Abstract:
A high speed electrical level converting circuit which converts an input signal into an output signal having a different potential from that of the input signal. A falling edge detector detects the falling edge of the input signal when the input signal changes from a high potential to a low potential. A short circuiting device serves to drain charge accumulated at the output terminal for a predetermined duration in response to a signal from the falling edge detector so as to allow the level converting circuit to make a transition from a high output to a low output in a very fast manner.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a level converting circuit for use in semiconductor integrated circuits or the like, and more particularly to a level converting circuit for reducing the amplitude, such as converting the CMOS level used in CMOS devices to the level used in ECL devices. 
     According to the prior art, as will be stated in detail afterwards, the conventional level converting circuit, composed of an MOS transistor, converts the high level of input signals into the positive source voltage of the level converting circuit and the low level, into a voltage resulting from the division of the positive source voltage by the sourcedrain impedance of the transistor. Such a level converting circuit, however, operates at a relatively low speed. Though the problem of the low speed in the circuit can be resolved by the increase in the current of the circuit, there arises another problem of consuming more power. 
     SUMMARY OF THE INVENTION 
     An object of the present invention, therefore, is to provide a level converting circuit capable of high speed operation with relatively less power consumption. 
     A level converting circuit of the invention converts an input signal having a predetermined potential into an output signal having a different potential. The level converting circuit includes a first power supply circuit for supplying a predetermined first potential. A second power supply circuit supplies a predetermined second potential which is lower than the first potential. A first inverter receives the input signal and outputs it as an inverted input signal. A first P type transistor, whose source electrode is connected to first power supply circuit, receives the inverted input signal at its gate electrode. A second P type transistor, of which the drain electrode and the gate electrode are connected to each other, the source electrode is connected to the first power supply circuit and the drain electrode is connected to the drain electrode of the first P type transistor, outputs the voltage of its gate electrode as the output signal. A first N type transistor, whose drain electrode is connected to the drain electrode of the first P type transistor, receives the inverted input signal at its gate electrode. A second N type transistor is provided, of which the gate electrode receives a reference potential having a predetermined level, the drain electrode is connected to the source electrode of the first N type transistor, and the source electrode is connected to the second power supply circuit. A falling edge detector detects the falling edge of the inverted input signal and generates a detection signal. A short-circuit is responsive to the detection signal for short-circuiting the source electrode and the drain electrode of the second N type transistor for a prescribed duration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a circuit diagram illustrating a level converting circuit according to the prior art; 
     FIG. 2 is a circuit diagram illustrating a level converting circuit according to a first preferred embodiment of the invention; and 
     FIG. 3 is a circuit diagram according to a second preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To facilitate understanding of the present invention, the configuration and operation of a level converting circuit according to the prior art will be described first. FIG. 1 is a circuit diagram illustrating a level converting circuit according to the prior art. As shown in FIG. 1, a signal input terminal 1 is connected to the gate electrode (gate) of a first P channel type MOS transistor (P type transistor) 2 and that of a first N channel type MOS transistor (N type transistor) 3, and a reference voltage input terminal 5 is connected to the gate of a second N type transistor 6. A positive power source 7 is connected to the source electrode (source) of the first P type transistor 2 and that of a second P type transistor 8. Meanwhile, the drain electrode (drain) of the first P type transistor 2 is connected to the drain and gate of the second P type transistor 8, a signal output terminal 9 and the drain of the first N type transistor 3. Further, the source of the first N type transistor 3 is connected to the drain of the second N type transistor 6, whose source is connected to a negative power source 11. 
     In such a level converting circuit, the drain and gate of the second P type transistor 8 are connected to each other, and this transistor can be regarded as a resistor. The second N type transistor 6 can be deemed to be a constant current source because of a reference voltage fed to its gate. Now it is supposed that the input signal has a high level of 0 V and a low level of -5.2 V, the voltages of the positive power source 7 and the negative power source 11 are 0 V and -5.2 V, respectively, and the reference voltage V REF  is -3 V. The input signal supplied to the input terminal 1, after being inverted by an inverter circuit 12, is fed to the gates of the first P type and N type transistors 1 and 3. Since the output of the inverter 12 is at its low level when the input signal is at its high level, the first P type transistor 2 is turned ON and the first N type transistor 3, OFF when the input signal is at a high level. As a result, 0 V which is the voltage of the positive power source 7 is supplied from the output terminal 9. On the other hand, when the input signal is at its low level, the first P type transistor 1 is turned OFF and the first N type transistor 3, ON. As a result, there is obtained at the output terminal 9 a value resulting from the division of the potential difference between the positive voltage source 7 and the negative voltage source 11 by the source-drain impedance (channel impedance) of the second P type transistor 8 and that of the series circuit of the first and second N type transistors 3 and 6. As the channel impedance is variable with the channel width, the level converting circuit is so designed as to give any desired output voltage. 
     If it is attempted to increase the operating speed of the level converting circuit illustrated in FIG. 1, when the output voltage changes from the low level to the high level, the length of time required for the change (rising time) can be shortened by increasing the driving capacity of the first P type transistor 2. When the output voltage changes from the high level to the low level, the falling time can be shortened by increasing the current flowing into the constant current source consisting of the reference voltage and the second N type transistor 6. However, the reduction of the falling time requires a greater constant current and accordingly involves the consumption of more power. 
     FIG. 2 is a circuit diagram illustrating a level converting circuit according to the present invention. In FIG. 2, the same constituent elements as in FIG. 1 are assigned respectively the same reference numerals. In this preferred embodiment of the invention, the configuration of FIG. 1 is augmented with a third N type transistor 10 connected in parallel with the second N type transistor 6, and a second inverter 4 for inverting the output of the first inverter 12 and supplying the inverted output to the gate of the third N type transistor 10. 
     Next, the operation of the first embodiment of the invention will be described. The operation which takes place when the input signal rises from the low level to the high level is the same as in the prior art circuit of FIG. 1. Thus, as the input signal is at its high level, the first inverter 12 provides a low level output, and the first P type transistor 2 is turned ON. The third N type transistor 10, whose gate is supplied with the high level by the second inverter 4, is also turned ON, but both the first N type and the second P type transistors 3 and 8 are OFF, so that the voltage of the positive voltage source 7 is supplied from the output terminal 9. Meanwhile, when the input signal falls from the high level to the low level, the output of the first inverter 12 rises from the low level to the high level, the first P type transistor 2 is turned OFF and the first N type transistor 3, ON. At this time, the output of the second inverter 4 changes from the low level to the high level with a delay by its gate delay time, and turns the third N type transistor 10 from ON to OFF. Thus, the third N type transistor 10 is kept ON during the gate delay time of the second inverter 4 after the fall of the input signal. When the input signal falls, the charge accumulated at the output terminal 9 flows at high speed into the negative power source 11 via the first N type transistor 3 and the third N type transistor 10, which have been turned ON. As in the prior art circuit shown in FIG. 1, a voltage resulting from the division of the negative voltage of the power source 11 by the source-drain impedance of the first and second N type transistor 3 and 6 is supplied from the output terminal 9. Thus, in the level converting circuit according to the present invention, the third N type transistor 10 is provided for bypassing the second N type transistor 6 as the constant source and, by giving it continuity for a certain length of time when the input signal falls, the fall of the output voltage can be accelerated without consuming extra power. 
     FIG. 3 is a circuit diagram illustrating another preferred embodiment of the invention. This level converting circuit has a configuration in which the second inverter 4 and the third N type transistor 10 in FIG. 2 are replaced with a transfer gate 13 and a third P type transistor 14, respectively. Thus in this alternative embodiment, the same benefit as that of the embodiment shown in FIG. 2 is achieved by keeping the third P type transistor 14 ON by the delay time of the transfer gate 13 after the rising of the output of the first inverter 12.