Differential Input Buffer Using Local Reference Voltage and Method of Construction

This invention relates in general to the field of electronic circuits, and more particularly to a differential input buffer using a local reference voltage and method of construction.

Integrated circuits often include input buffers for receiving input signals and for driving signals to other parts of the integrated circuit. For example, input buffers operate to distinguish between logic low and logic high on the input signal based upon comparison to a reference voltage, and the input buffers then drive internal signals appropriately. Conventional input buffers can require the use of external reference generators that use power and consume space. In addition, some conventional input buffers can have difficulty with accurately distinguishing between logic low and logic high when the input signal is near the level of the reference voltage.

In accordance with the present invention, a differential input buffer using a local reference voltage and method of construction are provided that provides benefits over previously developed input buffers.

According to one embodiment of the present invention, a differential input buffer and method of construction are provided. The differential input buffer includes a differential amplifier connected to receive an input signal. A local reference voltage generator is connected to the differential amplifier and is connected to receive an external voltage reference and to provide a local reference voltage to the differential amplifier. The local reference generator is adjustable during construction to produce a desired level for the local reference voltage. The differential input buffer also includes a hysteresis element that is connected to provide feedback to the differential amplifier and includes a buffer stage that is connected to receive an output of the differential amplifier and to drive an output signal. The buffer stage is also connected to the hysteresis element to provide a basis for the feedback to the differential amplifier.

A technical advantage of the present invention is better high and low input voltage characteristics. The differential input buffer locally generates a reference voltage, which can be adjusted for individual cases to favor either high or low inputs. An existing on-chip voltage reference, such as the bit line reference (BLR), is used as a voltage reference for generating the reference voltage used by the differential input buffer. Another advantage is that the local generation of the internal reference voltage consumes relatively low current when compared to conventional external reference generators.

A further technical advantage of the present invention is that the differential input buffer, as a whole, uses relatively low power. The differential input buffer also contains a hysteresis element to provide stability when input signals are at voltage levels near the reference voltage generated and used by the input buffer.

A more complete understanding of the present invention and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:

FIG. 1 is a block diagram of one embodiment of an integrated circuit that receives signals using differential input buffers constructed according to the present invention;

FIG. 2 is a circuit diagram of one embodiment of the differential input buffer of FIG. 1;

FIG. 3 is a plot showing one embodiment of change in the reference voltage generated by the differential input buffer of FIG. 2; and

FIG. 4 is a plot showing another embodiment of change in the reference voltage generated by the differential input buffer of FIG. 2.

FIG. 1 is a block diagram of one embodiment of an integrated circuit, indicated generally at 10, that receives input signals using differential input buffers 14 constructed according to the teachings of the present invention. In the embodiment of FIG. 1, each input buffer 14 receives signals from a system bus 12. Each input buffer 14 also receives an enable signal, CAEN, generated by an enable signal generator 16. Each input buffer 14 further receives a reference voltage generated by a reference generator 18. In the embodiment of FIG. 1, reference generator 18 is a bit line reference (BLR) generator in a DRAM device, and the BLR reference voltage is typically one-half the positive power supply, V.sub.DD. Each input buffer 14 provides an output signal to other parts of integrated circuit 10, such as to a memory array, address buffers, clocks or other circuitry. According to the present invention, each differential input buffer 14 uses an internal reference generator to locally generate a reference voltage. In addition, the reference voltage can be adjusted for individual cases to favor either high or low input voltage levels.

FIG. 2 is a circuit diagram of one embodiment of differential input buffer 14 of FIG. 1. As shown, differential input buffer 14 receives the input signal, IN, the enable signal, CAEN, and the voltage reference, BLR. The input signal, IN, is provided to a p-channel transistor 50 and an n-channel transistor 52, as shown. A p-channel transistor 54 is connected to the positive power supply and also receives the enable signal, CAEN. P-channel transistor 54 is then connected to p-channel transistor 56 which is connected to p-channel transistor 50 and to an n-channel transistor 58. An n-channel transistor 60 is connected to p-channel transistor 50 and n-channel transistor 52 and receives the enable signal CAEN, as shown.

A p-channel transistor 62 and an n-channel transistor 64 are connected between p-channel transistor 56 and n-channel transistor 58. The gates of p-channel transistor 62 and n-channel transistor 64 are connected to a locally generated voltage reference, VREF, produced by n-channel transistor 68, p-channel transistor 70 and p-channel transistor 72. The gate of n-channel transistor 66 is connected to n-channel transistor 74 which receives the reference voltage, BLR, at its gate. The drain of n-channel transistor 60 is connected to three output buffer stages, as shown. The first output buffer stage is formed from p-channel transistor 76 and n-channel transistor 78. The second is formed from p-channel transistor 80 and n-channel transistor 82, and the third is formed from p-channel transistor 84 and n-channel transistor 86. The output of differential input buffer 14 is then provided as the output signal, OUT.

As can be seen, differential input buffer 14 of FIG. 3 has essentially three parts: a local reference voltage generator formed by transistors 68, 70, and 72; an amplifier formed by transistors 54, 56, 50, 52, 62, and 64; and a hysteresis element formed by transistors 66 and 74. The local reference voltage generator produces the reference voltage, VREF, which is adjustable and determined by the threshold voltage, V.sub.TP, of transistor 72. The node, NO, has a voltage level at a voltage close to the voltage reference, BLR, minus the threshold voltage, V.sub.TP. As mentioned above, the voltage reference, BLR, is one of the inputs to input buffer 14 and, in one embodiment, BLR is approximately half the voltage of the power supply, V.sub.DD.

At lower input voltages, the node, NO, will be at a voltage level that is less than VREF. As a result, the gate voltage for device 70 will be greater than the threshold voltage, V.sub.TP, of transistor 72. Therefore transistor 70 will operate in its cut-off region. When the gate voltage (which is equal to VREF less the voltage level of the node, NO) reaches the threshold voltage, V.sub.TP, or less, transistor 70 will begin to turn-on. Due to coupling effects of parasitic capacitances of the nodes of differential amplifier to VREF, transistor 70 will act as a stabilizer, keeping VREF close to the initial voltage.

The amplifier portion of the circuit receives the enable signal, CAEN. In this embodiment, if the enable signal, CAEN, is high, it indicates that the integrated circuit 10 is disabled. If the enable signal, CAEN, is low, then differential input buffer 14 will sense the level of the input signal, IN, relative to the reference voltage, VREF, and determine what the output will be. The reference voltage, VREF, comes from the local reference generator described above and is connected to the gates of transistor 62 and 64. As can be seen, transistors 62 and 64 make an inverter that drives the node, N1, to a voltage level dependent upon the voltage of VREF.

As an example, TTL input levels are typically 2.4 volts and 0.8 volts for high and low signals. In a TTL environment, for differential input buffer 14 to sense effectively, the level of reference voltage, VREF, should be close to the average of these two values. In the illustrated embodiment, VREF varies between 1.49 volts and 1.9 volts. As a result of this relatively low voltage level for VREF, p-transistor 62 will turn on more than does transistor 64. In other words, transistor 62 passes more current than transistor 64. Also, the node, N1, will be close to the value of the node, N3. The node, N1, is also driving transistor 56, which causes transistor 56 not to be turned on as much. As a result, the node, N3, will not be at a full V.sub.DD. Conversely, because the voltage of the node, N1, is closer to V.sub.DD than it is to ground potential, transistor 58 will be turned on sufficiently to pull the node, N4, all the way to ground potential.

Compared to CMOS voltage levels, the TTL input level of 2.4 volts is relatively low. Therefore, by favoring transistor 58 with a high voltage on the node, N1, the input buffer effectively favors reading a logic high. For example, the TTL input of 2.4 volts would be sensed as a logic high at the output of input buffer 14. When the TTL input is low, that is, less than approximately 1.4 volts, transistor 52 will be very close to the cutoff region of operation, and transistor 50 will pull the node, N2, high and therefore the output would be a logic low.

The hysteresis element of input buffer 14 includes transistor 66 which is controlled by the node, N6, through transistor 74. Transistor 74 limits the voltage of the node, N6, to the voltage reference, BLR, less the threshold voltage, V.sub.TP, of transistor 72 when the node, N6, is high. Also when the node, N6, is high, transistor 66 will be partially on and help the reference side of the differential input buffer 14 pull the node, N1, lower. When the node, N8, is low, transistor 66 is off. This effectively produces what is known as a hysteresis effect. Depending on what state the differential input buffer 14 is in initially, it will react differently to inputs. If the node, N6, is high initially, then a slightly higher input level can be applied at the input node, IN, to switch the differential input buffer 14. Once switched, a lower level can be applied at the input node, IN, to switch the amplifier back the other way. Effectively, a buffer zone is created where differential input buffer 14 does not switch. If the hysteresis element were not there, the differential input buffer 14 could start oscillating when the input node, IN, is sitting at the exact reference voltage, VREF, and the differential amplifier can go either way.

FIG. 3 is a plot showing the change in the reference voltage, VREF, with temperature giving a V.sub.BB of -3.5 volts and the V.sub.DD shown. This plot shows how the transistor 72 is essentially programmable to adjust the threshold voltage level of that transistor. By adjusting the threshold voltage of transistor 72, the reference voltage, VREF, produced and supplied to transistor 62 and transistor 64 is modified according to the drawing in FIG. 3. FIG. 3 shows the results when transistor 72 having width of 10 and a length of 2. According to the present invention, the width of transistor 72 is adjustable from 2 to 10 and the length is adjustable from 0.9 to 2. It should be understood, however, that other values could be used in accordance with the present invention.

FIG. 4 shows a plot of the change in the reference voltage, VREF, for a V.sub.BB of -3.5 volts and for changing V.sub.DD, as shown. The values shown in FIG. 4 are for transistor 72 having a width of 4 and a length of 1.4. As can be seen in comparison with FIG. 3, the value for the reference voltage, VREF, has increased at the same temperature. The adjustability of transistor 72 is important to ensuring that the output of the differential input buffer 14 has the appropriate levels based upon the inputs received.

Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made thereto without departing from the spirit and scope of the invention and is defined by the appended claims.