BUFFER CIRCUIT OPERABLE WITH REDUCED POWER CONSUMPTION

Abstract: A buffer circuit which can operate with reduced power consumption and at a high speed is disclosed. The buffer circuit is of the type having a flip-flop circuit with a pair of output terminals (NA, NB) and a pair of boot-strap circuits (Q3, C1; Q4, C2) each provided for each of the output terminals and having a capacitor (C1, C2) and a precharge circuit for precharging the capacitor, and is featured in that a pair of control circuits (11, 12) are provided to the pair of boot-strap circuits. Each of the control circuits allows the capacitor (C1, C2) to be charged when the associated output terminal is about to produce a high level signal and inhibits the capacitor from being charged when the associated output terminal is about to produce a low level signal.

Description: BACKGROUND OF THE INVENTION: The present invention relates to a buffer circuit employing field effect transistors, and more particulary to a buffer circuit of a boot-strap type. Buffer circuits have been widely utilized in variety of digital ICs such as IC memories. A typical conventional buffer employes a flip-flop circuit having a pair of output nodes and a pair of boot-strap circuits provided to the pair of output nodes. Each of the boot-strap circuit includes a capacitor for achieving the boot-strap function. However, in the conventional buffer circuit, the capacitors of both the boot-strap circuits are simultaneously precharged by an activation signal each time when the buffer circuit is about to produce new contents of output signals, although the boot-strap circuit coupled to the output terminal from which a low level signal is produced is not required to operate. Therefore, unnecessary power is consumed by the above boot-strap circuit and the load of the activation signal is large because the activation signal is required to charge both of the capacitors, resulting in a low speed operation. SUMMARY OF THE INVENTION: It is an object to provide a buffer circuit operable with a low power consumption. It is another object of the present invent to provide a buffer circuit which can operate at a high speed. The buffer circuit according to the present invention is specified in claim 1. It is of the type having a flip-flop circuit receiving a first input signal and a second input signal complementary to the first input signal and a first output node generating a first output signal complementary to the first input signal and a second output node generating a second output signal complementary to the second input signal, and first and second boot-strap circuits coupled to the first and second output nodes, respectively, each of the boot-strap circuits including a capacitor and a precharge circuit for operatively charging the capacitor by an activation signal, and is featured in that first and second precharge control circuits are provided for the first and second boot-strap circuits, respectively. The first precharge control circuit receives at least the first input signal and enables the precharge circuit of the first boot-strap when the first input signal is a low level and disenables the precharge circuit of the first boot-strap circuit when the first input signal is a high level. The second precharge control circuit receives at least the second input signal and enables the precharge circuit of the second boot-strap circuit when the second input signal is at low in level and disenables the precharge circuit of the second boot-strap circuit when the second input signal is at high in level. The capacitor of the boot-strap circuit coupled to the output node which is about to produce a high level of the output signal is charged while the capacitor of the other boot-strap circuit coupled to the other output node which is about to produce a low level of the output signal is not charged. Therefore, a power consumption required by the capacitor of the boot-strap circuit coupled to the output node which generates a low level output signal is effectively removed. Also, the charge of the capacitor of the boot-strap circuit coupled to the high level output node can be made rapidly than the case where both capacitors are charged so that a high speed operation can attained. BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1 is a schematic circuit diagram showing a buffer circuit according to a prior art; Fig. 2 is a wave-form diagram showing an operation of the circuit in Fig. 1; Fig. 3 is a schematic circuit diagram showing a buffer circuit according to a preferred embodiment of the invention; and Fig. 4 is a wave-form diagram showing an operation of the circuit in Fig. 3: DETAILED DESCRIPTION OF THE INVENTION: Referring to Fig. 1, an data output buffer circuit according to a prior art is explained. Throughout the following explanation, field effect transistors employed are all N channel type MOS transistors as representative example of field effect transistors. The buffer circuit includes a latch section 10 and a push-pull output section 20. The latch section 10 receives true and complementary input signals IN and IN and generates true and complementary buffered signals OUT and OUT . The push-pull output section 20 generates an data output signal D OUT in accordance with the buffered signals OUT and OUT . In the latch section 10, input transistors Q₇ and Q₈ receiving the input signals IN and IN, respectively and hold transistors Q₁₂ and Q₁₃ form a flip-flop circuit with reset transistors Q₅ and Q₆. Transistors Q₁, Q₃ and Q₁₀ and a boot-strap capacitor C₁ form an active load circuit for an output node NA from which the buffered signal OUT is produced, while transistors Q₂, Q₄ and Q₁₁ and a capacitor C₂ form an active load circuit for an output node NA from which the buffered signal OUT is produced. The active load circuits are reset by a reset signal ⊘ R and enabled or activated in response to an activation signal ⊘ A . With reference to Fig. 2, an operation of the circuit of Fig. 1 is explained. Prior to a time point t₁, the signal ⊘ A is at low and the signal ⊘ R is at high in level, while both of input signals IN and IN are at high in level. Therefore, both of the buffered signals OUT and OUT are at low (ground potential) in level because the transistors Q₅, Q₆, Q₇ and Q₈ are conducting. As a result, both transistors Q₁₄ and Q₁₅ in the output section 20 are non-conducting so that the output signal D OUT is at a high impedance state as illustrated by a dotted portion. At the time point t₁, the signal ⊘ A is raised from the ground potential toward a power voltage V cc . Also, development of the states of the input signals IN and IN are started and the signal IN is fallen to the ground potential while the signal IN is kept at high (V cc ) in level. In response to rise of the signal ⊘ A in potential, a node N₃ as the gates of the transistors Q₁ and Q₂ is raised above V cc by self-boost effect due to gate-source capacitances of the transistors Q₁ and Q₂ at a time point t₂. Thus, the transistors Q₁ and Q₂ are conducting in the non-saturated region to start charging of the nodes N₁ and N₂. In this instance, the capacitors C₁ and C₂ are charged. When the levels at the nodes N₁ and N₂ exceed the threshold voltage of the transistors Q₃ and Q₄ at a time point t₃, the transistors Q₃ and Q₄ become conductive. Thus, the level of the signal OUT at the node NA is determined according to the ratio of the on-resistance of the transistor Q₃ to that of the transistor Q₅, while the level of the signal OUT at the node NB is determined according to the ratio of the on-resistance of the transistor Q₄ to the parallel value of the on-resistances of the transistors Q₈ and Q₆. Therefore, the signal OUT is slightly lower than the signal OUT but they are relatively low levels. In this instance, it should be noted that DC current flows from V cc to the ground through the path of the transistors Q₃ and Q₅ and the path of the transistors Q₄, Q₆ and Q₈. Therefore, a relatively large power is uneffectively consumed. Then, when the reset signal ⊘ R becomes lower than the threshold voltage of the transistors Q₅ and Q₆, the transistors Q₅ and Q₆ become non-conductive. As a result, the node NA (OUT) starts to rise toward V cc and the potential at the node N₁ is raised above V cc by the boot-strap effect through the capacitor C₁. While the transistors Q₈ and Q₁₁ remain conducting to discharge the node N₂ and the transistor Q₂ becomes non-conducting so that the signal OUT falls in level. When the signal OUT exceeds the threshold voltage of the transistor Q₁₄ in the output section at a time point t₅, the transistor Q₁₄ becomes conducting with the transistor Q₁₅ non-conducting so that the output signal D OUT becomes high in level. As explained above, the conventional buffer circuit of Fig. 1, inevitably consumes a large amount of power. Moreover, the signal ⊘ A is used to charge both the capacitors C₁ and C₂ and the load of the signal ⊘ A is large. Therefore, it requires a relative large time period to charge the capacitors, resulting in a low speed operation. Referring to Fig. 3, a buffer circuit according to a preferred embodiment of the present invention is explained. In Fig. 3, the elements corresponding to those in Fig. 1 are denoted by the same reference codes used in Fig. 1. As shown in Fig. 3, the latch section 10′ is different from the latch circuit in Fig. 1 in that the gates of the transistors Q₅ and Q₆ are connected to nodes N5 and N6, respectively and transistors Q 9A and Q 9B are connected between the gate N3A of the transistor Q₁ and the node N5, and between the gate N3B of the transistor Q₂ and the node N6, respectively. The node N5 is supplied with a selective reset signal ⊘ RA generated by a selective control circuit 11. The node N6 is supplied with a selective reset signal ⊘ RB generated by a selective control circuit 12. Namely, the present invention is featured in that the transistors Q₁ and Q₅ in the left side is controlled by the selective reset signal ⊘ RA while the transistors Q₂ and Q₆ in the right side are controlled by the selective reset signal ⊘ RB . The selective control circuit 11 receives a precharge signal ⊘ P , a timing control signal ⊘ A1 , the activation signal ⊘ A , and the input signals IN and IN , and includes transistors Q₁₇ to Q₂₂. The transistors Q₁₇ to Q₂₀ operate as a precharged type inverter for inverting the signal ⊘ A with a slight delay. The transistors Q₂₁ and Q₂₂ operate as a clocked inverter for inverting the input signal IN when the signal ⊘ A1 is at a high level. Therefore, the selective reset signal ⊘ RA can be substantially equivalent to the NOR result of ⊘ A and IN . Since the timing signal ⊘ A1 is such signal that rises in potential when the precharge signal ⊘ P falls and before the rise of the activation signal ⊘ A , the selective control signal ⊘ RA falls in level at a time slightly after ⊘ A is raised in level in the case where the input signal IN is at a low level, and at an early time when ⊘ A1 is raised in level and before the rise of ⊘ A in the case where the input signal IN at a high level. Therefore, the selective signal ⊘ RA is deactivated to the low level before the rise of ⊘ A when the input signal IN to the left side of the latch circuit 20′ is high in level and the buffered signal OUT is about to be a low level thereby to block DC current through the transistors Q₁, Q₇ and Q₅. To the contrary, when the input signal IN is at a high level and the low level of the buffered signal OUT is to be developed, the selective reset signal ⊘ RA falls in level after the activation signal is raised in level so that the transistors Q₁ and Q₅ are conducting to charge the node N₁ and the capacitor C₁. Similarly, the selective control circuit 12 composed of transistors Q₂₃ to Q₂₈, receives ⊘ A , ⊘ A1 , ⊘ P and the input signals IN and IN and generates the NOR result of the activation signal ⊘ A and the input signal IN. The selective reset signal ⊘ RB applied to the right side of the latch circuit 20′ falls in level at a time when the signal ⊘ A is raised in level in the case of the low level of IN and at an early time when the signal ⊘ A1 is raised in level but before the rise of ⊘ A in the case of the high level of IN. Accordingly, the capacitor C₂ and the node N₂ are charged only when the input signal IN is at a low level and the buffered signal OUT is about to be high in level. With reference to Fig. 4, an operation of the circuit in Fig. 3 is explained. Prior to a time point t₁, the precharge signal ⊘ P is at a high (V cc ) level so that the nodes N5 and N6 are kept high in level. Therefore, the gate nodes N3A and N3B of the transistors Q₁ and Q₂ are charged through the transistors Q 9A and Q 9B , respectively, while the buffered signals OUT (NA) and OUT are low in level due to the conducting transistors Q₅ and Q₆. In this instance, the input signals IN and IN are high level so that the transistors Q₇ and Q₈ are conducting. At a time point t₂, the precharge signal ⊘ P is charged to the low (ground) level and the signal ⊘ A1 changes to a high level from the low level to turn the transistors Q₂₂ and Q₂₅ conducting. At a time point t₃, the states of the input signals IN and IN are developed, and in this case the signal IN is changed to the low level with the input signal IN at a high level. Thereafter, the charge at the node N₆ is discharged to the low level of the input signal IN through the transistors Q₂₄ and Q₂₅ so that the selective reset signal ⊘ RB falls towards the low level so that the transistors Q₂ and Q₆ are made non-conductive. In this instance, the charge at the node N₅ is still maintained and the transistors Q₁ and Q₅ are still conducting because the transistor Q₂₁ is non-conducting. At a time point t₄, the activation signal ⊘ A is changed to a high level and the signal ⊘ A is transmitted to the node N₁ through the conductive transistor Q₁ but not transmitted to the node N₂ because the transistor Q₂ is non-conductive. Accordingly, the node N₁ and the capacitor C₁ is charged approximately by the voltage of ⊘ A . In this instance, the capacitor C₂ is not charged. After a delay time due to the transistors Q₁₇ to Q₂₀ from the rise of ⊘ A , the charge at the node N₅, i.e. the selective reset signal ⊘ RA is changed to the low level so that the transistors Q₁ and Q₅ are turned off. Accordingly, the node NA, i.e. the buffered signal OUT is raised in potential through the transistor Q₃ and such rise at the node NA is positively fed back to the node N₁ through the charged capacitor C₁ by the boot-strap effect. Thus, the buffered signal OUT is raised to the power voltage. While the transistor Q₈ is conducting and the charge at the node N₂ is discharged, and therefore the buffered signal OUT at the node NB is kept at the low level. As a result, the transistor Q₁₄ is conducting with the transistor Q₁₅ non-conducting so that the output D OUT is raised to the high level (Vcc - Vth, Vth being a threshold voltage of Q₁₄). As explained above, according to the present invention, the activation signal ⊘ A is applied only to the capacitor C₁ which is used to produce the high level signal OUT. Therefore, the consumption required by the signal ⊘ A is remarkably reduced to the half and the load of ⊘ A is also reduced. Also, the charge of the capacitor (C₁) is made at a high speed so that a high speed operation can be achieved.

Claims: A buffer circuit comprising a flip-flop circuit (Q7, Q8, Q10-Q13) having first and second input terminals (IN IN ), and first and second output terminals ( OUT OUT); a first boot-strap circuit (Q2, Q4, Q6, C2) coupled to said first output terminal ( OUT ), and including a first capacitor (C2) and a first precharge circuit (Q6, Q2) for operatively charging said capacitor; a second boot-strap circuit (Q1, Q3, Q5, C1) coupled to said second output terminal (OUT) and including a second capacitor (C1) and a second precharge circuit (Q5, Q1) for operatively charging said second capacitor; characterized by a first control circuit (12) coupled to said first precharge circuit for enabling said first precharge circuit when said first output terminal ( OUT ) is to be made high in level and disenabling said first precharge circuit when said first output terminal is to be made low in level; and a second control circuit (11) coupled to said second precharge circuit for enabling said second precharge circuit when said second output terminal is to be made high in level and disenabling said second precharge circuit when said second output terminal is to be made low in level. The circuit according to claim 1, characterized in that said flip-flop circuit includes a pair of cross-coupled transistors (Q12, Q13) having gates and drains cross-coupled at said first and second output terminals and sources coupled to a reference voltage terminal (GND), a first input transistor (Q8) coupled between said first output terminal and said reference voltage terminal and having a gate connected to said first input terminal, and a second input transistor (Q7) coupled between said second output terminal and said reference voltage terminal and having a gate connected to said second input terminal. The circuit according to claim 1, characterized in that said first boot-strap circuit includes a first load transistor (Q4) coupled between a power voltage terminal (VCC) and said first output terminal, said first capacitor (C2) being coupled between the gate of said first load transistor and said first output terminal, and said second boot-strap circuit includes a second load transistor (Q3) coupled between said power voltage terminal and said second output terminal, said second capacitor (C1) being coupled between the gate of said second load transistor and said second output terminal.

Inventors: KANEKO, SHOUJI C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1987
Main classification: G11C11
IPC codes: G11C11, H03K19, G11C7