounces. When considered at the ground pin, this is referred to as ground bounce. Ground bounce occurs as a result of parasitic inductance of the integrated circuit and packaging interconnections. Ground bounce occurs when the pull down transistor switches from an off to an on state.  
           [0005]    Referring to FIG. 1, when the pull down transistor N 116  is turned ON, the potential developed across the capacitor C 122  is coupled by the transistor N 116  to the inductor L 120 . As a result, a transient is generated across inductor L 120 . A sudden increase of current flows from the output terminal  0112  through the pull-down transistor N 116  and through the parasitic inductance L 120  to ground.  
           [0006]    Due to the above noted properties of an inductor, the voltage at the source of the pull down transistor rises. This decreases the gate-source voltage of the pull down transistor. In the case where this rise in source voltage is very large, it can cause ringing, which is reflected in the output of other buffers which are connected to the same ground pin and whose outputs are stable at a low level. The worst case is when all of the buffers, except one whose output is stable at a low level, are connected between the same supply pins and switch from high to low, which may lead to false triggering if the ground bounce is not kept within certain limits. This, in turn, imposes a limit on the number of output buffers that can be connected to a single ground pin, thus increasing the number of ground pins on a chip.  
           [0007]    Various techniques have been used to reduce ground bounce. For example, U.S. Pat. No. 5,124,579 discloses the use of a resistive device for delaying the turn-on time of the output transistors to limit the rate of increase of ground current. Yet, this method is limited in its ability to dynamically adjust to changing output conditions. Furthermore, the delays produced are manufacturing process dependent.  
           [0008]    Another approach is disclosed in U.S. Pat. No. 5,148,056, in which feedback is taken from the output terminal of the buffer. However, this technique has poor sensitivity to the actual ground bounce, especially when it is produced by the switching of other buffers. Further, U.S. Pat. No. 5,604,453 teaches an approach which relies on the matching of the geometries of various individual devices rather than feedback. As a result, this approach is incapable of dynamically adjusting to changing output conditions. Mismatches arising out of process variations would also influence the effectiveness of this approach.  
         SUMMARY OF THE INVENTION  
         [0009]    An object of the present invention is to overcome the above drawbacks and to provide a CMOS buffer with reduced ground bounce.  
           [0010]    These and other objects, features, and advantages in accordance with the invention are provided by a CMOS buffer with reduced ground bounce which may include feedback means or circuit for sensing the ground bounce voltage at a ground terminal. The feedback circuit may be connected to the input of a controlling means or circuit for dynamically adjusting the rate of increase of the ground current in a manner that reduces the sensed ground bounce voltage to a level below a threshold while maintaining a desired speed of operation.  
           [0011]    The feedback circuit may include an amplifier that amplifies the difference between the sensed output ground voltage and an internal reference ground voltage. The controlling circuit may include a slew-rate controlling circuit, for example. In particular, the slew-rate controlling circuit may dynamically adjust the gate voltage of the output NMOS transistor to limit the rate of increase of the current through the ground terminal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The invention will now be described with reference to the accompanying drawings, in which:  
         [0013]    [0013]FIG. 1 is a schematic diagram of a basic inverter with parasitic package inductances in accordance with the prior art;  
         [0014]    [0014]FIG. 2 is a schematic diagram of CMOS output buffers in accordance with the present invention;  
         [0015]    [0015]FIG. 3 is a schematic block diagram of a control element configuration for use with the CMOS output buffers of FIG. 2;  
         [0016]    [0016]FIG. 4 is a schematic block diagram of an alternate control element configuration for use with the CMOS output buffers of FIG. 2; and  
         [0017]    [0017]FIG. 5 is a flowchart illustrating operation of a CMOS output buffer in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Referring to FIG. 2, three output buffers BUFFER 11 , BUFFER 22 , and BUFFER 33  in accordance with the invention are connected between common supplies VDD and GND through package inductances on the VDD and GND pins illustratively represented as inductors L 218  and L 220 , respectively. The inputs to the buffers are IN 11 , IN 22 , and IN 33 , respectively, and the outputs are OP 11 , OP 22 , and OP 33 , respectively. Each buffer BUFFER 11 , BUFFER 22 , and BUFFER 33  has its input connected to its pull-down transistor through a respective control element CE 11 , CE 22 , and CE 33 .  
         [0019]    One configuration of a control element is illustrated in FIG. 3. Here, only BUFFER 11  is considered for clarity of illustration. Input IN 11  is connected to one end of the slew rate control element  305 , while it receives its other input  302  from an amplifier  304 . The amplifier  304  receives as its input  301  feedback from the inductor L 220 . The voltage at the input  301  varies dynamically according to ground bounce. This voltage is used to keep the bounce under control and at a selected level.  
         [0020]    When the ground bounce at input  301  increases to a specific level, it increases the slew of the output signal on output  303  provided to the pull-down transistor N 11 . Further, when the ground bounce is not present, the input signal IN 11  passes through the control element CE 11  without any changes and reaches the gate of pull-down transistor N 11 .  
         [0021]    An alternate control element configuration is illustrated in FIG. 4. The output  303  of control element CE 11  is processed according to a given formula which depends upon the type of package and technology used.  
         [0022]    A steady state condition will now be considered with reference to FIG. 2 where the input signal IN 11  of the BUFFER 11  is low, the input signal IN 22  of the BUFFER 22  is high, and the input signal IN 33  of the BUFFER 33  is also high. The pull-up transistor P 11  is ON, P 22  is OFF, and P 33  is OFF. The pull-down transistor N 11  is OFF, N 22  is ON, and N 33  is ON. The output of control element CE 11  is low, as at this moment there is no bounce at the inductor L 220 . This pulls up the node OP 11  high and also charges the load connected thereto. As the pull-down transistors N 22  and N 33  are ON, OP 22  and OP 33  are pulled down and stable at a low level.  
         [0023]    Now we will consider the case when the input IN 11  is switching from a low to high state. During this switching, as the bounce is produced in the inductor L 220  it is fed back to the control element CE 11 . After the feedback has reached a particular selected level, the control element CE 11  circuitry controls the output provided to the pull-down transistor N 11  by increasing the slew of the signal on the output  303 , thus regulating the current therethrough which decreases the ground bounce at L 220 . Due to this decrease in ground bounce, feedback magnitude also decreases and the input to the gate of the transistor N 11  rises faster (i.e., with decreased slew), which again increases ground bounce. This cycle is repeated until the voltage at IN 11  reaches its high state.  
         [0024]    The above will be further understood with reference to the flow diagram of FIG. 5. The selected level of feedback (which is low as compared to the maximum tolerable ground bounce) at which the control element circuitry becomes active is determined based upon the delay of the control element circuitry. This configuration decreases the sensitivity of the circuitry to process parameters, as well as different voltages and temperatures, because it mainly depends on the feedback from the package inductance. If process models are slow, the bounce at the inductor L 220  will be low and the circuit will be faster. Yet, if the process models are fast, the bounce at the inductor L 220  will be greater, and the circuit will be slower, thus trying to neutralize the effect of process conditions on propagation delays.  
         [0025]    It will be appreciated by those skilled in the art that the circuitry explained above is for reducing ground bounce. It will also be appreciated that similar circuitry may be used for controlling VDDBUMP, bounce at the VDD pin, and the inductance L 218  in accordance with the present invention.