Abstract:
A level shifter circuit includes a level shifter unit and a latch unit. The level shifter unit receives two complementary input signals and converts the voltage levels of two complementary input signals. The latch unit latches the state of two output nodes before the low voltage supply is turned off. On condition that the low voltage supply is off, the level shifter circuit avoids current drainage and ensures the voltage level of the output. The invention has the advantages of small circuit size and being easy to design.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to an integrated circuit, and in particular, to a level shift circuit with the function of latching the state of an output node before the low voltage supply is off.  
         [0003]     2. Description of the Related Art  
         [0004]     For saving power, some of integrated circuits are operated at two different voltage levels or power domains. For instance, the circuits in the core of a integrated circuit often operates at a lower voltage (VDDL) of 3.3V than input/output circuits operating at a voltage (VDDH) of 5V in order to reduce power consumption and to enable the use of smaller transistors, thereby reducing the overall die size. Therefore, the integrated circuits frequently use level shifter circuits to adjust the voltage of the input signal so as to be correctly interpreted in the new power domain operating at a higher or lower voltage level.  
         [0005]      FIG. 1  is a circuit diagram of a prior art level shifter circuit. Referring to  FIG. 1 , a level shifter circuit  100  includes an input unit  11 , a level shifting unit  12  and an output unit  13 . The voltage level (VDDL) of a first power supply provided for the input unit  11  is lower than the voltage level (VDDH) of a second power supply provided for the level shifting unit  12  and the output unit  13 . After receiving a front-end input signal INP, the input unit  11  generates an input signal IN and a complementary input signal XIN. To save the power consumption, the first power source operating at a low voltage level is shut off. After the first power source is shut off, the voltages of the gates of both N-channel transistors  105  and  107  are dropped below the threshold voltage V t  such that the N-channel transistors  105  and  107  are shut off and the output node  112  and the complementary output node  111  are floated. In the worst case, the nodes  111  and  112  could float to the voltage of VDDH/2, thereby causing a static current drain through the inverters  108  and  109  in the output unit  13 .  
         [0006]     Because only one of both nodes  111  and  112  can be pulled to VDDH by using two P-channel transistors  104  and  106 , the other node stays at the voltage of VDDH/2 for lack of a leaking path to the ground so that the P-channel and N-channel transistors of inverters  108  and  109  are simultaneously turned ON and there is a static current drain through the second power supply operating at a high voltage. Besides, the voltage of the output terminal  110  becomes indeterminable, which may result in errors of the following stages.  
         [0007]      FIG. 2  is a circuit diagram of another prior art level shifter circuit. Referring to  FIG. 2 , a level shifter circuit  200  also includes an input unit  11 , a level shifting unit  22  and an output unit  13 . Additionally, in the level shifting unit  22 , two P-channel transistors  204  and  206  are inserted to the original level shifting unit  12  in  FIG. 1 , allowing nodes  111  and  112  to pull down more quickly. As mentioned above, when the first power source operating at a low voltage is turned off, N-channel transistors  105  and  107  are dropped below the threshold voltage V t  such that the N-channel transistors  105  and  107  are shut off and P-channel transistors  204  and  206  are turned on. As regards other situations, nodes  111  and  112  may float just like  FIG. 1 . If the voltage of the nodes  111  and  112  is staying at VDDH/2, there is a static current drain through the second power supply operating at a high voltage and the voltage of the output terminal  110  becomes indeterminable.  
         [0008]      FIG. 3  is a circuit diagram of still another prior art level shifter circuit. In U.S. Pat. No. 6,600,358, Chan discloses a level shifter circuit  300  as shown in  FIG. 3 , which includes a low voltage detector  320  for detecting the low voltage supply, and eliminates the current drain when the low voltage supply is off. When the low voltage supply is turned off, the input terminal  101  is isolated from the output terminal  110  to avoid the current drain caused by a floating of the gate. However, after the low voltage supply is turned off, the output terminal  110  of the level shifter circuit  300  is fixed at a certain voltage level, but not maintained at a state right before the low voltage supply was turned off. Besides, a lot of transistors are required in the low voltage detector  320 . Meanwhile, the number of transistors in the low voltage detector  320  is increased as the voltage difference between two different power domains become larger, so that more stages are necessary for detecting the low voltage supply. Further, the circuit layout size of the level shifter circuit  300  is larger than those of conventional level shifter circuits.  
         [0009]      FIG. 4  is a circuit diagram of another prior art level shifter circuit. In U.S. Pat. No. 6,819,159, Lencioni discloses a level shifter circuit  400  as shown in  FIG. 4 , which includes two level shifters  430  and  440  and two transistors  405  and  407 . Two transistors  405  and  407  are used to improve the pull-down speed of the output terminal  110  in the level shifter circuit  400 . When the first power supply is turned off, if the two level shifters  430  and  440  match with each other, two transistors  405  and  407  are used as a leaking path to the ground for both the complementary output node  111  and the output node  112 , and there is no static current drain. However, owing to including two level shifters  430  and  440 , the level shifter circuit  400  occupies a larger circuit size than conventional level shifter circuits do. Also, the circuit layouts of the two level shifters  430  and  440  necessarily match each other, thereby increasing the complexity of circuit design.  
         [0010]     Conventional level shifter circuits are numerous. However, the primary object of a level shifter circuit is to correctly adjust the voltage level of the input signal. Thereby, when the low voltage supply is turned off, it is important that there are no static current drain and a definite voltage level of the output terminal in a practical level shifter circuit, which should be small in layout size and easy in circuit design.  
       SUMMARY OF THE INVENTION  
       [0011]     In view of the above-mentioned problems, an object of the invention is to provide a level shifter circuit to latch the state of an output node before the low voltage supply is turned off.  
         [0012]     To achieve the above-mentioned object, the level shifter circuit converts both an input signal and a complementary input signal driven by a first power supply into an output signal driven by a second power supply. The level shifter circuit comprises a level shifting unit and a latch unit. After receiving the input signal and the complementary input signal, the level shifting unit driven by the second power supply and having an output node and a complementary output node outputs the output signal. The latch unit, which is driven by the second power supply and is connected to the output node, the complementary output node and a ground terminal, latches the state of the output node and the complementary output node before the first power supply is switched off.  
         [0013]     Comparing with conventional level shifter circuits, the invention solves the current drainage when the low voltage supply is turned off, by just adding two N-channel transistors. With the minimum circuit size increment, the aim of saving the power consumption is achieved. Moreover, the output voltage is held at a voltage level right before the low voltage supply was turned off so that the following stages can operate normally. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a circuit diagram of a prior art level shifter circuit.  
         [0015]      FIG. 2  is a circuit diagram of another prior art level shifter circuit.  
         [0016]      FIG. 3  is a circuit diagram of another prior art level shifter circuit.  
         [0017]      FIG. 4  is a circuit diagram of another prior art level shifter circuit.  
         [0018]      FIG. 5  is a circuit diagram of a level shifter circuit according to the invention.  
         [0019]      FIG. 6  is a circuit diagram of a level shifter circuit according to a first embodiment of the invention.  
         [0020]      FIG. 7  is a circuit diagram of a level shifter circuit according to a second embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The level shifter circuit of the invention will be described with reference to the accompanying drawings.  
         [0022]      FIG. 5  is a circuit diagram of a level shifter circuit according to the invention. Referring now to  FIG. 5 , the level shifter circuit  500  includes a level shifting unit  12  and a latch unit  54  according to the invention. The level shifting unit  12  converts two complementary input signals (i.e. an input signal IN and a complementary input signal XIN) having a voltage amplitude ranging from 0 to VDDL into an output signal having a voltage amplitude ranging from 0 to VDDH. Wherein, the two complementary input signals IN and XIN are driven by a first power supply operating at a low voltage while both the level shifting unit  12  and the latch unit  54  are driven by a second power supply operating at a high voltage. The latch unit  54  latches the state of the output node  112  and the complementary output node  111  before the first power supply is turned off.  
         [0023]     The level shifting unit  12  includes a first P-channel transistor  104 , a second P-channel transistor  106 , a first transistor device  505  and a second transistor device  507 . The latch unit  54  is connected to the output node  112 , the complementary output node  111  and a ground terminal  113 .  
         [0024]     The first P-channel transistor  104  and the second P-channel transistor  106  are respectively connected to the second power supply. The first transistor device  505  receives the input signal IN, and is connected to the drain of the first P-channel transistor  104 , the gate of the second P-channel transistor  106  and the latch unit  54 . The second transistor device  507  receives the complementary input signal XIN, and is connected to the gate of the first P-channel transistor  104 , the drain of the second P-channel transistor  106  and the latch unit  54 .  
         [0025]      FIG. 6  is a circuit diagram of a level shifter circuit according to a first embodiment of the invention. Referring to  FIG. 6 , the level shifter circuit  600  includes an input unit  11 , a level shifting unit  12 , an output unit  13  and a latch unit  54  according to the first embodiment of the invention. The input unit  11  is driven by the first power supply operating at a low voltage VDDL. The level shifting unit  12 , the output unit  13  and the latch unit  54  are driven by the second power supply operating at a high voltage VDDH, which voltage is higher than the voltage of the first power supply.  
         [0026]     The input unit  11 , as well as conventional level shifter circuits, also includes two inverters  102 ,  103  in series. The first inverter  102  receives the front-end input signal INP and then generates the complementary input signal XIN while the second inverter  103  receives the complementary input signal XIN and then generates the input signal IN. The output unit  13 , as well as conventional level shifter circuits, also includes two inverters  108 ,  109  in series. The inverters  102 ,  103 ,  108  and  109  can be implemented by using a complementary transistor pair made up of a N-channel transistor and a P-channel transistor.  
         [0027]     In this embodiment, the first and the second transistor devices are implemented by using N-channel transistor  105  and  107  respectively. Therefore, the level shifting unit  12  includes a first P-channel transistor  104 , a second P-channel transistor  106  and two N-channel transistors  105  and  107 . The latch unit  54  includes two N-channel transistors  605  and  607 . The N-channel transistor  605  has a drain connected to the complementary output node  111 , a gate connected to the output node  112  and a source connected to ground. The N-channel transistor  607  has a drain connected to the output node  112 , a gate connected to the complementary output node  111  and a source connected to ground.  
         [0028]     When the first voltage VDDL is turned on, if the electric potential of the input signal IN is at a logic high of voltage VDDL and the electric potential of the complementary input signal XIN is at a logic low of GND, the N-channel transistor  105  is turned on and the complementary output node  111  is pulled down to a logic low of GND. Next, the second P-channel transistor  106  is turned on, and then the output node  112  is pulled up to a logic high of voltage VDDH so as to switch the N-channel transistor  605  on and improve the pull-down speed of the complementary output node  111 . On the other hand, if the electric potential of the input signal IN is at a logic low of GND and the electric potential of the complementary input signal XIN is at a logic high of voltage VDDL, the N-channel transistor  107  is turned on and then the output node  112  is pulled down to a logic low of GND. Next, the first P-channel transistor  104  is turned on, and then the complementary output node  111  is pulled to a logic high of voltage VDDH so as to switch the N-channel transistor  607  on and improve the pull-down speed of the output node  112 . Thus, the level shifter circuit  600  including the latch unit  54  reduces the falling time of the complementary output node  111  and the output node  112 , thereby, effectively increasing the maximum operating frequency of the level shifter circuit  600 .  
         [0029]     Suppose that the complementary output node  111  and the output node  112  are at a logic low and a logic high respectively before the first power supply is turned off. After the first power supply is turned off for lowering the power consumption, the voltages of the gates of both N-channel transistors  105  and  107  are dropped below the threshold voltage V t  so that the N-channel transistors  105  and  107  are shut off. At this time, the N-channel transistor  605  is turned ON and serves as a leaking path from the complementary output node  111  to the ground. Meanwhile, the second P-channel transistor  106  is also turned on and serves as a charging path from the output node  112  to the second power supply. Therefore, in turn, the complementary output node  111  and the output node  112  stay at a logic low and a logic high respectively.  
         [0030]     On the other hand, suppose that the complementary output node  111  and the output node  112  are at a logic high and a logic low respectively before the first power supply is turned off. After the first power supply is turned off, the N-channel transistor  607  is turned on and serves as a leaking path from the output node  112  to the ground. Meanwhile, the first P-channel transistor  104  is also turned on and serves as a charging path from the complementary output node  111  to the second power supply. Therefore, the complementary output node  111  and the output node  112  stay at a logic high and a logic low respectively.  
         [0031]     Because the level shifter circuit  600  includes the latch unit  54 , the electric potentials of the complementary output node  111  and the output node  112  can be swiftly pulled to VDDH or GND, and be latched before the first power supply is off. Hence, the following stages can operate normally even after the first power supply is turned off. The level shifter circuit of the present invention solves the current drainage problem caused by the floating output nodes  111 ,  112  at the inputs of inverters  108  and  109  in conventional level shifter circuits. Further, the aim of saving the power consumption by turning off the first power supply is achieved with the minimum circuit size increment.  
         [0032]      FIG. 7  is a circuit diagram of a level shifter circuit according to a second embodiment of the invention. With reference now to  FIG. 7 , the level shifter circuit  700  includes an input unit  11 , a level shifting unit  22 , an output unit  13  and a latch unit  54  according to the second embodiment of the invention. The first transistor device of the level shifting unit  22  is implemented by using a complementary transistor pair made up of a N-channel transistor  105  and a P-channel transistor  204  while the second transistor device is implemented by using a complementary transistor pair made up of a N-channel transistor  107  and a P-channel transistor  206 . Thus, the level shifting unit  22  includes a first P-channel transistor  104 , a second P-channel transistor  106 , two N-channel transistors  105  and  107  and two P-channel transistors  204  and  206 . The output terminal of the complementary transistor pair  105  and  204  is electrically connected to both the gates of the second P-channel transistor  106  and N-channel transistor  607  via the complementary output node  111 . The output terminal of the complementary transistor pair  107  and  206  is electrically connected to both the gates of the first P-channel transistor  106  and N-channel transistor  605  via the output node  112 .  
         [0033]     Compared with the level shifter circuit  600  of the first embodiment, the level shifter circuit  700  is additionally inserted two P-channel transistors  204  and  206 , which achieves the effect of improving the pull-down speed of the output node  111  and  112 . Because the other circuits in the second embodiment are the same as the first embodiment, the description is omitted.  
         [0034]     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention should not be limited to the specific construction and arrangement shown and described, since various other modifications may occur to those ordinarily skilled in the art.