Power supply switch reference circuitry

Abstract

An apparatus and method for switching between two power supplies, a primary power supply and a secondary power supply. The present invention generates a first reference voltage using the voltage of the primary power supply and the secondary power supply, wherein the primary power supply voltage is variable. The present invention also generates a second reference voltage based on the voltage of the primary power supply. The first and second reference voltages each have a different slope and the crossing point between these two reference voltages indicate that a switch between the primary power supply and the secondary power supply should occur.

What is claimed is:  
     
       1. A power supply switching circuit for switching power supplies to a load, the power supply switching circuit comprising: 
       a switching circuit connected to two power supplies supplying two voltages, a primary power supply and a secondary power supply, the primary power supply providing a first voltage and the secondary power supply providing a second voltage, the first voltage being a variable voltage wherein the switching circuit is controlled by a control signal, the control signal causing one of the two power supplies to be connected to the load, wherein one of the two voltages is connected to the load;  
       a reference circuit including:  
       means for generating a first reference voltage utilizing the first voltage and a switched voltage, the switched voltage being switched between the first voltage and the second voltage, wherein the first voltage reference has a first slope; and  
       means for generating a second reference voltage utilizing the first voltage, wherein the second reference voltage has a second slope that is selected to provide a crossing point with the first reference voltage,  
       wherein the switching circuit switches between the two power supplies in response to an occurrence of a crossing point between the first reference voltage and the second reference voltage.  
     
     
       2. The power supply switching circuit of  claim 1 , wherein both the first reference voltage and the second reference voltage are lower than the secondary power supply voltage. 
     
     
       3. The power supply switching circuit of  claim 2 , wherein the secondary power supply is a battery power supply. 
     
     
       4. The power supply switching circuit of  claim 1 , wherein both the first and second reference voltages are less than the first voltage. 
     
     
       5. The power supply switching circuit of  claim 1 , wherein the slope of the second reference voltage is selected as follows:          R   2         R   1     +     R   2                       
       wherein R 1  and R 2  are resistor values. 
     
     
       6. The power supply switching circuit of  claim 2 , wherein the secondary power supply voltage is greater than the primary power supply voltage. 
     
     
       7. A power supply switching circuit for switching power supplies to a load, the power supply switching circuit comprising: 
       a switching circuit connected to two power supplies supplying two voltages, a primary power supply and a secondary power supply, wherein the primary power supply generates a first voltage and the secondary power supply generates a second voltage, wherein the switching circuit is controlled by control signal, the control signal causing one of the two power supplies to be connected to the load, wherein one of the two voltages is connected to the load;  
       a comparator circuit having a first input connected to a first input reference voltage, a second input connected to a second reference voltage, and an output, connected to the switching circuit, wherein the control signal is generated at the output;  
       a reference voltage circuit including:  
       a first circuit comprising a first transistor having a base, a collector, and an emitter, the collector being  having a connection that is switched between the first voltage and the second voltage in response to the switching circuit connecting one of the two power supplies to the load, a second transistor having a gate, a source, and a drain, and a resistor, having a first end and a second end wherein the drain of the second transistor is connected to the first end of the resistor and the second end of the resistor is connected to the emitter of the first transistor, wherein the first reference voltage is equal to voltage drop across the second transistor  present with respect to the lower power supply voltage, wherein the drain of the second transistor is connected to the first input of the comparator circuit; and  
       a second circuit comprising a first resistor and a second resistor connected in series, wherein the first resistor has a first end connected to the first voltage and a second end connected to a first end of the second resistor, wherein the second reference voltage is equal to a voltage drop across the second resistor and wherein the first end of the second transistor  resistor is connected to the second input of the comparator circuit.  
     
     
       8. The power supply switching circuit of  claim 7 , further comprising a current source including: 
       a first transistor and a second transistor, each transistor having a source, a drain, and a gate, the sources of the first transistor and the second transistor being connected to the first voltage, the gate of the first transistor and the second transistor being connected to the drain of the second transistor;  
       a third transistor and a fourth transistor, each transistor having a source, a drain, and a gate, the drain of the third transistor being connected to the drain of the first transistor, the drain of the fourth transistor being connected to the drain of the second transistor, and the gates of the third transistor and the fourth transistor being connected to the drain of the third transistor; and  
       a fifth transistor and a sixth transistor, each transistor having a source, a drain, and a gate, the drain of the fifth transistor being connected to the source of the third transistor, the drain of the sixth transistor being connected to the source of the fourth transistor, the sources of the fifth transistor and the sixth transistor being connected to a lower power supply voltage, and the gates of the fifth transistor and the sixth transistor being connected to the drain of the fifth transistor,  
       wherein the gate of the fifth transistor in the current source is connected to a gate of the second transistor in the reference circuit and wherein the source of the sixth transistor is connected to the lower power supply by a resistor having a first end connected to the source of the sixth transistor and a second end connected to the lower power supply voltage.  
     
     
       9. The power supply switching circuit of  claim 7 , wherein in the second circuit, the second end of the second resistor is connected to the lower power supply voltage by a transistor having a drain connected to the second end of the resistor and a source connected to the lower power supply voltage, wherein the transistor has a gate controlled by the first reference voltage. 
     
     
       10. The power supply switching circuit of  claim 7 , wherein the first transistor is a PNP transistor. 
     
     
       11. The power supply switching circuit of  claim 7 , wherein the first transistor is an NPN transistor. 
     
     
       12. The power supply switching circuit of  claim 10 , wherein the second transistor is an N-channel MOSFET. 
     
     
       13. The power supply switching circuit of  claim 9 , further comprising a current source including a first transistor, a second transistor, a third transistor, and a fourth transistor, wherein the first reference voltage is generated as followings  follows:          V   rref     =       V   CC1     -     V   BE     -     K                   (     kT   q     )                     (     Rb   Rs     )                   ln                       S   4          S   1           S   2          S   3                             
       wherein V ref  is the first reference voltage measured with respect to the lower power supply voltage, V cc1  is the first voltage, V BE  is a base emitter voltage of the first transistor in the first circuit, S 1  is the device size of the first transistor in the current source, S 2  is the device size of the second transistor in the current source, S 3  is the device size of the third transistor in the current source, S 4  is a device size of the fourth transistor in the current source, kT/q is the thermal voltage, Rb is the resistor in the first circuit, and Rs is the resistor in the current source. 
     
     
       14. The power supply switching circuit of  claim 7 , further comprising: 
       a current source including:  
       a first transistor and a second transistor, each transistor having a source, a drain, and a gate, the sources of the first transistor and the second transistor being connected to the first voltage and the gate of the first transistor and the second transistor being connected to the drain of the second transistor; and  
       a third transistor and a fourth transistor, each transistor having a source, a drain, and a gate, the drain of the third transistor being connected to the drain of the first transistor, the drain of the fourth transistor being connected to the drain of the second transistor, the sources of the third transistor and the fourth transistor being connected to the lower power supply, and the gates of the third transistor and the fourth transistor being connected to the drain of the third transistor;  
       wherein the gate of the third transistor in the current source is connected to a gate of the second transistor in the reference circuit.  
     
     
       15. The power supply switching circuit of  claim 14 , wherein in the second circuit, the second end of the second resistor is connected to the lower power supply voltage by a transistor having a drain connected to the second end of the resistor and a source connected to the lower power supply voltage, the transistor having a gate controlled by the reference voltage. 
     
     
       16. The power supply switching circuit of  claim 15 , wherein the first reference voltage is determined as follows:          V   ref     =       V   CC1     -     V   BE     -     K                   (     kT   q     )                     (     Rb   Rs     )                   ln                       S   4          S   1           S   2          S   3                             
       wherein V ref  is the first reference voltage, V cc1  is the first voltage, V BE  is a base emitter voltage of the first transistor in the first circuit, S 1  is the device size of the first transistor in the current source, S 2  is the device size of the second transistor in the current source, S 3  is the device size of the third transistor in the current source, S 4  is a device size of the fourth transistor in the current source, kT/q is the thermal voltage, Rb is the resistor in the first circuit, and Rs is the resistor in the current source. 
     
     
       17. The power supply switching circuit of  claim 16 , wherein the second reference voltage is determined as follows:          V   a     =         R   2         R   1     +     R   2              V   cc1                       
       wherein V a  is the second reference voltage, R 1  is a resistor value of the first resistor in the second circuit, R 2  is a resistor value of the second resistor in the second circuit, and V cc  is the first voltage. 
     
     
       18. A power supply switching circuit for switching power supplies to a load, the method  power supply switching circuit comprising: 
       a switching circuit connected to two power supplies, a primary power supply and a secondary power supply, wherein the switching circuit causes one of the two power supplies to be connected to the load;  
       a circuit including:  
       generating means for generating a first reference voltage utilizing the first voltage and a switched voltage, the switched voltage being switched between the first voltage and the second voltage, wherein the first voltage reference has a first slope; and  
       generating means for generating a second reference voltage utilizing the first voltage, wherein the second reference voltage has a second slope that is selected to provide a crossing point with the first reference voltage,  
       wherein the switching means for switching circuits switches between the two power supplies in response to an occurrence of a crossing point between the first reference voltage and the second reference voltage.  
     
     
       19. A method for switching between a primary power supply and a secondary power supply, wherein the primary power supply voltage provides a first voltage and the secondary power supply voltage provides a second voltage, the method comprising: 
       generating a first reference voltage utilizing the first voltage and a switched voltage, the switched voltage being switched between the first voltage and the second voltage, wherein the first reference voltage has a first slope;  
       generating a second reference voltage utilizing the first voltage, wherein the second reference voltage has a second slope, wherein the second slope is selected to provide a crossing point with the first reference voltage; and  
       switching between the primary and secondary power supplies in response to an occurrence of the crossing point between the first reference voltage and the second reference voltage.  
     
     
       20. A switching reference circuit for selecting a first power supply or a second power supply to power a load, the switching reference circuit comprising: 
       
         first, second, and third supply nodes;  
       
       
         a current source operable to generate a constant source current;  
       
       
         a first transistor having a first node coupled to the first supply node, a second node, and a control node coupled to the second supply node;  
       
       
         a first impedance element having a first node coupled to the second node of the first transistor and having a reference node for providing a first reference voltage;  
       
       
         a second transistor having a first node coupled to the reference node of the impedance element, a second node coupled to the third supply node, and a control node coupled to the current source, the second transistor operable to conduct a constant current that is proportional to the constant source current; and  
       
       
         a voltage divider having a first node coupled to the second supply node, a second node coupled to the third supply node, and a reference node for providing a second reference voltage such that the first and second reference voltages select the first power supply when the first reference voltage is greater than the second reference voltage and select the second power supply when the second reference voltage is greater than the first reference voltage. 
       
     
     
       21. The switching reference circuit of  claim 20  wherein the first supply node is operable to be directly coupled to the second supply node. 
     
     
       22. The switching reference circuit of  claim 20  wherein the first transistor comprises a bipolar transistor, the first node of the bipolar transistor comprises a collector, the second node comprises an emitter, and the control node comprises a base terminal. 
     
     
       23. The switching reference circuit of  claim 20  wherein the first transistor comprises an NPN bipolar transistor, the first node of the NPN bipolar transistor comprises a collector, the second node comprises an emitter, and the control node comprises a base. 
     
     
       24. The switching reference circuit of  claim 20  wherein: 
       
         the first supply node is the same as the second supply node; and  
       
       
         the first transistor comprises a PNP bipolar transistor, the first node of the PNP bipolar transistor comprises a collector, the second node comprises an emitter, and the control node comprises a base terminal. 
       
     
     
       25. The switching reference circuit of  claim 20  wherein the impedance element comprises a resistor. 
     
     
       26. The switching reference circuit of  claim 20  wherein the first node of the second transistor comprises a drain, the second node comprises a source, and the control terminal comprises a gate. 
     
     
       27. The switching reference circuit of  claim 20  wherein the voltage divider comprises: 
       
         a second impedance element having a first node coupled to the second supply node and a second node coupled to the reference node of the voltage divider; and  
       
       
         a third impedance element having a first node coupled to the reference node of the voltage divider and a second node coupled to the third supply node. 
       
     
     
       28. The switching reference circuit of  claim 20  wherein the voltage divider comprises: 
       
         a second impedance element having a first node coupled to the second supply node and a second node coupled to the reference node of the voltage divider;  
       
       
         a third impedance element having a first node coupled to the reference node of the voltage divider and having a second node; and  
       
       
         a transistor having a first node coupled to the second node of the third impedance element, a second node coupled to the third supply node, and a control node coupled to the first reference voltage. 
       
     
     
       29. The switching reference circuit of  claim 20 , further comprising a start-up circuit coupled to the current source. 
     
     
       30. The switching reference circuit of  claim 20  wherein: 
       
         the first supply node is operable to be coupled to the first power supply; and  
       
       
         the second supply node is operable to be coupled to the first power supply when the first reference voltage is greater than the second reference voltage and to the second power supply when the second reference voltage is greater than the first reference voltage. 
       
     
     
       31. A method for switching between a primary power supply that provides a first voltage and a secondary power supply that provides a second voltage, the method comprising: 
       
         generating a constant current;  
       
       
         generating from the first voltage and the constant current a first reference voltage having a first slope;  
       
       
         generating from the first voltage a second reference voltage having a second slope that is different than the first slope; and  
       
       
         switching between the primary and the secondary power supplies in response to a comparison of the first and second reference voltages. 
       
     
     
       32. The method of  claim 31  wherein generating the first reference voltage comprises generating the first reference voltage from a voltage that is switched between the first and the second voltages. 
     
     
       33. The method of  claim 31  wherein: 
       
         first reference voltage has the first slope with respect to a third voltage; and  
       
       
         the second reference voltage has the second slope with respect to the third voltage. 
       
     
     
       34. The method of  claim 31  wherein: 
       
         the first reference voltage has the first slope with respect to the first voltage; and  
       
       
         the second reference voltage has the second slope with respect to the first voltage. 
       
     
     
       35. The method of  claim 31  wherein the absolute value of the first slope is greater than the absolute value of the second slope. 
     
     
       36. The method of  claim 31  wherein the absolute value of the second slope is greater than the absolute value of the first slope.