Abstract:
A circuit configuration generates an internal supply voltage for integrated circuits at two different levels, each of which are constant. The selection of the levels is made solely on the basis of the magnitude of an external supply voltage. As a result, it is possible to switch back and forth between an operating mode, in which the internal supply voltage is at a usual value for operation, and a test mode, in which the internal supply voltage is at an elevated value. The invention is used particularly in semiconductor memories.

Description:
BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     The invention relates to a circuit configuration for generating an internal supply voltage, with which integrated circuits can be operated. 
     As the scale of integration in integrated circuits increases, the dimensions of the integrated components become smaller and smaller. It is highly important that the space requirement per memory cell be slight, especially in semiconductor memories, in which the storage capacity and thus the number of memory cells are being increased more and more. 
     However, the increased scale of integration has meant that the electrical field intensity at the individual components of the integrated circuit, for instance at the gate oxides of transistors, is greater in comparison with memories having a lesser scale of integration. Thus the stress exerted on the components also rises, leading to a growth in failure rates. In order to avoid that, the cell fields of semiconductor memories are operated with an internal supply voltage. As a rule, it is below the external supply voltage with which the external circuit located outside the cell fields is operated. For instance, in the case of the cell field, the voltage of 5 V of the outer circuit is reduced to the internal supply voltage of 3.3 V. Various circuits for reducing the voltage are known. 
     The dependency of the service life of the cell field on the internal supply voltage being applied and on the resultant electrical field can be exploited in a so-called burn-in test. In it, the cell field is operated at a higher voltage than the internal supply voltage used for proper operation. The resultant failures of the memories make quality control possible. 
     Only the external supply voltage can be applied to the semiconductor memory from outside. The internal supply voltage, which should be as constant as possible and as independent as possible of external interfering factors, is generated by a voltage generator provided specifically for that purpose. Since the internal supply voltage is regulated to a certain value by the voltage generator, increasing the external supply voltage does not at the same time lead to an increase in the internal supply voltage. It is therefore not possible to perform the burn-in test with conventional voltage generators. 
     A voltage generator which furnishes a regulated, constant internal supply voltage as long as the external supply voltage is below a certain value, is known from German Published, Non-Prosecuted Patent Application DE 42 26 048 A1. If the external supply voltage exceeds that certain value, then the internal supply voltage rises with the external supply voltage. That is attained due to the fact that either a constant comparison voltage or the external supply voltage, depending on whether the external supply voltage is below or above that certain value, is supplied to a closed control loop that generates the internal supply voltage. 
     The disadvantage of that voltage generator is that a relatively complicated and expensive device is needed for the burn-in test in which semiconductor memories with that voltage generator are to be tested. That is because in order to expose the semiconductor memory to a defined stress, the external supply voltage must be kept at a very specific value, which to that end should be as constant as possible. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a circuit configuration for generating an internal supply voltage, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and with which a defined, disproportionately increased internal supply voltage is furnished in a simple way. 
     With the objects of the invention in view, there is also provided a circuit configuration, comprising a device for generating an internal supply voltage derived from an external supply voltage for operating an integrated circuit; and a reference voltage generator for detecting a voltage proportional to the external supply voltage and for generating a reference voltage as a function of a magnitude of the voltage proportional to the external supply voltage, to control the device for generating the internal supply voltage, the reference voltage generator generating at least two constant voltage values of the reference voltage. 
     The invention has the advantage of ensuring that the disproportionately increased internal supply voltage is not sensitive to fluctuations in the external supply voltage. Testing the semiconductor memories into which the circuit configuration of the invention is integrated makes only slight demands of a testing device, for instance for performing the burn-in test. 
     In accordance with another feature of the invention, the reference voltage generator has at least two voltage sources. 
     In accordance with a further feature of the invention, the reference voltage generator has a switching transistor connected through a circuit node in series with a diode chain. 
     In accordance with an added feature of the invention, at least one of the voltage sources has a double current mirror circuit. 
     In accordance with an additional feature of the invention, there is provided a circuit device controllable as a function of a potential at the circuit node for causing one of the voltage sources to assume the reference voltage. 
     In accordance with yet another feature of the invention, the diode chain has ends, and the switching transistor has a channel side connected to the external supply voltage and to one of the ends of the diode chain. 
     In accordance with a concomitant feature of the invention, the internal supply voltage is proportional to a voltage value of the reference voltage. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a circuit configuration for generating an internal supply voltage, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are graphs showing the course of an internal supply voltage in known circuit configurations; 
     FIG. 2 is a schematic and block diagram showing one possible embodiment of a circuit configuration of the invention; and 
     FIG. 3 is a graph showing the course of an internal supply voltage and a reference voltage in the circuit configuration of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawings in detail and first, particularly, to FIGS. 1A and 1B thereof, there are seen typical courses of an internal supply voltage in voltage generators of the prior art, plotted as a function of an external supply voltage. After a linear rise in the internal supply voltage, that voltage remains constant within a certain range of the external supply voltage. Beyond a certain value of the external supply voltage, the internal supply voltage follows along with the external supply voltage. It is thus possible that beyond that value, the internal supply voltage is either identical to the external supply voltage, as shown in FIG. 1A, or rises linearly with the external supply voltage, as shown in FIG.  1 B. 
     FIG. 2 shows one possible embodiment of a circuit configuration of the invention for generating an internal supply voltage V ccint . A reference voltage V Reference  which is generated by a reference voltage generator RG, and an external supply voltage V CCext , are supplied to a comparator VE. 
     An output of the comparator VE is connected to a control terminal of a controllable resistor P 10 . The controllable resistor P 10  is also connected to the external supply voltage V ccext  and to a terminal at which the internal supply voltage V ccint  can be picked up. The external supply voltage V CCext  is compared with the reference voltage V Reference , and the controllable resistor P 10  is triggered in such a way that the internal supply voltage V CCint  assumes either the value of the reference voltage V Reference  or a value that is proportional to the reference voltage V Reference . 
     The reference voltage generator RG has a first voltage source VREF 1  and a second voltage source VREF 2 . Both voltage sources VREF 1 , VREF 2  are connected to the external supply voltage V CCext . Each of the voltage sources may, for example, be made up of one double current mirror circuit. An output of the first voltage source VREF 1  is connected to one channel-side terminal of a first switching transistor P 1 . In the same way, an output of the second voltage source VREF 2  is connected to one channel-side terminal of a second switching transistor N 1 . The other channel-side terminals of the switching transistors N 1  and P 1  are interconnected and form an output of the reference voltage generator RG. The reference voltage V Reference  is present at this output. 
     Control terminals of the switching transistors N 1  and P 1  are connected to one another and to an output of an inverter INV. An input of the inverter INV is connected to a first circuit node K 1 . A resistor R is located between the circuit node K 1  and a reference potential V SS . This resistor R may be formed of a field-effect transistor, for instance. The channel side of a third switching transistor P 2  is connected between the external supply voltage V CCext  and the first circuit node K 1 . A control input of the third switching transistor P 2  is connected to a second circuit node K 2 . A diode chain DK is located between the second circuit node K 2  and the reference potential V SS . The diode chain DK includes at least one diode. In the present exemplary embodiment, the diode chain includes six transistors (P 3 -P 8 ) connected as diodes. 
     The second circuit node K 2  is also connected through the channel side of a fourth switching transistor P 9  to the external supply voltage V CCext . A control contact of the fourth switching transistor P 9  is connected to the first voltage source VREF 1 . A voltage which is furnished in the first voltage source VREF 1  and is proportional to the external supply voltage V CCext  is applied to the control terminal of the fourth switching transistor P 9 . 
     The mode of operation of the circuit configuration of the invention will be explained below in terms of two cases, which result from different values for V CCext . 
     If the magnitude of the supply voltage V CCext  is below a certain limit value, such as the usual operating voltage of the memory, then the circuit node K 2  is at low potential. The switching transistor P 2  switches through and the circuit node K 1  assumes a higher potential than the reference potential V SS . This is the same as saying that a signal value HIGH is present at the input of the inverter INV. The output of the inverter INV consequently assumes a signal value LOW, and as a result the first switching transistor P 1  switches through while the second switching transistor N 1  is blocked. The reference voltage V Reference  thus assumes the value of the voltage of the first voltage source VREF 1 . 
     If the external supply voltage V CCext  continues to increase, then the potential at the circuit node K 2  increases as well. If the external supply voltage V CCext  reaches the limit value, then the switching transistor P 2  blocks and the circuit node K 1  assumes a potential that is only slightly above the reference potential V SS . This is equivalent to a signal value LOW at the input of the inverter INV. The output of the inverter INV becomes HIGH. Thus the second switching transistor N 1  switches through, and the first switching transistor P 1  blocks. The reference voltage V Reference  then assumes the value of the voltage of the second voltage source VREF 2 . 
     The determination as to whether the reference voltage V Reference  is determined by the first voltage source VREF 1  or by the second voltage source VREF 2  accordingly depends solely on the magnitude of the external supply voltage V CCext . The first voltage source VREF 1  can thus be constructed in such a way that the reference voltage V Reference  assumes a value which is suitable for regulating the internal supply voltage V CCint  to the usual value for operation of the memory field, which is done through the comparator VE and the controllable resistor P 10 . Accordingly, the second voltage source VREF 2  can be constructed in such a way that the internal supply voltage V CCint  assumes a higher value than is usual for operation of the cell field. This disproportionately increased internal supply voltage is then used to perform a burn-in test. 
     The circuit configuration of the invention thus makes it possible for two different voltage levels of the internal supply voltage V CCint  to be selected solely by way of the external supply voltage V CCext . 
     FIG. 3 shows the dependency of the internal supply voltage V CCint  and the reference voltage V Reference  on the external supply voltage V CCext  in the circuit configuration of the invention. The internal supply voltage V CCint  assumes two defined, different values, depending on the magnitude of the external supply voltage V CCext .