Reference Voltage Generator

This invention relates to circuits for generating reference voltages. More particularly, the invention relates to a reference voltage generator (RVG) circuit whose output voltage varies inversely with its supply voltage. For the purposes of this invention, two voltages Va and Vb will be said to vary inversely if they can be related by the equation Vb=C-M Va where C is a constant and M is a positive constant.

Many electronic circuits make use of reference voltage generators in order to maintain a desired operating state. In most cases, the RVG's output voltage is required to remain essentially constant, independent of its supply voltage, but other RVGs having different relationships between the supply voltage and the output voltage are also possible.

RVGs whose output voltages vary inversely with supply voltage have some very useful applications. For example, the performance of many circuits depends critically on the supply voltage. Increasing the supply voltage has the undesirable effect of increasing the power consumed by the circuit, but coupled with this increased power dissipation is the desirable effect of increasing the speed of the circuit. Thus, in some voltage-controlled devices, it is desirable to increase or decrease the voltage supplying the device as the main supply voltage decreases or increases respectively. In this way, the variation of power dissipation in the device due to supply voltage fluctuations is minimized, and an optimum operating power level is maintained.

In view of the foregoing, it is an object of this invention to provide an RVG circuit whose output voltage decreases as the supply voltage increases, and whose output voltage increases as the supply voltage decreases.

This and other objects of the invention are accomplished by a circuit which includes a voltage divider, an inverting amplifier, and an optional level shifter. The voltage divider produces an intermediate voltage which is a predetermined fraction of the supply voltage. This intermediate voltage is fed to an inverting amplifier whose properties are such that its output voltage varies inversely with its input voltage over a certain range of supply voltages. The output of the inverting amplifier is fed to a level shifter which shifts the output voltage to a level which is more convenient for the end user and increases the output current available from the RVG.

Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.

FIG. 1 is a schematic block diagram of a preferred embodiment of the invention;

FIG. 2 is a schematic drawing of a Complementary Metal-Oxide-Semiconductor (CMOS) embodiment of the invention;

FIG. 3 is a graph showing the transfer characteristics of an inverter for two different supply voltages; and

FIG. 4 is a graph showing the output voltage of the RVG as a function of the supply voltage.

The block schematic diagram, FIG. 1, shows how the three parts of the circuit are interconnected to form the RVG. Voltage divider 11 produces an intermediate voltage V.sub.1 which is a fixed fraction of the supply voltage V.sub.cc. This intermediate voltage is passed on to inverting amplifier 12 whose characteristics are such that a simultaneous decrease of its input voltage V.sub.1 and its supply voltage V.sub.cc will result in an increase in its output voltage V.sub.2 over a certain supply voltage range. Similarly, a simultaneous increase in V.sub.1 and V.sub.cc will result in a decrease in V.sub.2 over a certain supply voltage range. The supply voltage range over which an inverse relationship exists between V.sub.2 and V.sub.cc will be termed the "operating range." The voltage V.sub.2 is passed on to the input of level shifter 13 which acts as an output buffer and voltage level shifter. The reference voltage V.sub.REF is obtained at the output of the level shifter.

FIG. 2 shows a schematic diagram of a CMOS embodiment of the RVG. The n-channel transistors 201, 202, and 203 form the voltage divider 11. If transistors 201 through 203 are identical, the intermediate voltage V.sub.1 is approximately one third of the supply voltage V.sub.cc. Other values of V.sub.1 can be obtained by changing the number or the geometry of transistors in the voltage divider. In an alternative embodiment, the voltage divider could be implemented as a number of resistors in series. However, in Metal-Oxide-Semiconductor technology, the preferred embodiment of the voltage divider is in the form of a series connection of transistors.

Voltage V.sub.1 is fed to the input of the inverting amplifier formed by load transistor 204, drive transistor 205, and resistor 206. Resistor 206 provides negative feedback to stabilize the gain of the inverting amplifier. Typical input/output characteristics of the inverting amplifier shown in FIG. 3. The two curves represent the inverters output voltage V.sub.2 as a function of its input voltage V.sub.1 for two different values of supply voltage V.sub.cc. Observe that input voltages V.sub.in1 and V.sub.in2, with V.sub.in1 less than V.sub.in2, produce output voltages V.sub.out1 and V.sub.out2 with V.sub.out1 greater than V.sub.out2. Unlike the operation of a conventional inverting amplifier, this inversion of voltage variation is not simply a consequence of a shift in one direction of the input voltage leading to a shift in the opposite direction of the output voltage; it is a combined effect due to the simultaneous change in input and supply voltages. Implicit in the argument is the fact that the change in V.sub.1 from V.sub.in1 to V.sub.in2 is a result of the change in V.sub.cc from V.sub.cc1 to V.sub.cc2. Thus, the voltage V.sub.2 depends only on V.sub.cc. If the supply voltage V.sub.cc falls below a certain critical level, or rises above another critical level, the desired inverse relationship between voltages V.sub.1, V.sub.2, and V.sub.cc will not be retained. The limits between which this inverse relationship exists define the operating range.

Having produced voltage V.sub.2 which varies inversely with V.sub.cc, a desirable enhancement of the circuit is the addition of a level shifting stage 13. This stage translates the voltage V.sub.2 to a more convenient value V.sub.REF and, in addition, increases the amount of output current that the RVG can supply. Transistors 207 through 211 form such a level shifter. The level shifting stage 13 comprises a buffer amplifier made up of transistors 207 and 208, a current mirror made up of transistors 207 and 209, and load transistors 210 and 211. The p-channel transistor 209 is chosen to be larger than transistor 207 so that current amplification is obtained. Two load transistors, 210 and 211, are shown in FIG. 2, but more or less than this number can be used. The lowest output voltage obtainable from the circuit is limited to the number of load transistors multiplied by their threshold voltage. If the optional level shifter is not used, V.sub.2 is taken to be the reference voltage.

The overall operation of the circuit is summarized in FIG. 4 which shows how the reference voltage V.sub.REF varies with supply voltage V.sub.cc. The FIG. shows that the operating range is in the supply voltage range between V.sub.x and V.sub.y.

Thus, in the operating range, the desired inverse relationship between output voltage V.sub.REF and supply voltage V.sub.cc is obtained. One skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments which are presented for purposes of illustration and not of limitation.