Patent Publication Number: US-2010111137-A1

Title: Temperature sensing circuit using cmos switch-capacitor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a temperature sensing circuit, and more particularly, to a temperature sensing circuit using CMOS switch-capacitor. 
     2. Description of the Prior Art 
     In recent years, rapid developments in integrated circuit technology have reached the stage where a single-packaged chip may contain millions of transistors. As such, when an integrated circuit configured with a large number of transistors operates at a high clock rate, the amount of heat dissipated will be enormous to the extent that the operating temperature may exceed 100 degrees centigrade. Due to the change in temperature, all components in the chip will be adversely affected, since temperature and conductivity have an inversely proportional relationship. Therefore, when temperature rises, the electrical characteristics of components will change accordingly. The most evident effect is that operating speed and overall efficiency are reduced. 
     Please refer to  FIG. 1 .  FIG. 1  is a schematic diagram of a conventional temperature sensing circuit. The temperature sensing circuit  10  includes a current mirror  11  and a Widlar current source  12 . By matching transistors in the current mirror  11 , the temperature sensing circuit  1  will have equal currents I 1 , I 2 , I 3 , i.e., I 1 =I 2 =I 3 . When the transistor Q 2  of the Widlar current source  12  operates in the forward active region, the current  12  flowing through the transistor Q 2  will be 
     
       
         
           
             
               
                 
                   
                     I 
                      
                     
                         
                     
                      
                     2 
                   
                   = 
                   
                     
                       1 
                       
                         R 
                          
                         
                             
                         
                          
                         1 
                       
                     
                      
                     
                       V 
                       T 
                     
                      
                     
                       ln 
                        
                       
                         ( 
                         n 
                         ) 
                       
                     
                   
                 
               
               
                 
                   EQU 
                    
                   
                       
                   
                    
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
     wherein n is the emitter-base junction ratio between the transistor Q 2  and the transistor Q 1 , and the thermal voltage V T =26 mV*T/300° K. Since the voltage V TEMP =I 3 *R 2 =I 2 *R 2 , the following equation can be obtained: 
     
       
         
           
             
               
                 
                   
                     V 
                     TEMP 
                   
                   = 
                   
                     
                       
                         R 
                          
                         
                             
                         
                          
                         2 
                       
                       
                         R 
                          
                         
                             
                         
                          
                         1 
                       
                     
                      
                     
                       V 
                       T 
                     
                      
                     
                       ln 
                        
                       
                         ( 
                         n 
                         ) 
                       
                     
                   
                 
               
               
                 
                   EQU 
                    
                   
                       
                   
                    
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
     Therefore, the amount of change in the voltage V TEMP  is determined by the values of n and R 2 /R 1 . For example, the emitter-base junction ratio between the transistor Q 2  and the transistor Q 1  is (n=4), the resistor R 1 =3.6K, R 2 =30K. By substituting these parameters into EQU (2), the following equation can be obtained: 
     
       
         
           
             
               
                 
                   
                     V 
                     TEMP 
                   
                   = 
                   
                     300 
                      
                     
                         
                     
                      
                     mV 
                     * 
                     
                       T 
                       
                         300 
                          
                         ° 
                          
                         
                             
                         
                          
                         K 
                       
                     
                   
                 
               
               
                 
                   EQU 
                    
                   
                       
                   
                    
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
           
         
       
     
     From EQU (3), when the temperature rises by 1.degree.K, the voltage V TEMP  rises by 1 mV. As such, when the temperature sensing circuit  7  is electrically connected to a main circuit (not shown) the operating temperature of the main circuit can be monitored by observing the voltage V TEMP  from the temperature sensing circuit  7  so that thermal protection of the main circuit can be activated when appropriate. 
     However, the foregoing analysis was made under ideal conditions in practice, due to manufacturing constraints, the actual output of the temperature sensing circuit  10  usually differs from the original design. It is noted that the accuracy of the voltage V TEMP  depends on the actual values of n and R 2 /R 1 . Therefore, during manufacturing, if a lower value of R 2 /R 1  is desired, a higher value of n must be provided for compensation. For example, if R 2 /R 1 =2, the value of n must be as high as 320 to satisfy the condition that when the temperature rises by 1.degree.K, the voltage V TEMP  rises by 1 mV. Nevertheless, the value of n is determined by the physical characteristics of the transistors Q 2  and Q 1  and cannot be adjusted. If manufacture of the transistors Q 2  and Q 1  is based simply on the calculated values, the outcome will be a mismatch in the current gains  13  of the transistors Q 2  and Q 1 , thereby resulting in failure of the temperature sensing circuit  10  to operate normally and inability of the temperature sensing circuit  10  to serve the purpose of temperature measuring. Thus, to ensure the accuracy of the characteristic curve of the circuit, a value smaller than  10  is usually adopted for n. This introduces another design problem since the value of R 2 /R 1  must be correspondingly increased to satisfy the aforesaid requirement. However, in view of manufacturing constraints, it is known that the resistance values of resistors cannot be accurately controlled. Due to the requirement of a high resistance ratio R 2 /R 1 , the resultant error tends to be too high. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, a temperature sensing circuit using CMOS switch-capacitor comprises a PNP bipolar junction transistor (BJT), a comparator, a amplifier, a first current source, a second current source, a first capacitor, a second capacitor, a first switch, a second switch, a third switch, a fourth switch, a fifth switch, and a sixth switch. The PNP bipolar junction transistor (BJT) has an emitter, a collector electrically connected to a ground, and a base electrically connected to the collector. The comparator has a positive input end, a negative input end, and an output end. The amplifier has an input end and an output end electrically connected to the positive input end of the comparator. The first current source is used for providing a first current. The second current source is used for providing a second current. The first capacitor has a first end electrically connected to the emitter of the PNP BJT, and a second end electrically connected to the input end of the amplifier. The second capacitor has a first end electrically connected to the input end of the amplifier, and a second end. The first switch has a first end electrically connected to the first current source, and a second end electrically connected to the emitter of the PNP BJT. The second switch has a first end electrically connected to the second current source, and a second end electrically connected to the emitter of the PNP BJT. The third switch has a first end electrically connected to the emitter of the PNP BJT, and a second end electrically connected to the negative input end of the comparator. The fourth switch has a first end electrically connected to the input end of the amplifier, and a second end electrically connected to the output end of the amplifier. The fifth switch has a first end electrically connected to the second end of the second capacitor, and a second end electrically connected to the output end of the amplifier. The sixth switch has a first end electrically connected to the second end of the second capacitor, and a second end electrically connected to the ground. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a conventional temperature sensing circuit. 
         FIG. 2  is a circuitry of a temperature sensing circuit using CMOS switch-capacitor according to the present invention. 
         FIG. 3  is a schematic diagram of the temperature sensing circuit operating in the initial/sample duration according to the present invention. 
         FIG. 4  is a schematic diagram of the temperature sensing circuit operating in the hold/compare duration according to the present invention. 
         FIG. 5  is a graph of the voltage to the temperature of the temperature sensing circuit according to the present invention. 
     
    
    
     DETAILED DESCRIPTION  
     Please refer to  FIG. 2 .  FIG. 2  is a circuitry of a temperature sensing circuit using CMOS switch-capacitor according to the present invention. The temperature sensing circuit  20  comprises a PNP bipolar junction transistor (BJT)  22 , a hysteresis comparator  24 , a transconductance amplifier  26 , a first current source  31 , a second current source  32 , a first capacitor C 1 , a second capacitor C 2 , a third capacitor C 3 , a first switch SW 1 , a second switch SW 2 , a third switch SW 3 , a fourth switch SW 4 , a fifth switch SW 5 , and a sixth switch SW 6 . The base of the PNP BJT  22  is electrically connected to the collector of the PNP BJT  22 , and the collector of the PNP BJT  22  is electrically connected to the ground. The negative input end of the hysteresis comparator  24  is electrically connected to the emitter of the PNP BJT  22  via the first switch SW 1 , and the positive input end of the hysteresis comparator  24  is electrically connected to the output end of the transconductance amplifier  26 . The output end of the transconductance amplifier  26  is electrically connected to the input end of the transconductance amplifier  26  via the fourth switch SW 4 , and the input end of the transconductance amplifier  26  is electrically connected to the emitter of the PNP BJT  22  via the first capacitor C 1 . The first current source  31  is electrically connected to the emitter of the PNP BJT  22  via the first switch SW 1 . The second current source  32  is electrically connected to the emitter of the PNP BJT  22  via the second switch SW 2 . The first end of the second capacitor C 2  is electrically connected to the input end of the transconductance amplifier  26 , and the second of the second capacitor C 2  is electrically connected to the input end of the transconductance amplifier  26  via fifth switch SW 5 . Besides, the second end of the second capacitor C 2  is electrically connected to the ground via sixth switch SW 6 . The third capacitor C 3  is electrically connected between the output end of the transconductance amplifier  26  and the ground. The first switch SW 1 , the third switch SW 3 , and the fifth switch SW 5  are controlled by a first control signal. The second switch SW 2 , the fourth switch SW 4 , and the sixth switch SW 6  are controlled by a second control signal. The first control signal and the second control signal are complementary control signals. The first current source  31  can provide the current I, and the second current source  32  can provide the current nI. 
     Please refer to  FIG. 3  and  FIG. 4 .  FIG. 3  is a schematic diagram of the temperature sensing circuit operating in the initial/sample duration according to the present invention.  FIG. 4  is a schematic diagram of the temperature sensing circuit operating in the hold/compare duration according to the present invention. As shown in  FIG. 3 , when the temperature sensing circuit  20  operates in the initial/sample duration, the first switch SW 1 , the third switch SW 3 , and the fifth switch SW 5  are turned off, and the second switch SW 2 , the fourth switch SW 4 , and the sixth switch SW 6  are turned on. The second current source  32  provides the current nI to the node N 1  via second switch SW 2 . Thus, the voltage at the emitter of the PNP BJT  22  can be represented as: 
     
       
         
           
             
               
                 
                   
                     V 
                     EB 
                   
                   = 
                   
                     
                       V 
                       T 
                     
                      
                     ln 
                      
                     
                       nI 
                       
                         I 
                         S 
                       
                     
                   
                 
               
               
                 
                   EQU 
                    
                   
                       
                   
                    
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
           
         
       
     
     As shown in  FIG. 4 , when the temperature sensing circuit  20  operates in the hold/compare duration, the first switch SW 1 , the third switch SW 3 , and the fifth switch SW 5  are turned on, and the second switch SW 2 , the fourth switch SW 4 , and the sixth switch SW 6  are turned off. The first current source  31  provides the current I to the node N 1  via first switch SW 1 . Thus, the voltage at the emitter of the PNP BJT  22  can be represented as: 
     
       
         
           
             
               
                 
                   
                     V 
                     EB 
                   
                   = 
                   
                     
                       V 
                       T 
                     
                      
                     ln 
                      
                     
                       I 
                       
                         I 
                         S 
                       
                     
                   
                 
               
               
                 
                   EQU 
                    
                   
                       
                   
                    
                   
                     ( 
                     5 
                     ) 
                   
                 
               
             
           
         
       
     
     After the initial/sample duration and the hold/compare duration, the electric charge Q 1  stored in the first capacitor C 1  and the electric charge Q 2  stored in the second capacitor C 2  can be represented respectively as: 
         Q 1 =C 1 *V   T ln ( n )   EQU (6) 
         Q 2 =C 2 *Vg    EQU (7) 
     The voltage at the node N 1  decreases, so that the electric charge Q 1  flows from the node N 2  to the node N 1 . When the voltage at the node N 2  decreases, the electric charge Q 2  flows from the node N 3  to the node N 2 . The node N 2  and the node N 3  form a feedback loop by the transconductance amplifier  26 , so the electric charge Q 1  and the electric charge Q 2  will achieve the balance in the end; that is, Q 1 =Q 2 . Thus, the output voltage Vg of the transconductance amplifier  26  can be represented as: 
     
       
         
           
             
               
                 
                   Vg 
                   = 
                   
                     
                       
                         C 
                          
                         
                             
                         
                          
                         1 
                       
                       
                         C 
                          
                         
                             
                         
                          
                         2 
                       
                     
                      
                     
                       V 
                       T 
                     
                      
                     
                       ln 
                        
                       
                         ( 
                         n 
                         ) 
                       
                     
                   
                 
               
               
                 
                   EQU 
                    
                   
                       
                   
                    
                   
                     ( 
                     8 
                     ) 
                   
                 
               
             
           
         
       
     
     Please refer to  FIG. 5 .  FIG. 5  is a graph of the voltage to the temperature of the temperature sensing circuit according to the present invention. In  FIG. 5 , the vertical coordinates represent the voltage, and the horizontal coordinates represent the temperature. V CTAT  represents the voltage at the emitter of the PNP BJT  22 . V PTAT  represents the output voltage of the transconductance amplifier  26 . Vout represents the output voltage of the temperature sensing circuit  20 . From EQU (4), the voltage V EB  of the emitter of the PNP BJT  22  is complementary to absolute temperature (CTAT), which is represented as V CTAT . From EQU (8), the output voltage Vg of the transconductance amplifier  26  is proportional to absolute temperature (PTAT), which is represented as V PTAT . When the temperature increases, the voltage V CTAT  will decrease and the voltage V CTAT  will increase. The voltage V CTAT  and the voltage V CTAT  intersect at the temperature T 1  in the horizontal coordinates. The T 1  value can be adjusted according to the capacitance ratio C 1 /C 2  of the first capacitor C 1  and the second capacitor C 2 . In the present semiconductor process, the capacitance can be controlled in a smaller error than the resistance. Thus, the temperature sensing circuit  20  outputs the low voltage level when the temperature is smaller than T 1 ; the temperature sensing circuit  20  outputs the high voltage level when the temperature is greater than T 1 . In addition, the hysteresis comparator  24  can prevent the output voltage of the sensing circuit from oscillating between the low voltage level and the high voltage level. 
     In conclusion, the temperature sensing circuit using CMOS switch-capacitor according to the present invention comprises a PNP BJT, a hysteresis comparator, a transconductance amplifier, two current sources, two capacitors, and six switches. The first switch, the third switch, and the fifth switch are controlled by a first control signal. The second switch, the fourth switch, and the sixth switch are controlled by a second control signal. The first control signal and the second control signal are complementary control signals. A voltage complementary to the absolute temperature (CTAT) is generated according to the PNP BJT, and a voltage proportional to the absolute temperature (PTAT) is generated according to two capacitors and the transconductance amplifier. After the temperature sensing circuit completes the initial/sample duration and the hold/compare duration by controlling the switches, the voltage complementary to absolute temperature is transmitted to the negative input end of the hysteresis comparator, and the voltage proportional to absolute temperature is transmitted to the positive input end of the hysteresis comparator. Thus, when the voltage proportional to absolute temperature is greater than the voltage complementary to absolute temperature as the temperature increasing, the hysteresis comparator outputs a high level signal. The temperature sensing circuit of the present invention uses the capacitance ratio of the first capacitor and the second capacitor to determine the sense temperature value so as to increase the accuracy. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.