Voltage sequencing circuit for sequencing voltages to an electrical device
BACKGROUND OF THE INVENTION 
     Many semiconductor circuits can be damaged by the improper application of voltage to the devices thereof. Field Effect Transistor devices are particularly sensitive to the application of a substantial positive voltage without the application of a balancing negative voltage. However, these devices are not as sensitive in the case where application of a negative voltage is effected without a compensating positive voltage. It is, therefore, desirable to arrange for the application of a negative voltage to the circuit both before application of a positive voltage and after removal of the positive voltage. 
     In an FET integrated circuit memory which employs both negative and positive voltage sources, the negative voltage is used to reverse bias the FET substrate. If the negative voltage is not available before the positive voltage, excessive current drain may occur in the FET device. Excessive current drain in the FET integrated circuit device can permanently damage the device. In voltage/time sensitive circuits it is often necessary to sequence the voltages to prevent spurious oscillations or erroneous timing signals from occurring. 
     Past attempts to resolve this problem have required that both the positive and negative voltages be derived from the same winding of a transformer. This solution is adequate for most applications if the failure occurs in the transformer or in the supply to the transformer, however, it does not ensure protection if the voltage failure occurs at some other point in the circuit. Furthermore, even if the same winding of a transformer is used for all circuit voltages one is still not assured that one voltage will not rise before another. 
     SUMMARY OF THE INVENTION 
     The present invention obviates the problems of the prior art and positively assures the sequencing of the application of voltages so as to protect the semiconductor circuit from improperly applied voltages or eliminate spurious timing signals which previously resulted from the improper sequencing of supply voltages. The invention provides circuitry which facilitates the application of the negative voltage to the semiconductor circuitry prior to the application and subsequent to the removal of the positive voltage. 
     The invention provides a voltage controlled semiconductor switch in the voltage supply lead furnishing the voltage to be last applied and first removed from an electrical device or circuit. A charge storage device receives charging current from the voltage source to be first applied to and last removed from the electrical device or circuit. The charge storage device enables the semiconductor switch only after the first applied voltage source is supplying a voltage of a predetermined level to the electrical device or circuit. A charge storage device in the supply lead from the first applied and last removed voltage source supplies voltage to the electrical device or circuit for a short time after interruption of the power supply. 
     The foregoing and other features and advantages of the invention will be apparent from the following more particular description of the preferred embodiment of the invention, as illustrated in the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In order that the full advantage of the invention may be obtained preferred embodiments thereof are described in detail hereinafter with reference to the accompanying drawings in which: 
     FIG. 1 is a block diagram showing the environment of the invention; 
     FIG. 2 shows a preferred embodiment of the invention; 
     FIG. 3 is a graphical representation of the voltage wave forms during the application and removal of the supply voltages; and 
     FIG. 4 shows an alternative embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Supply voltages are supplied to an electrical device or circuit 1 through power sequencing circuit 2. Electrical device or circuit 1 may be of any type, however, the power sequencing circuit 2 is most useful where the electrical device or circuit 1 is a FET integrated circuit digital memory or a voltage/time sensitive circuit. 
     The power sequencing circuit 2 is more fully shown in FIG. 2. First variable voltage level V 1  is applied to the emitter terminal of transistor T 1  via conductor 3. The collector terminal of transistor T 1 , is connected to a first output terminal 5 which supplies a voltage level V 1  out to the electrical device or circuit 1 (shown in FIG. 1). A second variable voltage level V 2  is applied via conductors 4 and 7 to the timing control network 8 and to terminal 6 via diode D 2 . The output terminal of the timing control network 8 is connected to the base of transistor T 2 . The emitter of transistor T 2  is connected to the base of transistor T 1 . The collector of transistor T 2  is grounded through resistor R 3 . The base of transistor T 2  is connected by resistor R 2  to the emitter of transistor T 1 . 
     The operation of the sequencing circuit will now be described with reference to the voltage curves shown in FIG. 3. 
     In the initial state, no voltage is available to either of the input terminals of conductors 3 or 4 and, therefore, both V 1  out and V 2  out are at zero potential. If the potential on conductor 3 is raised, V 1  out will remain at zero potential so long as conductor 4 remains at zero potential since transistor T 1  is still turned off. Transistor T 1  can only be turned on when transistor T 2  is conducting. To turn transistor T 2  on, a sufficiently high negative voltage must be applied to terminal 4. This ensures that no voltage can be supplied to terminal 5 unless a countervailing voltage has already been supplied to terminal 6. When a negative voltage is applied to conductor 4, the voltage V 2  out is brought up as shown at time T 1  in FIG. 3. At the same time capacitor C 1  starts to charge through resistor R 1  to conductor 4. When the voltage at node 9 is sufficiently negative with respect to the voltage at the base of transistor T 2  to reverse bias Zener diode Z 1  transistor T 2  will turn on. When transistor T 2  becomes conductive current flows in the base of transistor T 1  thus turning on transistor T 1  and thereby supplying a positive potential to terminal 5 as shown at time T 2  in waveform V 1  out in FIG. 3. Resistor R 2  is inserted between the emitter electrode of transistor T 1  and the base electrode of transistor T 2  to compensate for any leakage in Zener diode Z 1  and ensures that transistor T 2  is off when voltage V 2  is off. 
     Zener diode Z 1  is chosen with a reverse break-down potential which is two or three volts less than the voltage drop between conductors 3 and 4 minus the voltage drop across the resistor R 1  and the emitter-base junctions of the transistors T 1  and T 2 . In the present embodiment this voltage is approximately 4 to 5 volts less than the maximum voltage difference between conductors 3 and 4. 
     When the voltage V 2  at conductor 4 falls to a level V T  which is insufficient to draw current through Zener diode Z 1 , the transistor T 2  will turn off. When transistor T 2  becomes non-conductive it immediately turns off transistor T 1  and disconnects voltage source V 1  at conductor 3 from output terminal 5 thereby dropping voltage V 1  out to zero as shown at time T 4  in FIG. 3. As voltage V 2  drops the diode D 2  is reverse-biased and the output voltage V 2  out decays exponentially as the load draws current from capacitor C 2 . The voltage necessary to drive the load is the minimum operating voltage V M . Therefore, the turn-off delay in the circuit is the difference between time T 4  and T 5  which is the time it takes for the voltage to drop from the threshold voltage V t  to the minimum operating voltage V m . This delay is determined by the size of the capacitor C 2  and the current drain at terminal 6. 
     In one embodiment of the invention the following devices and parameters were used: 
     
         ______________________________________                                    
Voltage V.sub.1                                                           
               +8.5 Volts                                                 
Voltage V.sub.2                                                           
               -12.0 Volts                                                
Resistor R.sub.1                                                          
               1 KOHMS                                                    
Resistor R.sub.2                                                          
               10 KOHMS                                                   
Resistor R.sub.3                                                          
               120 OHMS                                                   
Capacitor C.sub.1                                                         
               100 Microfarads                                            
Capacitor C.sub.2                                                         
               150 Microfarads                                            
Zener diode Z.sub.1                                                       
               16 Volts                                                   
Diode D.sub.1  Standard Switching Diode                                   
Diode D.sub.2  Standard Switching Diode                                   
Transistor T.sub.1                                                        
               3 AMP Power Transistor                                     
Transistor T.sub.2                                                        
               0.5 AMP Medium Power Transistor                            
______________________________________                                    
 
    
     In some situations, it may be desirable or necessary to ensure that a negative voltage is applied to the electrical device or circuit 1 only when a positive voltage is also applied to the device or circuit 1. This can be readily accomplished by reversing the polarity of the power sequencing circuit 2. This reversed polarity circuit is shown in FIG. 4. For ease of understanding elements of FIG. 4 corresponding to elements in FIG. 2 have been designated with the same reference characters except for the addition of the suffix a. In operation, the circuit of FIG. 4 would operate in the same manner as the circuit shown in FIG. 2 except that the positive and negative voltages at the output would be transposed so that V 1  out shown in FIG. 3 corresponds to V 2a  out and V 2  out in FIG. 3 corresponds to V 1a  out. 
     Other modifications to the invention are readily apparent to one skilled in the art. For example, one could easily control the application of several other voltages simply by duplicating the power sequencing circuit 2 for each voltage to be controlled. 
     It is also possible to construct a circuit with two or more negative voltages controlling the application of a positive voltage. In this embodiment, additional transistors such as T 1  are cascaded, one for each additional negative potential. In other words, the collector of one transistor feeds the emitter of the next transistor. Suitable power sequencing networks comprised to T 2 , timing control network 8, D 2  and C 2  control the base of each T 1  transistor. 
     Further modifications and variations of the invention are readily apparent to one skilled in the art. 
     While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.