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:
RELATED APPLICATION  
       [0001]    The present application is a continuation-in-part of U.S. patent application Ser. No. 10/206,135 filed Jul. 26, 2002, the entire disclosure of which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to the field of integrated circuits, and, more particularly, to a CMOS buffer.  
         BACKGROUND OF THE INVENTION  
         [0003]    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,  
       
           [0004]    where dV is the voltage generated, L is the inductance, and dI/dT is the rate of change of the current.  
           [0005]    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.  
           [0006]    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 O 112  through the pull-down transistor N 116  and through the parasitic inductance L 120  to ground.  
           [0007]    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.  
           [0008]    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.  
           [0009]    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  
         [0010]    An object of the present invention is to overcome the above drawbacks and to provide a CMOS buffer with reduced ground bounce.  
           [0011]    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 a 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 control 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.  
           [0012]    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  
       [0013]    The invention will now be described with reference to the accompanying drawings, in which:  
         [0014]    [0014]FIG. 1 is a schematic diagram of a basic inverter with parasitic package inductances in accordance with the prior art;  
         [0015]    [0015]FIG. 2 is a schematic diagram of CMOS output buffers in accordance with the present invention;  
         [0016]    [0016]FIG. 3 is a schematic block diagram of a control element configuration for use with the CMOS output buffers of FIG. 2;  
         [0017]    [0017]FIG. 4 is a schematic block diagram of an alternate control element configuration for use with the CMOS output buffers of FIG. 2;  
         [0018]    [0018]FIG. 5 is a flowchart illustrating operation of a CMOS output buffer in accordance with the present invention.  
         [0019]    [0019]FIG. 6 is a simplified diagram of the output buffer showing control circuit as block  101  according to the present invention.  
         [0020]    [0020]FIG. 7 is a simplified diagram of output buffer along with control circuit shown in detail.  
         [0021]    [0021]FIG. 8 is a voltage waveforms diagram showing the operation of circuit.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternative embodiments.  
         [0023]    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 .  
         [0024]    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.  
         [0025]    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 .  
         [0026]    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.  
         [0027]    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.  
         [0028]    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.  
         [0029]    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.  
         [0030]    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.  
         [0031]    A more detailed embodiment of an output buffer and control circuit according to the present invention will now be described with reference to FIGS.  6 - 8 . FIG. 6 illustrates a CMOS output buffer, including a pre-driver  102 , a pad driver circuit  100 , a control circuit  101  for controlling ground bounce and an AND gate A 1 . The output buffer also includes IO PAD  103 . One input of A 1  is connected to configuration bit CB while other input is connected to NIN 3  which is coming from pre driver  102 . Output driver  100  includes PMOS P 1  with it&#39;s drain connected to the output pad  103  and source connected to power supply VDD. Output driver  100  also includes NMOSs N 1  and N 2  with their drains connected to output pad  103  and their sources connected to C 2 . C 2  is connected to ground GND via parasitic inductor L 1 . PIN 1  and NIN 1  are coming from predriver  102  and connected to the gates of transistor P 1  and N 1  respectively. NIN 3  is coming from predriver  102  which is connected to one of the inputs of AND gate A 1 . NIN 2  is gate voltage for N 2  coming from control circuit  101 .  
         [0032]    More specifically, the output buffer as shown in FIG. 7 includes pad driver  100 , Pre-driver  102 , control circuit  101 , AND gate A 1  and pad  103 . Control circuit  101  includes NMOSs N 3 , N 4 , N 5  and inverter G 1 . The source of N 4  is connected to ground while it&#39;s drain is connected to NIN 2 . The gate of N 4  is connected to line FB. The output of inverter G 1  is connected to the gate of N 3 . The source of N 3  is connected to NIN 2  while it&#39;s drain is connected to line CC. Drain of N 5  is connected to node C 2 . Gate of N 5  is connected to VDD and its source is connected to line FB. The input of inverter G 1  is connected to line FB while it&#39;s output is connected to the gate of N 3 . Feedback is taken from node C 2  which is connected to line FB via N 5 . N 5  is used to protect the gates of G 1  and N 4  from any occasional high voltage noise at C 2 . N 5  will never allow a voltage greater than VDD−Vt(N 5 ) to pass through it.  
         [0033]    Vmtp is maximum tolerable peak voltage. This is the maximum amplitude of ground bounce pulse that can be tolerated for a particular pulse width. Vtrip(G 1 ) is the trip point voltage of inverter G 1 . Depending on the current sinking capability required either N 1  is conducting or both N 1  and N 2  are conducting. This is decided by configuration bit CB. It is presumed that for lower sinking capability (CB=0) i.e when only N 1  is conducting, ground bounce remains in acceptable limits. With CB=0 line CC remains at 0V.  
         [0034]    With only N 1  ON, voltage at node C 2  is low enough (lower than Vtrip(G 1 )) to keep the gate of N 3  at logic 1. This keeps NIN 2  at 0V and hence N 2  OFF. For higher sinking capability CB=1 Where both N 1  and N 2  are ON. In this case the current flowing through inductor L 1  is high which raises the voltage at C 2  above tolerable limit. The control circuit  101  controls the voltage at C 2  so that it always remains within tolerable limits. Considering a stable condition when output from pre driver  102  i.e NIN 1 , PIN 1  and NIN 3  are all 0V. With CB=1 and NIN 3 =0V line CC remains at 0V. NIN 1  is 0V which makes N 1  OFF. Node C 2  and line FB remains at 0V. The input to G 1  is 0V while its output which is connected to the gate of N 3  is at VDD. This makes N 3  ON and hence makes NIN 2  0V. The gate of N 4  is connected to 0V which makes N 4  OFF. With PIN 1 =0V P 1  is ON, keeping PAD  103  at VDD. Now considering NIN 1 , PIN 1  and NIN 3  all makes a transition from logic low to high. This makes P 1  OFF. Sizing of Predriver is such that slew rate of voltage (dV/dt) at NIN 3  is much faster than NIN 1 . Sinking is faster as the control circuitry never allows ground bounce to exceed Vmtp. Also sinking capability of N 1  is such that if only N 1  is ON ground bounce never exceeds beyond maximum tolerable value Vmtp.  
         [0035]    Now voltage at NIN 3  and NIN 1  starts increasing. Increase in voltage at NIN 1  turns ON N 1 . At the same time the voltage at NIN 3  also starts increasing and increases at a rate faster than NIN 1 . This makes line CC to go at logic 1. With N 3  ON the voltage at line CC is transmitted to NIN 2 . This makes N 2  ON. Now N 1  and N 2  both are ON to pull down PAD  103 . This increases the current flowing through L 1 . Because of this voltage at C 2  starts increasing the current flowing through inductor L 1  is not constant therefore the voltage at point C 2  is given by V(C 2 )=L 1 [di(t)/dt] i(t) is the current flowing through L 1 .  
         [0036]    Depending on the maximum tolerable peak voltage (Vmtp) at C 2  the trip point of G 1  is adjusted. The threshold voltage of N 4  is less as compare to the trip point of G 1 . As mentioned earlier Vmtp will be defined for a particular noise pulse width. The size of N 4  is small in comparison to that of N 3 . As the voltage at C 2  approaches threshold voltage of N 4 , it starts conducting. N 4  tries to slow down the increase in voltage at NIN 2 . Now depending on the operating conditions and the type of models used two things can happen.  
         [0037]    Firstly, under best operating conditions when both N 1  and N 2  starts conducting, the voltage at C 2  starts increasing. The trip point of G 1  is higher than that of threshold voltage of N 4 . Increase in the voltage at C 2  first of all turns N 4  ON. Now both N 4  and N 3  are ON. But the size of N 4  is much smaller than that of N 3 . Conducting N 4  slightly reduces the voltage slew rate (dv/dt) at NIN 2 . But still the slew rate is enough high and the voltage at C 2  is still increasing. As the voltage at C 2  reaches to the trip point of G 1 , the output of G 1  becomes zero which makes N 3  OFF. With N 3  OFF and N 4  ON the magnitude of voltage at NIN 2  starts decreasing. This will reduce the conductivity of N 2  and hence the voltage at C 2  also starts decreasing. Reduction in voltage at C 2  again trips G 1  which again turns ON N 3  and voltage at NIN 2  starts increasing. This will again increase current through inductor L 1 . If the voltage at C 2  again exceeds the trip point of G 1 , the above explained process is repeated again.  
         [0038]    [0038]FIG. 3 shows the voltage waveforms at different nodes. NIN 2  starts increasing from 0V at time T0. As N 2  becomes ON ground bounce starts increasing. At time T1 V(C 2 ) crosses the threshold level of N 4 . This will reduce the slew rate of NIN 2 . Reduction in the slew rate of NIN 2  can be seen from time T1 to T2. Even with the reduction in slew rate ground bounce (V(C 2 )) still increasing. At time (T2−dt) G 1  trips and makes N 3  OFF. At time T2 voltage at line NIN 2  starts falling because of N 4 . This reduces the current flowing through L 1  because of which voltage at node C 2  starts decreasing. At time (T2+dt) G 1  again trips making N 3  ON. NIN 2  doesn&#39;t start increasing instantaneously as N 3  has some delay and also N 4  is still conducting to stop NIN 2  from increasing. At T3 NIN 2  starts increasing which again results in increase in the ground bounce. But this time the magnitude of voltage at V(C 2 ) remains well below Vtrip(G 1 ). The control circuit is working on feedback principle so it never allows ground bounce to cross Vmtp.  
         [0039]    Secondly, under worst operating conditions magnitude of voltage at C 2  is less than Vtrip(G 1 ). The output of G 1  always remains VDD and hence N 3  always remains ON. During slow operating conditions its N 4  which slightly reduces the slew rate at NIN 2 . The above explained circuitry not only controls the ground bounce but it also tries to equalize delays under different operating conditions. Under fast operating conditions as the bounce approaches Vmtp, N 3  becomes OFF which controls the bounce from further increase. With N 3  OFF and N 4  ON, the voltage at NIN 2  actually starts falling as shown in FIG. 3. This reduces the current flowing through L 1  because of which voltage at node C 2  starts decreasing. This makes N 4  less conducting. After a time 2dt N 3  again turns ON but voltage at NIN 2  starts increasing only after a delay of T3−(T2+dt) as shown in FIG. 3 whereas under slow operating conditions N 3  is always ON and a slight reduction in the slew rate by N 4  is sufficient to control the bounce. Thus in best operating conditions the bounce is controlled by actually decreasing the voltage at NIN 2  whereas in worst operating conditions the bounce is controlled by slightly reducing the slew rate of voltage at NIN 2 .  
         [0040]    Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.