Power-Saver Differential Input Buffer

This invention relates to integrated circuits and more particularly to input buffer circuits.

Present complementary metal oxide semiconductor (CMOS) circuits are frequently used for a variety of computer applications including lap top and notebook computer systems which are battery powered. These applications impose practical limitations such as speed, power, and feature size on semiconductor design. Optimal performance of a system depends on an effective balance of these factors.

Input buffers in semiconductors such as dynamic random access memories (DRAM) act as an interface between external input signals and internal logic circuits. Input buffer designs, as disclosed in FIGS. 126, 138, and 139 of U.S. Pat. No. 5,208,776, entitled PULSE GENERATION CIRCUIT, translate external transistor-transistor logic (TTL) logic level signals to internal logic level signals. These TTL input buffers are designed to minimize operating power consumption during a standby mode. However, they are sensitive to transistor threshold voltage variation. Therefore, TTL input buffer speed degrades significantly with variations in operating voltage and temperature. Furthermore, the ability of the TTL input buffer to correctly determine an external input signal level may degrade significantly due to internal noise on supply voltage bus lines during an active mode.

Differential input buffers disclosed in U.S. Pat. No. 4,280,070, entitled BALANCED INPUT BUFFER CIRCUIT FOR SEMICONDUCTOR MEMORY, and U.S. Pat. No. 4,288,706, entitled NOISE IMMUNITY IN INPUT BUFFER CIRCUIT FOR SEMICONDUCTOR MEMORY, compare external logic signals to a stable internal reference signal to produce an internal logic level signal. These differential input buffers offer greater speed and noise immunity than TTL input buffers. However, use of differential input buffers greatly increases operating power consumption during a standby mode.

These issues are resolved by an input circuit for a semiconductor device. A first input buffer receives a control signal, an input signal, and a reference signal, for producing a first output signal in response the control signal and a difference between the input signal and the reference signal. A second input buffer receives the control signal and the input signal, for producing a second output signal in response to the control signal and the input signal. A control circuit produces the control signal, in response to a predetermined output state.

The present invention provides high speed operation and excellent noise immunity during an active mode. Noise immunity is uncompromised during a standby mode when internal noise is negligible. Power is conserved over previous methods in a standby mode.

A more complete understanding of the invention may be gained by reading the subsequent detailed description with reference to the drawings wherein:

FIG. 1 is block diagram of a first embodiment of the present invention;

FIG. 2 is block diagram of a second embodiment of the present invention;

FIG. 3 is a differential input buffer which may be used in FIGS. 1 and 2;

FIG. 4 is a TTL input buffer which may be used in FIG. 1; 15 FIG. 5 is a control circuit which may be used in FIGS. 1 and 2.

Referring now to FIG. 1, a first embodiment of the input circuit will be described in detail. Differential input buffer 14 receives input signal IN, reference signal VREF, and control signal EN, for producing a first output signal OUT.sub.1. TTL input buffer 16 receives input signal IN and control signal EN, for producing a second output signal OUT.sub.2. Control circuit 22 has input terminals 18 and 20 for receiving output signals OUT.sub.1 and OUT.sub.2, respectively. Control circuit 22 produces control signal EN at terminal 24 having low and high logic states corresponding to active and standby modes, respectively, of the semiconductor device.

Input signal IN is low and control signal EN is high in a standby mode. Differential input buffer 14 is disabled, TTL input buffer 16 is enabled, and output signals OUT.sub.1 and OUT.sub.2 are high and low, respectively. Thus, no power is consumed by differential input buffer 14. Internal noise is negligible in standby mode while TTL input buffer 16 is enabled, so noise immunity is uncompromised.

At the beginning of an active mode, input signal IN goes high causing output signal OUT.sub.2 to go high. Control signal EN goes low in response to this predetermined output state, thereby enabling differential input buffer 14 and disabling TTL input buffer 16. Output signal OUT.sub.2 is reset low. Furthermore, control signal EN enables other circuits (not shown) on the semiconductor device for operation in the active mode. Thus, excellent noise immunity and high speed of the differential input buffer are operative during the active mode. Power consumption of differential input buffer 16 is negligible with respect to total power consumption during the active mode.

At the beginning of a standby mode, input signal IN goes low causing output signal OUT.sub.1 to go low. Control signal EN goes high in response to this predetermined output state, thereby disabling differential input buffer 14 and enabling TTL input buffer 16.

Referring now to FIG. 2, a second embodiment of the input circuit will be described in detail. Differential input buffer 14 receives input signal IN, reference signal VREF, and control signal EN, for producing a first output signal OUT.sub.1. TTL input buffer 16 receives input signal IN and control signal EN, for producing a second output signal OUT.sub.2. Control circuit 22 has input terminals 18 and 22 for receiving output signals OUT.sub.1 and OUT.sub.2, respectively. Control circuit 22 produces complementary control signals EN and EN at terminals 24 and 26, respectively. Furthermore, control signal EN enables other circuits (not shown) on the semiconductor device for operation in the active mode. Thus, operation of the input circuit is similar to that of the previous embodiment except that TTL input buffer 16 is enabled when control signal EN at terminal 26 is low. This is effected by substitution of a NOR gate for a NAND gate in TTL input buffer 16, as is well known by those having ordinary skill in the art.

Referring now to FIG. 3, a differential input buffer which may be used in the input circuit of FIGS. 1 and 2 will be described in detail. N-channel current mirror transistors 226 and 228 are connected to P-channel input transistors 216 and 218, respectively, to form a differential amplifier. The common source of P-channel input transistors 216 and 218 is connected to positive supply V.sub.DD through P-channel transistor 214. Reference signal VREF is coupled to the gate of P-channel input transistor 216 through resistor 210. Resistor 210 and P-channel capacitor 212 stabilize the gate voltage of P-channel input transistor 216. Input signal 12 is coupled to the gate of P-channel input transistor 218. The output of the differential amplifier at terminal 224 is coupled to the input of a high threshold inverter formed by P-channel transistor 232 and N-channel transistors 236 and 238. The output of the high threshold inverter at terminal 234 is buffered by inverters 240 and 242 to produce output signal OUT.sub.1.

When control signal EN is high, P-channel transistor 214 is off and N-channel transistor 230 is on. Thus, the differential amplifier is disabled, input terminal 224 of the high threshold inverter is low and output signal OUT.sub.1 is high.

When control signal EN goes low, P-channel transistor 214 is on, N-channel transistor 230 is off, and the differential amplifier is enabled. Input signal IN is compared with reference signal VREF by P-channel input transistors 218 and 216, respectively. If input signal IN is greater than reference signal VREF, P-channel input transistor 218 is less conductive than P-channel input transistor 216, and the output at terminal 224 goes low causing output signal OUT.sub.1 to go high. If input signal IN is less than reference signal VREF, output signal OUT.sub.1 goes low.

Referring now to FIG. 4, a TTL input buffer which may be used in the input circuit of FIG. 1 will be described in detail. A two input NAND gate is formed by P-channel transistors 310 and 318 and N-channel transistors 314 and 316. The output of the NAND gate at terminal 312 is coupled to the input of buffering inverter 320, which produces output signal OUT.sub.2.

When control signal EN is low, P-channel transistor 318 is on and N-channel transistor 314 is off. Thus, the NAND gate is disabled, and output signal OUT.sub.2 is low. When control signal EN goes high, P-channel transistor 318 is off and N-channel transistor 314 is on. Thus, the NAND gate is enabled, and output signal OUT.sub.2 is the same logic state as input signal IN.

Referring now to FIG. 5, a control circuit which may be used in the input circuit of FIGS. 1 and 2 will be described in detail. Output signal OUT.sub.1 is coupled to one input terminal 18 of a cross-coupled latch formed by NAND gates 434 and 430. Output signals OUT.sub.1 and OUT.sub.2 are coupled by CMOS pass gates 410 and 414, respectively, to the input of inverter 424. The output of inverter 424 is coupled to the other input of the cross-coupled latch at terminal 426. An output of the cross-coupled latch at terminal 436 is buffered by inverters 438 and 440 to produce control signal EN at terminal 24. Control signal EN is fed back to the gates of CMOS pass gates 410 and 414 through inverters 428 and 418.

When control signal EN is high in a standby mode, CMOS pass gate 410 is off and CMOS pass gate 414 is on. When input signal IN (FIG. 1) goes high at the beginning of an active mode, output signal OUT.sub.2 goes high, and the cross-coupled latch is reset when terminal 426 goes low. Both inputs of NAND gate 434 are high and control signal EN goes low, thereby enabling the differential input buffer 14 and disabling the TTL input buffer 16 (FIG. 1). Thus, noise immunity of the input circuit of FIG. 1 is greatly improved, because the signal level of input signal IN is determined by comparison with reference signal VREF. Common mode noise on supply voltage bus line V.sub.SS (FIG. 3) has little effect on the differential input buffer operation. Input circuit speed is improved, because a small change of input signal IN with respect to reference signal VREF will effect a change in output signal OUT.sub.1.

Referring again to FIG. 5, when control signal EN is low in an active mode, CMOS pass gate 410 is on and CMOS pass gate 414 is off. When input signal IN (FIG. 1) goes low at the beginning of a standby mode, output signal OUT.sub.1 is immediately driven low by differential input buffer 14. The cross-coupled latch (FIG. 5) is set, and control signal EN goes high, thereby disabling the differential input buffer and enabling the TTL input buffer (FIG. 1). Thus, power consumption is greatly reduced in the standby mode. Noise is negligible in the standby mode, and noise immunity is uncompromised.

Although the invention has been described in detail with reference to its preferred embodiment, it is to be understood that this description is by way of example only and is not to be construed in a limiting sense. It is to be further understood that numerous changes in the details of the embodiments of the invention will be apparent to persons of ordinary skill in the art having reference to this description. For example, in another embodiment TTL input buffer 14 remains enabled during both active and standby modes. In yet another embodiment, control signal EN is a logic function of output signal OUT.sub.2 only. It is contemplated that such changes and additional embodiments are within the spirit and true scope of the invention as claimed below.