Abstract:
The present invention provides an output buffer providing multiple voltages including an arrangement of bootstrapping capacitors, and a charge replenishing mechanism which provides continuous pulses to the arrangement of bootstrapping capacitors, thereby, maintaining voltage on the bootstrapping capacitors.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an output buffer providing multiple voltages at an I/O interface. More specifically, the present invention relates to an output buffer for driving an I/O pad at the desired high voltage level along with providing circuitry for tolerating higher voltages.  
         [0003]     2. Discussion of the Related Art  
         [0004]     Implementation of integrated circuits using lower dimension feature sizes entails the need of scaling the supply voltages to a lower value. In certain applications, interfacing of circuits operating at different voltages is required. The output buffer of an I/O circuit uses a totem pole output for driving the pad. In a totem pole structure, the source of a P-channel device is connected to the supply voltage. Similarly, the source of an N-channel device is connected to the ground with the drain of both transistors connected together. Whenever a device operating at a lower voltage is connected to a node, which is also driven by some other logic operating at a higher voltage, the common node attains the higher voltage level. As a result, the diode formed by the drain of the P-channel transistor and the N-well in which the said P-channel device is formed, becomes forward biased. The P-channel transistor may also turn on. If any of these things happen, the circuit starts dissipating power and the common node clamps at the lower operating voltage.  
         [0005]     Several approaches to solve this problem have been tried. One among them was to use an N-channel device instead of a P-channel device in the totem pole and the gate of the N-channel device was driven at a higher voltage level using a level translator to have full-swing voltage at the pad.  
         [0006]     U.S. Pat. 5,952,847 is described with the help of  FIG. 1 . It illustrates a prior art output buffer. The output buffer circuit includes an output totem pole, a level shifter and logic enable block. The gate of the N-channel device in totem pole  12  used to drive the pad at 3.3V is kept at 5V through a level shifter. The oxide of the N-channel device of the totem pole  12 , N-channel and P-channel devices of the level shifter  10  are rated for 5V. These transistors are having LDD structure to prevent short channel effect. The structure will prevent any power dissipation when the common node  14  is driven at 5V by any other logic. The output totem pole gets input from logic enable block  8 , which is operating at 3.3V. However, the disadvantage of this circuit is that it needs two power supplies to achieve the goal.  
       SUMMARY OF THE INVENTION  
       [0007]     In the present invention, the results are achieved using a single power supply and the gate of the said N-channel device is maintained at the desired higher voltage level through sustained bootstrapping. Bootstrapping utilizes a capacitance called Miller capacitance to raise the potential of the gate of a MOS device which is greater than the source potential applied to the terminals of the device in order to compensate for the threshold voltage drop while passing logic high at the pad.  
         [0008]     To overcome at least some of the above limitations, the present invention provides a method for driving an I/O pad at the desired high voltage level along with providing circuitry for tolerating higher voltages.  
         [0009]     An object of the present invention is to provide a circuit that can help interface devices operating at different voltages.  
         [0010]     Another object of the present invention is to provide a totem pole output having a pull-up stage that will not turn on or whose junction diode will not turn on when the output is driven to a voltage higher than the operating voltage of the circuit of interest.  
         [0011]     Another object of the present invention is to provide an output buffer that is resistant to latch-up.  
         [0012]     Another object of the present invention is to provide a high voltage tolerant output buffer.  
         [0013]     Another object of the present invention is to provide an output buffer operating at a single supply voltage along with having a capability of being connected to a common node which may be driven to a higher voltage without current sinking.  
         [0014]     Another object of the present invention is to provide an I/O buffer that achieves rail-to-rail swing at the output through bootstrapping.  
         [0015]     Another object of the present invention is to sustain the bootstrap by converting DC energy to AC energy through an oscillating means.  
         [0016]     To achieve these and other objects, the present invention provides an output buffer providing multiple voltages comprising: 
        an arrangement of bootstrapping capacitors, and;     a charge replenishing mechanism that provides continuous pulses to the arrangement of bootstrapping capacitors, 
 
 thereby maintaining voltage on said bootstrapping capacitors. 
       
 
         [0019]     According to one embodiment of the invention, a compensating element is used to improve the rise time of output signal for capacitive loads.  
         [0020]     According to one embodiment of the invention, at least some of the bootstrapping capacitors are parasitic capacitances.  
         [0021]     According to one embodiment of the invention, the charge replenishing mechanism is a ring oscillator.  
         [0022]     The present invention further provides a method for providing multiple voltages comprising: 
        bootstrapping the intermediate and output nodes using bootstrapping capacitor and;     replenishing the charge on the bootstrapping capacitor at periodic intervals so as to maintain said bootstrap voltage.        
 
         [0025]     According to one embodiment of the invention, the bootstrapping is used to improve the rise time of the output signal for capacitive loads.  
         [0026]     According to one embodiment of the invention, at least some of said bootstrapping capacitors are parasitic capacitances.  
         [0027]     According to one embodiment of the invention, replenish of charge is implemented using a ring oscillator. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]     The invention will now be explained with reference to the accompanying drawings.  
         [0029]      FIG. 1  illustrates a prior art output buffer implemented using a level shifter.  
         [0030]      FIG. 2  is a block diagram of the present invention.  
         [0031]      FIG. 3  is a ring oscillator used for sustaining bootstrapping at the gate of the output buffer.  
         [0032]      FIG. 4  illustrates the output buffer according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0033]      FIG. 2  illustrates a block diagram of the present invention. It contains an oscillator circuit  18  to provide continuous pulses to bootstrapping circuit  20 . It also contains a logic enable circuit  16 , which is used to drive the totem pole circuit  22 .  
         [0034]      FIG. 3  illustrates a ring oscillator. It includes 5 inverters connected in a ring. The temperature and process variation don&#39;t affect the performance of the output buffer and the frequency of the oscillator is also immaterial as long as the bootstrap voltage does not go below a minimum level required for threshold drop compensation. Frequency of the oscillator is decided by the gain of the inverters (i.e. sizes of the inverters) and the number of stages. A minimum number of inverter stages are required to sustain oscillation. In the present invention, the number of stages is 5. The bootstrap voltage achieved should not exceed a certain level so that the circuit remains high-voltage tolerant.  
         [0035]      FIG. 4  is a schematic diagram of the output buffer of the invention. According to the present invention, pad  34  is pulled up by a p-channel device  50  in conjunction with bootstrapped N-channel device  52  and the pad is pulled low by permanently-on N-channel device  54  in series with N-channel device  56 . Bootstrapping circuit  28  contains a P-channel device and an N-channel device to get the bootstrap voltage at the gate of the transistor  52 . The transistor  54  is added to the totem pole output to make the circuit high-voltage tolerant.  
         [0036]     Referring to the bootstrapping circuit  28 , nodes  58  and  60  are the nodes that get bootstrapped by miller capacitances  62 ,  64 ,  66  and  68 . There is a cumulative bootstrapping at node  60  which helps achieve a voltage exceeding 5V which would otherwise have been achieving a voltage close to 3.6 V, had the N-channel device  48  been removed from the circuit. Oscillator  26  helps sustain higher voltage at the gate of N-channel device  52  by providing pulses at regular intervals to provide transition at the gate of P-channel device  46 . The gate of the N-channel device  52  will no longer remain at higher voltage when the pad  34  is driven low. This will keep the output buffer 5V tolerant. Oxide of the N-channel devices  52  and  48  are rated for higher voltage. It can also be observed that whenever data changes from low to high, it enhances the bootstrapped voltage.  
         [0037]     Referring to the enable logic block  30 , whenever global enable and output enable signals are raised high, data is passed onto the P-channel device  50  and the N-channel device  56 . When data is low, transistor  50  is ON and pad  34  is pulled high. Similarly, whenever data is high, transistor  56  is ON and the pad  34  is pulled low. The output buffer gets restarted whenever a low signal comes at either of a global enable or output enable signal.  
         [0038]     Referring to the pad driver totem pole block  32 , it contains a P-channel device  50 , a N-channel device  52 , a N-channel device  54  and a N-channel device  56 . Data is provided at the gate of  50  and  56 . Whenever data goes high, transistor  50  passes 3.3 V, which, in turn, is passed onto the pad  34  by virtue of the bootstrapped voltage at the gate of transistor  52 . Similarly, whenever data goes low, pad  34  is driven low through transistors  54  and  56 .  
         [0039]     If pad  34  is driven at 5V by some other logic, node  62  gets a voltage that is one threshold voltage (Vt) less than 3.3 V. At this voltage, transistor  50  does not turn on and pad  34  remains at 5V i.e. there is no power dissipation. Similarly, to make circuit 5 V tolerant, the output buffer needs to be restated, whenever the pad is driven at 5V by some other logic. In this case, drain to source voltage, gate to source and gate to drain voltage of transistors  50 ,  52 ,  54  and  56  do not exceed the tolerant voltage level. The oxide of the transistors  48  and  52  is rated for higher voltage. Transistors  46 ,  48  and  52  are required to have LDD structure. The clock signal can be applied at the gate of the transistor  46  instead of the oscillator circuit. As clock signals are available in I/O circuit for I/O registers, they can easily be routing at the desired place and hence can replace the oscillator circuit. If the bootstrapped buffer is used to drive a dynamic signal like clock, the oscillator circuit is not needed.  
         [0040]     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.