Abstract:
An output buffer circuit is provided, which is capable of obtaining a large drive power when the level of an input signal changes, while allowing a through current to flow in suppressed amounts. A first P-channel MOS transistor and a first N-channel MOS transistor are connected in series with a power supply. The pair of transistors are exclusively switched on and off by an input signal such that the first and second switching elements are not simultaneously on or off, to deliver an output signal corresponding to the input signal, from a common junction between the first and second switching elements. A second P-channel MOS transistor is connected in parallel with the first P-channel MOS transistor as an auxiliary transistor. A second N-channel MOS transistor is connected in parallel with the first N-channel MOS transistor as an auxiliary transistor. When the level of the input signal changes to switch one of the first P-channel MOS transistor and N-channel MOS transistor from an OFF state to an ON state, a drive switching control block delivers a signal to one of the auxiliary transistors connected in parallel with the switched one of the first P-channel MOS transistor and N-channel MOS transistor, for holding the one of the auxiliary transistors in an ON state over a predetermined time period.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an output buffer circuit for semiconductor integrated circuits. 
     2. Prior Art 
     In general, an LSI (Large Scale Integrated Circuit) has a large load connected to an output terminal thereof. Therefore, in order to drive such a large load, an output buffer circuit having a large drive power is usually provided in the output block of the LSI. 
     FIG. 1 shows the construction of a conventional output buffer circuit. 
     As shown in the figure, the output buffer circuit is comprised of a two-input NAND gate NAN 1 , an inverter IN 1 , a two-input NOR gate NOR 1 , a P-channel MOS transistor MP 1 , and an N-channel MOS transistor MN 1 . 
     The P-channel MOS transistor MP 1  and the N-channel MOS transistor MN 1  provide a sufficient drive power for driving an external load. 
     In the output buffer circuit constructed as above, if an input signal i changes from a low level to a high level when an enable signal en is at a low level, an output signal na 1  from the two-input NAND gate NAN 1  changes from a high level to a low level, and an output signal nr 1  from the two-input NOR gate NOR 1  also changes from a high level to a low level. Accordingly, the P-channel MOS transistor MP 1  is switched from an OFF-state to an ON-state, while the N-channel MOS transistor MN 1  is switched from the ON-state to the OFF-state. As a result, an output signal x changes from a low level to a high level. 
     During this transition of the output signal from the low level to the high level, there occurs a time period when the P-channel MOS transistor MP 1  and the N-channel MOS transistor MN 1  are both held in the ON-state simultaneously. During this time period, a large current (through current) flows between a power supply VDD and ground GND, which generates noise on the power line and the ground line, and can lead to erroneous operation of the LSI. 
     The above problem will be described more in detail with reference to FIG.  2 . 
     FIG. 2 shows the P-channel MOS transistor MP 1  and the N-channel MOS transistor MN 1  each having an inductance L connected thereto. 
     These inductances L are parasitic inductances interposed, respectively, between the power supply VDD outside the LSI and the source of the MOS transistor MP 1  inside the LSI and between the ground GDN outside the LSI and the source of the MOS transistor MN 1  inside the LSI. When the through current i flows between the P-channel MOS transistor MP 1  and the N-channel MOS transistor MN 1  each connected to the corresponding inductance L, a spike noise is generated across each of the inductances L. 
     The noise level of the spike noise can be expressed in terms of a spike voltage (ΔV) by the following equation (1): 
     
       
           ΔV=−L·di/dt   (1) 
       
     
     In the output buffer circuit, since the P-channel MOS transistor MP 1  and the N-channel MOS transistor MN 1  each have a large load-driving power, a large through current i flows between them, which generates a large spike noise. 
     The generation of the big spike noise leads to erroneous operations of other circuits within the LSI. 
     Further, the FIG. 1 output buffer circuit suffers from a problem that the through current causes an increased current consumed by the buffer circuit. 
     An output buffer circuit intended for preventing generation of the through current and noise described above has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 05-327444. 
     FIG. 3 shows the construction of the proposed output buffer circuit. 
     The output buffer circuit is comprised of an input terminal  1 , an output terminal  2 , a pre-driver  3 , a delay circuit block  4 , and a final driver  5 . 
     The final driver  5  is comprised of P-channel MOS transistors P 1 , P 2 , and N-channel MOS transistors N 1 , N 2 . The P-channel MOS transistors P 1 , P 2  each have a source thereof connected to a positive power supply VDD, while the N-channel MOS transistors N 1 , N 2  each have a source thereof grounded. The P-channel MOS transistors P 1 , P 2  and the N-channel MOS transistors N 1 , N 2  each have a drain thereof connected to the output terminal  2 . 
     The delay circuit block  4  is interposed between the input terminal  1  and the final driver  5  and composed of a delay block  6 , a two-input NAND gate  11 , and a two-input NOR gate  12 . The delay block  6  delays an input signal thereto by a predetermined delay amount td. An output signal from the delay block  6  is delivered to one of the input terminals of the two-input NAND gate  11  and one of the input terminals of the two-input NOR gate  12 . The other input terminal of the two-input NAND gate  11  and that of the two-input NOR gate  12  are each supplied with an input signal i. The two-input NAND gate  11  supplies an output signal to the gate of the P-channel MOS transistor P 2  of the final driver  5 , while the two-input NOR gate  12  supplies an output signal to the gate of the N-channel MOS transistor N 2  of the same. 
     The pre-driver  3  is interposed between the input terminal  1  and the final driver  5 . The pre-driver  3  delivers a signal formed by inverting the polarity of the input signal i to the respective gates of the P-channel and N-channel MOS transistors P 1  and N 1  of the final driver  5 . 
     Next, the operation of the above output buffer circuit will be described with reference to FIGS. 4A to  4 J. 
     FIGS. 4A to  4 J collectively form a timing chart which is useful in explaining the operation of the FIG. 3 output buffer circuit. 
     First, assuming that the input signal i changes from a low level to a high level at a time t 01 , the output signal from the pre-driver  3  changes from a high level to a low level (see FIGS. 4A,  4 B). As a result, the P-channel MOS transistor P 1  is switched from the OFF-state to the ON-state, while the N-channel MOS transistors N 1  is switched from the ON-state to the OFF-state (see FIGS. 4F,  4 G). Accordingly, an output signal from the final driver starts changing from a low level to a high level (FIG.  4 J). 
     After the lapse of a delay time td from the time t 01  (i.e. at a time t 02 ), the output signal from the delay block  6  changes from a low level to a high level (see FIG.  4 C). At the same time, the output signal from the two-input NAND gate  11  changes from a high level to a low level (see FIG.  4 E), whereby the P-channel MOS transistor P 2  is switched from the OFF-state to the ON-state (see FIG.  4 H). 
     Consequently, the P-channel MOS transistors P 1  and P 2  perform additional driving operation to cause an output signal level to rise sharply. 
     In the output buffer circuit described above, between the time t 01  at which the input signal i changes and the time t 02  after the lapse of the delay time td from the time t 01 , there exists no time period over which the P-channel MOS transistor P 2  and the N-channel MOS transistor N 2  are both held in the ON-state simultaneously (see FIGS. 4H,  4 I), which prevents a through current from flowing between the two transistors P 2  and N 2 . 
     However, when a large capacity load connected to the output buffer circuit is to be charged or discharged, a large drive power is required upon switching between the charge and the discharge. In the above output buffer circuit, when the input signal i changes from the low level to the high level, one of the P-channel MOS transistors parallel-connected to the load, i.e. the P-channel MOS transistor P 1 , is switched from the OFF-state to the ON-state (see FIG.  4 F), while the P-channel MOS transistor P 2  is switched from the OFF-state to the ON-state after the lapse of the delay time td (see FIG.  4 H). 
     That is, immediately after the input signal i changes as described above, the P-channel MOS transistor P 1  alone can provide a drive power. Therefore, it is impossible to obtain a sufficient drive power for charging or discharging the large capacity load, which results in delayed response. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an output buffer circuit which is capable of obtaining a large drive power when the level of an input signal changes, while allowing a through current to flow in suppressed amounts. 
     Further, since the output signal x is at the low level, the output signal x 2  from the delay block  210  is held at a high level, and the output signal a 2  from the AND gate AND 1  and the output signal r 2  from the OR gate OR 1  are also each held at a high level. 
     To attain the above object, there is provided an output buffer circuit comprising a pair of first and second switching elements that are connected with each other at a common junction and connected in series with a power supply, for being exclusively switched on and off in response to an input signal such that the first and second switching elements are not simultaneously on or off, to deliver an output signal corresponding to the input signal, from the common junction, a first auxiliary switching device that is connected in parallel with the first switching element, a second auxiliary switching device that is connected in parallel with the second switching element and a drive switching control block that operates when a change occurs in level of the input signal and one of the first and second switching elements is switched from an OFF state to an ON state in response to the change in level of the input signal, to deliver auxiliary drive control signals to the of the first and second auxiliary switching devices which is connected in parallel with the switched one of the first and second switching elements, for holding the one of the first and second auxiliary switching devices in an ON state over a predetermined time period. 
     More preferably, the first and second auxiliary switching devices are each formed by a plurality of switching elements, the output buffer circuit including a selection control signal supply device that supplies the drive switching control block with selection control signals for selecting or not selecting at least one of the plurality of switching elements to be supplied with auxiliary drive control signals, and when the level of the input signal changes to switch one of the first and second switching elements from an OFF state to an ON state, the drive switching control block delivers auxiliary drive control signals to the selected at least one of the plurality of switching elements of one of the first and second auxiliary switching devices which is connected in parallel with the switched one of the first and second switching elements, for holding the selected at least one off the plurality of switching elements in the ON state over the predetermined time period. 
     More preferably, the selection control signal supply device supplies selection control signal such that more of the plurality of switching elements are selected to be supplied with auxiliary drive control signal to thereby increase a speed of rise of the output signal. 
     Preferably, the drive switching control block includes a delay circuit that delays the output signal delivered from the common junction between the first and second switching elements, and wherein the drive switching control block delivers auxiliary drive control signal over a time period during which a level of an output signal from the delay circuit and the level of the input signal are the same as each other. 
     Alternatively, the drive switching control block includes a delay circuit that delays the input signal, and the drive switching control block delivers auxiliary drive control signals over a time period during which a level of an output signal from the delay circuit and the level of the input signal are the same as each other. 
     More preferably, the delay circuit delays the output signal delivered from the common junction between the first and second switching elements by a delay time dependent on a speed of switching operation demanded of the output buffer circuit. 
     More preferably, the delay circuit delays the input signal by a delay time dependent on a speed of switching operation demanded of the output buffer circuit. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing the construction of a conventional output buffer circuit; 
     FIG. 2 is a diagram showing part of the conventional output buffer circuit; 
     FIG. 3 is a circuit diagram showing the construction of another conventional output buffer circuit; 
     FIGS. 4A to  4 J collectively form a timing chart useful in explaining the operation of the FIG. 3 output buffer circuit; 
     FIG. 5 is a circuit diagram showing the construction of an output buffer circuit according to a first embodiment of the present invention; 
     FIGS. 6A to  6 L collectively form a timing chart useful in explaining the operation of the FIG. 5 output buffer circuit; 
     FIG. 7 is a circuit diagram showing the construction of an output buffer circuit according to a second embodiment of the present invention; 
     FIGS. 8A to  8 L collectively form a timing chart useful in explaining the operation of the FIG. 7 output buffer circuit; 
     FIG. 9 is a circuit diagram showing the construction of an output buffer circuit according to a third embodiment of the present invention; 
     FIG. 10 is a diagram showing the construction of a control signal-generating block  600 ; 
     FIG. 11 shows a table which is useful in explaining the functions of the control signal-generating block  600 ; 
     FIG. 12 is a diagram showing changes in rise of an output signal x in each speed mode; and 
     FIG. 13 is a diagram showing the construction of a delay block  210 ′. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described in detail with reference to the drawings showing embodiments thereof. The following description is provided for the purposes of further ease of understanding of the present invention, and the embodiments described therein are by no means intended to limit the present invention, but given only by way of example. Therefore, further modifications and variations may be made without departing from the spirit and scope of the present invention. 
     FIG. 5 shows the construction of an output buffer circuit according to a first embodiment of the present invention. 
     The output buffer circuit is comprised of a combination of a first output block  100  which is identical with the FIG. 1 output buffer circuit, and a drive switching control block  200  and a second output block  300  which are newly added in the present embodiment. 
     The first output block  100  delivers a signal having a level corresponding to that of an input signal i to an output terminal  400  when an enable signal en is at a low level. The first output block  100  is constructed similarly to the FIG. 1 output buffer circuit, and therefore, component parts and elements corresponding to those of the FIG. 1 output buffer circuit described hereinbefore are designated by identical reference numerals, and detailed description thereof is omitted. 
     The drive switching control block  200  is comprised of a delay block  210 , a two-input AND gate AND 1 , a two-input OR gate OR 1 , an inverter IN 2 , a two-input NAND gate NAN 2 , and a two-input NOR gate NOR 2 . 
     The delay block  210  delays an output signal x from the output buffer circuit by a delay time td, and delivers a signal x 2  formed by inverting the polarity of the output signal x to one of the input terminals of the two-input AND gate AND 1  and one of the input terminals of a two-input OR gate OR 1 . 
     The other input terminal of the two-input AND gate AND 1  is supplied with a signal from the inverter IN 2  which inverts the polarity of an enable signal en, while the other input terminal of the two-input OR gate OR 1  is supplied with the enable signal en. 
     An output signal a 2  from the two-input AND gate AND 1  is supplied to one of the input terminals of the two-input NAND gate NAN 2 , while an output signal r 2  from the two-input OR gate OR 1  is supplied to one of the input terminals of the two-input NOR gate NOR 2 . 
     The other input terminal of each of the two-input NAND gate NAN 2  and the two-input NOR gate NOR 2  is supplied with an input signal i. The two-input NAND gate NAN 2  and the two-input NOR gate NOR 2  deliver respective output signals na 2  and nr 2  to the second output block  300 . 
     The second output block  300  is comprised of a P-channel MOS transistor MP 2 , and a N-channel MOS transistor MN 2 . 
     The P-channel MOS transistor MP 2  has a source thereof connected to a power supply VDD, while the N-channel MOS transistor MN 2  has a source thereof grounded. Further, the P-channel MOS transistor MP 2  and the N-channel MOS transistor MN 2  have respective drains thereof commonly connected to the output terminal  400  of the LSI. The P-channel MOS transistor MP 2  and the N-channel MOS transistor MN 2  cooperatively provide an augmenting drive power to make up for the drive power obtained from the P-channel MOS transistor MP 1  and the N-channel MOS transistor MN 1 . 
     FIGS. 6A to  6 L collectively form a timing chart which is useful in explaining the operation of the FIG. 5 output buffer circuit. In the following, the operation of the output buffer circuit according to the present embodiment will be described with reference to the timing chart. 
     First, let it be assumed that the enable signal en is at a low level, and the input signal i is also at a low level. In this state, the output signal na 1  from the NAND gate NAN 1  is held at a high level, and the output signal nr 1  from the NOR gate NOR 1  is also held at a high level, so that the N-channel MOS transistor MN 1  is held in an ON-state. Therefore, the output signal x from the output terminal  400  of the LSI is held at a low level (see FIGS. 6A to  6 E). 
     Further, since the output signal x is at the low level, the output signal x 2  from the delay block  210  is held at a high level, and the output signal a 2  from the AND gate AND 1  and the output signal r 2  from the OR gate OR 1  are also each held at a high level. 
     As a result, since the NAND gate NAN 2  receives the high-level signal a 2  and the low-level input signal i, an output signal na 2  from the NAND gate NAN 2  is held at a high level. 
     On the other hand, since the NOR gate NOR 2  receives the high-level signal r 2  and the low-level input signal i, an output signal nr 2  from the NOR gate NOR 2  is held at a low level. 
     Accordingly, the P-channel MOS transistor MP 2  and the N-channel MOS transistor MN 2  in the second output block  300  are each held in an OFF-state (see FIGS. 6F,  6 G,  6 H,  6 K and  6 L). 
     Next, let it be assumed that the input signal i changes at a time t 11  from the low level to a high level (see FIG.  6 A). 
     As a result, in the first output block  100 , the P-channel MOS transistor MP 1  is switched from the OFF-state to the ON-state, while the N-channel MOS transistor MN 1  is switched from the ON-state to the OFF-state (see FIGS.  6 I and  6 J). Accordingly, the output signal x from the output terminal  400  starts changing from the low level to a high level (FIG.  6 E). 
     During the transition of the output signal x from the low level to the high level, as long as the level of the output signal x is lower than a threshold value at the input of the delay block  210 , the output signal x 2  from the delay block  210  is held at the high level, so that the high-level signal a 2  is supplied to the NAND gate NAN 2 , and the high-level signal r 2  to the NOR gate NOR 2 . 
     Therefore, when the input signal i changes from the low level to the high level at the time t 11 , the output signal na 2  from the NAND gate NAN 2  changes from the high level to the low level in response to the change in the level of the input signal i. As a result, in the second output block  300 , the P-channel MOS transistor MP 2  is switched from the OFF-state to the ON-state. 
     Thus, at the time point the input signal i changes from the low level to the high level, charge of a load, not shown, connected to the output terminal  400  via the P-channel MOS transistor MP 1  in the first output block  100  and the P-channel MOS transistor MP 2  in the second output block  300  is started, and the level of the output signal x is progressively raised. 
     Then, after the level of the output signal x exceeds the threshold value at the input of the delay block  210  at a time t 12 , the output signal x 2  from the delay block  210  changes from the high level to a low level at a time t 13  when the delay time td has elapsed after the time point t 12  (see FIG.  6 F), whereby the output signal a 2  from the AND gate AND 1  and the output signal r 2  from the OR gate OR 1  are both changed to low levels. 
     As a result, the output signal na 2  from the NAND gate NAN 2  changes to a high level (see FIG.  6 G), whereby the P-channel MOS transistor MP 2  returns to the OFF-state (see FIG.  6 K). After the P-channel MOS transistor MP 2  having been thus turned off, only the P-channel MOS transistor MP 1  in the first output block  100  remains in the ON-state. 
     As described above, the P-channel MOS transistor MP 2  is not turned off until the delay time td has elapsed after the level of the output signal x exceeded the threshold value at the input of the delay block  210 . In this case, the delay time td is set to a time period over which the output signal x completes a rise in level after its level has exceeded the threshold value at the input of the delay block  210 . Therefore, at the time point the P-channel MOS transistor MP 2  is turned off, the output signal x has already changed to the high level. 
     Next, let it be assumed that the input signal i changes from the high level to the low level at a time t 14  (see FIG.  6 A). 
     As a result, in the first output block  100 , the P-channel MOS transistor MP 1  is switched from the ON-state to the OFF-state, while the N-channel MOS transistor MN 1  is switched from the OFF-state to the ON-state (see FIGS.  6 I and  6 J). 
     Accordingly, the output signal x from the output terminal  400  starts changing from the high level to the low level (FIG.  6 E). 
     During the transition of the output signal x from the high level to the low level, as long as the level of the output signal x is higher than the threshold value at the input of the delay block  210 , the output signal x 2  from the delay block  210  is held at the low level, so that the low-level signal a 2  is supplied to the NAND gate NAN 2 , and the low-level signal r 2  to the NOR gate NOR 2 . 
     Therefore, when the input signal i changes from the high level to the low level at the time t 14 , the output signal nr 2  from the NOR gate NOR 2  changes from the low level to the high level in response to the change in the level of the input signal i. As a result, in the second output block  300 , the N-channel MOS transistor MN 2  is switched from the OFF-state to the ON-state. 
     Thus, at the time point the input signal i changes from the high level to the low level, discharge of the load, not shown, connected to the output terminal  400  via the N-channel MOS transistor MN 1  in the first output block  100  and the N-channel MOS transistor MN 2  in the second output block  300  is started, and the level of the output signal x is progressively lowered. 
     Then, after the level of the output signal x becomes lower than the threshold value at the input of the delay block  210  at a time t 15 , the output signal x 2  from the delay block  210  changes from the low level to the high level at a time t 16  the delay time td has elapsed after the time point t 15  (see FIG.  6 F), whereby the output signal a 2  from the AND gate AND 1  and the output signal r 2  from the OR gate OR 1  are changed to the high levels. 
     As a result, the output signal nr 2  from the NOR gate NOR 2  changes to the low level (see FIG.  6 H), whereby the N-channel MOS transistor MN 2  returns to the OFF-state (see FIG.  6 L). After the N-channel MOS transistor MN 2  having been thus turned off, only the N-channel MOS transistor MN 1  in the first output block  100  is held in the ON-state. 
     As described above, the N-channel MOS transistor MN 2  is not turned off until the delay time td has elapsed after the level of the output signal x became lower than the threshold value at the input of the delay block  210 . In this case, the delay time td is set to a time period over which the output signal x completes a fall in level after its level has become lower than the threshold value at the input of the delay block  210 . Therefore, at the time point the N-channel MOS transistor MN 2  is turned off, the output signal x has already changed to the low level. 
     As described above, according to the present embodiment, even when the signal level of the input signal i changes, the P-channel MOS transistor MP 2  and the N-channel MOS transistor MN 2  in the second output block  300  are never simultaneously brought into the ON-state, which prevents a through current from flowing in the second output block  300 . 
     Further, after the input signal i has changed from the low level to the high level, the N-channel MOS transistor MN 2  in the second output block  300  is held in the OFF-state (see FIG.  6 L). 
     On the other hand, after the input signal i has changed from the high level to the low level, the P-channel MOS transistor MP 2  in the second output block  300  is held in the OFF-state (see FIG.  6 K). 
     Thus, after the level of the input signal i has changed, in the second output block  300 , either the P-channel MOS transistor MP 2  or the N-channel MOS transistor MN 2  is held in the OFF-state. This makes it possible to prevent an increase in electric power required for control of the output buffer circuit. 
     Still further, when the input signal i changes from the low level to the high level, the two P-channel MOS transistors in the first and second output blocks  100  and  300  start charging the load, while when the input signal i changes from the high level to the low level, the two N-channel MOS transistors in the first and second output blocks  100  and  300  start discharging the load. 
     Therefore, the output buffer circuit of the above embodiment makes it possible to charge or discharge a large-capacity load promptly in response to a change in the level of the input signal i. 
     Moreover, the output buffer circuit of the present embodiment makes it possible to reduce the sizes of transistors forming the NOR gate NOR 2  and the NAND gate NAN 2  in the drive switching control block  200 . The reasons for this are as follows: 
     (1) Transistors Forming the NOR Gate NOR 2   
     When the input signal i changes from the low level to the high level, the output signal nr 2  from the NOR gate NOR 2  is not changed but held at the low level (see FIG.  6 H). Therefore, it is possible to reduce the sizes of N-channel MOS transistors forming the NOR gate NOR 2  for switching the N-channel MOS transistor MN 2  in the second output block  300  from the ON-state to the OFF-state, thereby reducing power consumption. 
     (2) Transistors Forming the NAND Gate NAN 2   
     When the input signal i changes from the high level to the low level, the output signal na 2  from the NAND gate NAN 2  is not changed but held at the high level (see FIG.  6 G). Therefore, it is possible to reduce the sizes of N-channel MOS transistors forming the NAND gate NAN 2  for switching the P-channel MOS transistor MP 2  in the second output block  300  from the ON-state to the OFF-state, thereby reducing power consumption. 
     Next, an output buffer circuit according to a second embodiment will be described with reference to FIGS. 7 and 8. 
     As shown in FIG. 7, the output buffer circuit according to the present embodiment is distinguished from the output buffer circuit according to the first embodiment in which the output signal x is supplied to the delay block  210 , in that an input signal i is supplied to a delay block  210 . It should be noted that component parts and elements corresponding to those of the output buffer circuit according to the first embodiment are designated by identical reference numerals, and detailed description thereof is omitted. 
     FIGS. 8A to  8 L collectively form a timing chart which is useful in explaining the operation of the FIG. 7 output buffer circuit. In the following, the operation of the output buffer circuit according to the present embodiment will be described with reference to the timing chart. 
     First, let it be assumed that an enable signal en is at a low level, and the input signal i is also at a low level. The operation of the output buffer circuit in this state is similar to that of the output buffer circuit of the first embodiment, and therefore, description thereof is omitted. 
     Next, let it be assumed that the input signal i changes from the low level to a high level at a time t 31  (see FIG.  8 A). 
     As a result, in a first output block  100 , a P-channel MOS transistor MP 1  is switched from the OFF-state to the ON-state, while an N-channel MOS transistor MN 1  is switched from the ON-state to the OFF-state. Consequently, an output signal from the first output block  100  starts changing from a low level to a high level (FIG.  8 E). 
     In this embodiment, the input signal i is supplied to an input of the delay block  210 , which means that at the time t 31 , the input of the delay block  210  receives the high-level input signal. 
     However, the delay block  210  delivers an output signal x 2  formed by delaying the input signal i by a predetermined delay amount td′, so that at the time t 31  when the input signal i changes from the low level to the high level, the output signal x 2  from the delay block  210  remains at a high level. Therefore, a high-level signal a 2  from an AND gate AND 1  is supplied to a NAND gate NAN 2 , while a high-level signal r 2  from an OR gate OR 1  is supplied to a NOR gate NOR 2 . 
     On the other hand, an output signal na 2  from a NAND gate NAN 2  changes from a high level to a low level in response to the change in the level of the input signal i at the time t 31 . As a result, in a second output block  300 , a P-channel MOS transistor MP 2  is switched from the OFF-state to the ON-state. 
     Accordingly, at the time point the input signal i changes from the low level to the high level, charge of a load, not shown, connected to an output terminal  400  via the P-channel MOS transistor MP 1  in the first output block  100  and the P-channel MOS transistor MP 2  in the second output block  300  is started, and the level of the output signal x is progressively raised. 
     Then, at a time t 33  the delay time td′ has elapsed after the transition of the input signal i from the low level to the high level, the output signal x 2  from the delay block  210  changes from the high level to a low level (see FIG.  8 F), whereby the output signal a 2  from the AND gate AND 1  and the output signal r 2  from the OR gate OR 1  are both changed to low levels. 
     As a result, the output signal na 2  from the NAND gate NAN 2  changes to the high level (see FIG.  8 G), whereby the P-channel MOS transistor MP 2  returns to the OFF state (see FIG.  8 K). After the P-channel MOS transistor MP 2  having been thus turned off, only the P-channel MOS transistor MP 1  in the first output block  100  remains in the ON-state. 
     As described above, the output signal x 2  does not change from the low level to the high level until the delay time td′ has elapsed after the transition of the input signal i from the low level to the high level (FIG.  8 F). 
     In the present embodiment, the delay time td′ is set to a time period over which the output signal x completes a rise from the low level to the high level after the transition of the input signal i from the low level to the high level. 
     Therefore, at the time point the output signal x 2  from the delay block  210  changes from the high level to the low level (see FIG.  8 F), the output signal x has already changed to the high level. 
     When the input signal i changes from the high level to the low level, the output buffer circuit operates similarly to the output buffer circuit of the first embodiment, and therefore, description of the operation is omitted. 
     As described above, according to the present embodiment, even when the signal level of the input signal i changes from the low level to the high level, the P-channel MOS transistor MP 2  and the N-channel MOS transistor MN 2  in the second output block  300  are not both turned on. 
     Therefore, similarly to the first embodiment, the second embodiment makes it possible to prevent a through current from flowing in the second output block  300 , thereby reducing power consumption by the output buffer circuit. 
     Next, an output buffer circuit according to a third embodiment will be described with reference to FIGS. 9 to  13 . 
     FIG. 9 shows the construction of the output buffer circuit according to the present embodiment. 
     The output buffer circuit is comprised of a first output block  100 ′, a second output block  300 , a third output block  500 , and a drive switching control block  200 ′. 
     The first output block  100 ′ is distinguished from the first output block  100  of the output buffer circuit according to the first embodiment (shown in FIG. 5) in that a two-input OR gate OR 0  is provided at a location upstream of a two-input NOR gate NOR 1  and an inverter IN 1 . One of the input terminals of the two-input OR gate OR 0  is supplied with an enable signal en, while the other is supplied with a control signal OUT 2 , described in detail hereinafter. 
     Similarly to the output buffer circuit according to the first embodiment, the output buffer circuit of the second embodiment has the second output block  300  for providing an augmenting drive power to make up for the drive power obtained from the first output block  100 ′. 
     In the present embodiment, in addition to the second output block  300 , the third output block  500  also provides an augmenting drive power to make up for the drive power obtained from the first output block  100 ′. As shown in FIG. 9, the third output block  500  is comprised of a P-channel MOS transistor MP 3 , and an N-channel MOS transistor MN 3 . 
     According to the present embodiment, it is possible to drive a load connected to an output terminal  400  in the following three modes: 
     a. The load is driven only by the first output block  100 ′ (low-speed mode). 
     b. The load is driven by the first output block  100 ′ and the second output block  300  (middle-speed mode). 
     c. The load is driven by the first output block  100 ′, the second output block  300  and the third output block  500  (high-speed mode). 
     Which mode should be selected for driving the load is determined by the control signal OUT 2  and/or other control signals OUT 4 , OUT 8  from a control signal-generating block  600  shown in FIG.  10 . The control signal-generating block  600  functions as shown in a FIG. 11 truth value table. 
     The drive switching control block  200 ′ is comprised of a delay circuit  210 , a control circuit  220 , a second output block control circuit  230 , and a third output block control circuit  240 . 
     The control circuit  220  is comprised of an AND gate AND 1 , an inverter IN 2 , and an OR gate OR 1 . This circuit is similar in construction to corresponding circuits in the respective drive switching control blocks described in the first and second embodiments. 
     The delay circuit  210  is similar in construction to the delay block  210  in the first embodiment. 
     The second output block control circuit  230  is comprised of a NAND gate NAN 2 , a NOR gate NOR 2 , an AND gate AND 2 , an OR gate OR 2 , and an inverter IN 3 . 
     The control signal OUT 4  is supplied to one of the input terminals of the OR gate OR 2 , whereas the inverter IN 3  inverts the level of the control signal OUT 4  supplied thereto and delivers the resulting inverted signal to one of the input terminals of the AND gate AND 2 . Further, an output signal r 2  from the OR gate OR 1  in the control circuit  220  is delivered to the other input terminal of the OR gate OR 2 , and an output signal a 2  from the AND gate AND 1  in the control circuit  220  is delivered to the other input terminal of the AND gate AND 2 . 
     Further, an output signal r 3  from the OR gate OR 2  is delivered to one of the input terminals of the NOR gate NOR 2 , while an output signal a 3  from the AND gate AND 2  is delivered to one of the input terminals of the NAND gate NAN 2 . The other terminal of the NOR gate NOR 2  and that of the NAND gate NAN 2  are each supplied with an input signal i. 
     Output signals from the NAND gate NAN 2  and the NOR gate NOR 2  are supplied to the respective gates of the P-channel and N-channel MOS transistors MP 2 , MN 2  in the second output block  300 , respectively. 
     The second output block control circuit  230  is constructed as above. 
     The third output block control circuit  240  is arranged at a location upstream of the third control block  500 . The circuit  240  is constructed similarly to the second output block control circuit  230 . A NAND gate NAN 3 , a NOR gate NOR 3 , an AND gate AND 3 , an OR gate OR 3 , and an inverter IN 4  in the third output block control circuit  240  correspond to the NAND gate NAN 2 , the NOR gate NOR 2 , the AND gate AND 2 , the OR gate OR 2 , and the inverter IN 3  in the second output block control circuit  230 , respectively. However, the third output block control circuit  240  is distinguished from the second output block control circuit  230  in that the control signal OUT 8  is supplied to the OR gate OR 3  and the inverter IN 4 . 
     Next, the operation of the output buffer circuit of the present embodiment will be described. 
     a. Low-speed Mode 
     When input signals PCTL 0  and PCTL 1  delivered to the control signal-generating block  600  are both set to L levels, the control signal OUT 2  alone is set to an L level (low level), whereas the other control signals OUT 4  and OUT 8  are both set to an H level (high level) (see FIG.  11 ). In this state, when the enable signal en is set to an L level, the operational mode of the output buffer circuit is switched to the low-speed mode. 
     In the low-speed mode, the output signal r 3  and an output signal r 4  from the respective OR gates OR 2  and OR 3  are each forcibly held at an H level, and the output signal a 3  and an output signal a 4  from the respective AND gates AND 2  and AND 3  are each forcibly held at an L level. As a result, all the transistors forming the second output block  230  and the third output block  500  are brought into the OFF-state. On the other hand, an output signal from the OR gate OR 0  is held at an L level, and an output signal from the inverter IN 1  at an L level. 
     Thus, in the low-speed mode, the transistors MP 1  and MN 1  in the first output block  100  are turned on and off in response to the input signal i, and the load connected to the output terminal  400  is driven only by these transistors. 
     b. Middle-Speed Mode 
     When the input signal PCTL 0  delivered to the control signal-generating block  600  is set to an H level and the input signal PCTL 1  to the L level, the control signals OUT 2  and OUT 4  are both set to the L level, and the control signal OUT 8  alone is set to the H level (see FIG.  11 ). In this state, when the enable signal en is set to the L level, the operational mode of the output buffer circuit is switched to the middle-speed mode. 
     In the middle-speed mode, the output signal from the OR gate OR 3  is forcibly held at the H level, and the output signal from the AND gate AND 3  is forcibly held at the L level, so that all the transistors forming the third output block  500  are brought into the OFF-state. 
     On the other hand, the output signal from the OR gate OR 0  is held at the L level, and the output signal from the inverter IN 1  at an H level, and hence the transistors MP 1  and MN 1  in the first output block  100  are turned on and off in response to the input signal i. 
     Further, since the control signal OUT 4  is at the low level, the output signals a 2  and r 2  from the AND gate AND 1  and the OR gate OR 1  in the control circuit  220  are supplied to the NAND gate NAN 2  and the NOR gate NOR 2 , respectively, as the signal a 3  and the signal r 3 . 
     Accordingly, in the middle-speed mode, e.g. when the input signal i changes from the L level to the H level, the P-channel MOS transistor MP 2  in the second output block  300  is held in the ON-state over a predetermined time period after the level change. On the other hand, when the input signal i changes from the H level to the L level, the N-channel MOS transistor MN 2  in the second output block  300  is held in the ON-state over a predetermined time period after the level change. 
     Thus, in the middle-speed mode, the second output block  300  provides additional or augmenting drive power to make up for the drive power obtained from the first output block  100 . 
     c. High-speed Mode 
     When the input signals PCTL 0  and PCTL 1  delivered to the control signal-generating block  600  are both set to the H level, all the control signals OUT 2 , OUT 4  and OUT 8  are set to the L level (see FIG.  11 ). In this state, when the enable signal en is set to the L level, the operational mode of the output buffer circuit is switched to the high-speed mode. 
     In the high-speed mode, the output signal from the OR gate OR 0  is held at the L level, while the output signal from the inverter IN 1  is held at the H level. As a result, the transistors MP 1  and MN 1  in the first output block  100  are turned on and off in response to the input signal i. Further, since the control signals OUT 4  and OUT 8  are held at the L level, the output signal a 2  from the AND gate AND 1  in the control circuit  220  is supplied to the NAND gates NAN 2  and NAN 3 , while the output signal r 2  from the OR gate OR 1  is supplied to the NOR gates NOR 2  and NOR 3 . 
     Accordingly, in the high-speed mode, e.g. when the input signal i changes from the L level to the H level, the P-channel MOS transistor MP 2  in the second output block  300  and the P-channel MOS transistor MP 3  in the third output block  500  are held in the ON-state over a predetermined time period after the level change of the input signal i. On the other hand, when the input signal i changes from the H level to the L level, the N-channel MOS transistor MN 2  in the second output block  300  and the N-channel MOS transistor MN 3  in the third output block  500  are held in the ON-state over a predetermined time period after the level change of the input signal i. 
     Thus, in the high-speed mode, the second output block  300  and the third output block  500  provide additional or augmenting drive powers to make up for the drive power obtained from the first output block  100 . 
     d. High-impedance Mode 
     When the enable signal en is set to an H level, the output signals from the respective OR gates OR 1 , OR 2  and OR 3  are each forcibly changed to the H level, while the output signals from the inverter IN 1  and the AND gates AND 1 , AND 2  and AND 3  are each forcibly changed to the L level. As a result, all the transistors forming the first output block  100 ′, the second output block  300  and the third output block  500  are turned off, and the output terminal  400  is brought into a high-impedance state. 
     e. Changes in Level of Output Signal x in Low-Speed Mode, Middle-speed Mode and High-Speed Mode 
     FIG. 12 shows changes in rise of an output signal x in the respective speed modes in response to the change of the input signal i from the low level to the high level. 
     In the low-speed mode, the load connected to the output terminal  400  is charged by the P-channel MOS transistor MP 1  alone as described above, so that the level of the output signal x rises slowly. On the other hand, in the middle-speed mode, the load is charged by the P-channel MOS transistors MP 1 , MP 2 , and hence the output signal x rises faster than in the low-speed mode. Further, in the high-speed mode, the load is charged by the P-channel MOS transistors MP 1 , MP 2  and MP 3 , and hence the output signal x rises still faster than in the middle-speed mode. 
     As described above, according to the present embodiment, it is possible to switch the speed mode to thereby change the speed at which the output signal x rises. More specifically, it is possible to switch between a high-speed charge and a low-speed charge according to the capacity of a load connected to the output terminal  400  of the output buffer circuit. 
     In the output buffer circuit according to the present embodiment described above, the delay block  210  may be constructed such that its delay time can be set differently in the respective speed modes. For example, the output buffer circuit may be provided with a delay block  210 ′ constructed as shown in FIG.  13 . 
     The delay block  210 ′ is comprised of a speed mode-determining section  211 , and a variable delay circuit  212 . 
     The speed mode-determining section  211  is supplied with the control signals OUT 2 , OUT 4  and OUT 8  and determines the speed mode of the output buffer circuit based on the signal levels of the respective control signals. Then, the section  211  generates a delay stage number-selecting signal based on the result of the determination and delivers the generated delay stage number-selecting signal to the variable delay circuit  212 . 
     For example, when the received control signals OUT 2  and OUT 4  are both at the low level, and the control signal OUT  8  at the high level, the speed mode-determining section  211  determines that the output buffer circuit is in the middle-speed mode (see FIG.  11 ). Then, the speed mode-determining section  211  generates a delay stage number-selecting signal corresponding to the middle-speed mode and delivers the generated signal to the variable delay circuit  212 . 
     The variable delay circuit  212  changes setting of the delay time in response to the delay stage number-selecting signal. 
     The variable delay circuit  212  is comprised of a plurality of inverters, AND gates, OR gates, and so forth. The circuit  212  changes the delay time based on the delay stage number-selecting signal. More specifically, upon receiving the output signal x, the variable delay circuit  212  changes the number of delay stages to be used, in response to the delay stage number-selecting signal delivered from the speed mode-determining section  211 . Then, after the lapse of the delay time set by changing the number of the delay stages, the variable delay circuit  212  delivers an output signal x 2  formed by inverting the polarity of the output signal x to the control circuit  220 . 
     The number of the delay stages to be used in the variable delay circuit  212  is set such that the signal level of the output signal x 2  from the delay block changes simultaneously with a complete rise of the output signal x. 
     By providing the delay circuit  210 ′ constructed as above in the output buffer circuit, it is possible to change the level of the output signal x accurately. 
     Although in the output buffer circuit according to the present embodiment, the output signal x from the first output block  100 ′ is supplied to the drive switching control block  200 ′, this is not limitative, but the output buffer circuit may be constructed, similarly to the output buffer circuit according to the second embodiment, such that the input signal i is supplied to the drive switching control block  200 ′.