Abstract:
A voltage reference generator includes a voltage source and a differential amplifier. The voltage source supplies a stable voltage reference to a positive input of the differential amplifier which is configured as a follower having its output looped back to its negative input. The negative feedback loop is a variable-resistance loop that is controlled by the output of the differential amplifier. The variable-resistance feedback loop transiently imposes open-loop operation when the voltage reference generator is turned on so as to provide high current to the output before imposing closed-loop operation in follower mode.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a voltage reference generator, and more particularly to a voltage reference generator that generates a read voltage reference applicable to the cells of an EEPROM memory. 
     2. Discussion of the Related Art 
     The read voltage applied to the cells of a memory circuit is usually produced from the logic supply voltage of the circuit. It typically has a level below this supply voltage, for example, on the order of 2.5 volts. This read voltage should be stable to enable reliable reading of the memory cells, meaning that it should not vary with the supply voltage or the temperature. It should also be reproducible from one circuit to another despite the fact that the characteristics, for example the threshold voltage of the transistors, may vary dependent on the particular manufacturing method used. 
     The circuits used to generate a voltage reference of this kind generally use a current mirror structure with means to compensate for the different variations of temperature, supply voltage, etc. An example of such a structure is given for example in the French patent application No. FR 95 09023. Other circuits use a differential amplifier mounted as a follower amplifier to deliver, at the output, the reference voltage given by a voltage source at its positive input. 
     Usually, the read voltage reference generation circuit associated with a memory in an integrated circuit works continuously, once the integrated circuit is no longer in standby mode. However, it is a design goal to limit the consumption of the integrated circuits. 
     One way of limiting the consumption in an integrated circuit of this kind is to activate the read reference circuit only when it is needed, namely on demand, when there actually is a read operation to be carried out. However, using conventional read reference circuits, a problem arises. Because the read voltage reference is applied to a highly capacitive line, the build-up time of this read voltage is very long. This problem is further aggravated when these circuits are based on current mirror structures which require a long time to reach the state of equilibrium. Accordingly, conventional voltage reference circuits cannot be used on demand. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to resolve the afore-mentioned technical problems. It is a further object of the present invention to obtain a circuit for the generation, on demand, of a stable reference voltage that is lower than the logic supply voltage, in particular, to charge a capacitive line. 
     A solution to the afore-mentioned technical problems is found in an embodiment of the present invention that includes a structure using a voltage source to apply a reference voltage to the positive input of a differential amplifier mounted as a follower with its output looped back to its negative input. According to an embodiment of the present invention, the negative feedback loop is a variable-resistance loop controlled by the output of the amplifier in order to impose, transiently, an open loop operation when the voltage is turned on in such a way as to give high current at the output before imposing a closed-loop operation in follower mode. 
     According to an embodiment of the present invention, the reference voltage source does not have to source a large amount of current, rather, it simply should be stable. It is the amplifier that provides the high current necessary to quickly charge the capacitive line when the voltage is turned on, through its very high gain when the negative feedback loop is open. As soon as the capacitive line is sufficiently charged, the voltage level at the output becomes high, bringing the gain of the amplifier to unity (closed loop--follower assembly). The reference voltage level is then seen again at the output. Thus, instead of the two microseconds that would be necessary with a conventional voltage reference circuit, only 200 nanoseconds are necessary to obtain the level of the reference voltage at the output. Furthermore, the generator, according to the embodiments of the present invention, is quite stable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages and characteristics of a preferred, non-limiting embodiment of the present invention are described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 shows a block diagram of a circuit for generating a reference voltage according to an embodiment of the present invention; 
     FIG. 2 shows a more detailed diagram of this circuit; and 
     FIG. 3 shows a block diagram of an E 2  PROM memory to which a circuit according to an embodiment of the present invention can be applied. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a circuit for generating a reference voltage Vout according to an embodiment of the present invention. It includes a differential amplifier 1 and a loop circuit 2 for the looping of the output S of this amplifier to its negative (inverting) input e-. 
     The positive input e+ of the differential amplifier 1 receives a reference voltage VREF provided by a reference voltage source 3. The voltage source 3 does not have to supply significant loads, rather, it only needs to be capable of providing a stable reference. The voltage source 3 can be based, for example, on a P type diode-mounted transistor. 
     The loop circuit 2 includes a transistor Ti that is series-connected between the output S and the negative input e- of the amplifier. In the example, Transistor T1 is an N type transistor and its gate is biased by the logic supply voltage Vdd. The loop circuit 2 also includes a resistor R2 that is connected between the negative input e- and ground. FIG. 1 also shows a capacitor C representing the load at output of the generator. 
     The operation of the voltage reference generator depicted in FIG. 1 is as follows. Transistor T1 is equivalent to a variable resistor R1 that is controlled by the output S of the amplifier. The gain β of the amplifier is given by: 
     
         β=(R1=R2)/R2 
    
     When the reference voltage VREF is turned on, the output S is at zero volts. The transistor T1 is therefore not conductive and its equivalent resistance R1 is infinite. In practice, the gain of the amplifier is then very great (open loop operation). The amplifier therefore provides a very strong current that enables a very swift charging of the capacitor C. The output voltage Vout then increases and the transistor T1 becomes conductive. The equivalent resistance R1 becomes almost zero and the gain of the amplifier then tends gradually towards unity (closed loop-follower assembly operation). The reference voltage VREF is then seen again at output (Vout=VREF). 
     A more detailed drawing of a voltage reference generation circuit is given in FIG. 2. In the example of FIG. 2, the differential amplifier 1 is depicted as a CMOS assembly. It thus has a first P type transistor Tp1 and a second P type transistor Tp2 mounted as a current mirror. The transistor Tp1 has its gate connected to its drain and the gates of the two transistors Tp1 and Tp2 are connected together. The assembly also has a first N type transistor Tn1 and a second N type transistor Tn2. The transistors Tp1 and Tn1 are series-connected between the logic supply voltage Vdd and ground, and the transistors Tp2 and Tn2 are also series-connected between the logic supply voltage Vdd and ground. The gate of the first N type transistor Tn1 is the positive input of the differential amplifier and the gate of the second N type transistor Tn2 is the negative input of the differential amplifier. The output S of the differential amplifier is given by the common connection point of the transistors Tp2 and Tn2. The differential amplifier also has a third N type transistor Tn3 that is used as a current generator. The drain of this transistor Tn3 is connected in common to the sources of the transistors Tn1 and Tn2. 
     To control the differential amplifier on demand, two switching transistors Tp3 and Tn4 are provided. A first switching transistor, which is a P type transistor referenced Tp3, enables the switching over of the supply voltage Vdd to the sources of the transistors Tp1 and Tp2. Transistor Tp3 is controlled at its gate by the inverse of an enabling signal. This inverse signal is referenced /READ ENABLE. 
     A second N type switching transistor referenced Tn4 enables the connection of the source of the transistor Tn3 to ground. It is controlled at its gate by the enabling signal READ ENABLE. When this enabling signal is activated (READ ENABLE=1), the two switching transistors Tp3 and Tn4 are equivalent to short circuits and the differential amplifier is supplied by the logic supply voltage Vdd and ground. When this enabling signal is deactivated, the differential amplifier is no longer so supplied. 
     The positive input e+ of the differential amplifier receives the output VREF from a stable voltage source 3. In the example of FIG. 2, this voltage source 3 has a P type transistor Tp4 having its gate connected to its drain, biased by a resistor R3. Transistor Tp4 is supplied by the logic supply voltage Vdd when a switching transistor Tn5 is active. The transistor Tn5 is controlled by the standby signal STDBY of the circuit. When this signal is inactive, (STDBY=0) the transistor Tn5 is off and the circuit 3 is not supplied. 
     The resistor R3 is sized so that the output voltage VREF taken between the resistor R3 and the transistor Tp4 is stable with a voltage in the range of the threshold voltage of the transistor P. Thus in practice there is a voltage VREF of the order of 2.5 volts. This stable voltage VREF is applied to the positive input e+ of the differential amplifier. In the example, it is also used as a bias voltage of the transistor Tn3 (current source). Finally, an N type transistor Tn6 is provided and is connected between the output S of the differential amplifier and ground. Transistor Tn6 is controlled at its gate by the inverse signal /READ ENABLE for the enabling of the reference generation circuit. Transistor Tn6 sets the output S of the amplifier at zero so long as the generation circuit is not activated (READ ENABLE=0). 
     When the generator is activated (STDBY=1, READ ENABLE=1), the output S of the differential amplifier is initially at zero. The transistor T1 is therefore not conductive since its drain is at zero. The gate of the transistor Tn2 of the output arm of the differential amplifier is then at zero, and therefore, transistor Tn2 is off. Thus, all the current conducted by the transistor Tp2 of this arm is used to charge the output. No current is consumed by the transistor Tn2. When the generator is turned on, there is therefore a high current at the output that enables fast charging of a capacitive line (not shown). The output voltage S will therefore increase rapidly. As soon as the output voltage is fairly high, transistor T1 will become conductive, thus making the transistor Tn2 of the amplifier also conductive. The normal operation of the amplifier, in closed loop, is then recovered with a gain equal to unity. The level of the reference voltage VREF applied to the input e+ is then seen again at the output S. 
     In practice, it has been measured that the build-up time of the voltage Vout at output S of the amplifier is on the order of 200 nanoseconds once the READ ENABLE signal is activated. In contrast, with conventional voltage reference generator circuits, at least one microsecond is needed. Furthermore, the generator according to this embodiment of the invention gives a stable output voltage Vout. Indeed, if the temperature increases, the threshold voltage of the transistors tends to decrease. Hence, the reference voltage VREF tends to decrease. But since the threshold voltages of the N type and P type transistors of the amplifier also tend to decrease, the amplifier will compensate for the variation of the input voltage VREF. In a similar manner, variations of the threshold voltages of the transistors due to the manufacturing method are also compensated for by the amplifier. 
     The generator according to an embodiment of the present invention enables a reduction of the consumption of an integrated circuit, by consuming only when it is used. In point of fact, the voltage circuit 3 consumes continuously once the circuit is no longer in standby mode (STDBY=1), but the consumption of this circuit 3 is negligible. 
     Various improvements to the previously described embodiment of the present invention are shown in FIGS. 2 and 3. These improvements are aimed more particularly at the use of a reference generator to provide the read voltage to the cells of a memory. 
     If the memory capacity of a memory is high (namely with a large number of memory cells), it may be preferable to use a set 4 of output buffers (registers) 40 to 43 so that each one delivers the reference voltage Vout at the output of the reference generator to a group of cells (CGT0 to CGT3). This set then makes it possible to reduce the capacitance as seen from the output of the differential amplifier 1 of the generator. Each output buffer will then be such that the top transistor (connected to the supply voltage) is wide to give high current with the bottom transistor providing for the bias. In the example shown in FIG. 2, the top transistor is a native transistor and the bottom transistor is an enhanced transistor, both being N type transistors. 
     The output of each output buffer is applied to the associated group of cells. In the example, a respective transistor (50 to 53) is provided for each output buffer and controlled by the /READ ENABLE command to pull the lines CGT0 to CGT3 to the ground when the reading is not active. A set 4 of buffers of this kind is particularly useful for the application of the read voltage to a highly capacitive line, for it reduces the capacitance seen from the output of the generator. 
     An exemplary application to an E 2  PROM memory is shown in FIG. 3. It is recalled that, for these memories, a memory cell comprises a floating gate transistor TF and a selection transistor. This selection transistor is controlled at its gate by a signal for the selection of a word line (WLi) and connects a bit line BLO to the drain of the floating-gate transistor. Associated with a word line, another selection transistor Tsi is controlled at its gate by the word line selection signal (WLi) to switch an operational voltage, namely a reading, programming or erasure voltage, over to the gate. 
     In the recent structures of E 2  PROM memories, there is provision for dividing the memory into groups of bit lines. In the example shown, the memory comprises four groups: G0 (BL0 to BL7), G1 (BL8 to BL15), G2 (BL16 to BL23) and G3 (BL24 to BL31). A single group is accessed at a given time in read mode. It is the bit line decoder SELY associated with a gate circuit 6 that provides for the selection of this group. 
     In this structure, one control transistor TC per group and per word line has been associated to control the gate of the floating-gate transistor. Thus, for the word line WLi, there are the control transistors Tc0 to TC3, each respectively associated with a group G0 to G3. 
     All the control transistors of the first group are connected to one and the same control line CGT0. All the control transistors of the second group are connected to one and the same control line CGT1. The control lines CGT2 and CGT3 are each respectively associated with the group G2 and the group G3. 
     In an improved embodiment of the present invention, there is then provided an output buffer (40 to 43) for each group (G0 to G3) to apply the reference voltage Vout given by the generator according to the invention, to the control line of the associated group. In this example, the designator SV READ  is used to denote the voltage reference generators of FIGS. 1 or 2. However, it is also possible to directly apply the voltage Vout to all the control lines at the same time, without the output buffers 40 to 43. The use of output buffers is warranted when the capacitance seen from the reference generator is excessively great. 
     Although embodiments of the present invention have been described with reference to an application to E 2  PROM memories, this is not its only application. They can be used more particularly as reference generators to charge capacitive lines. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.