Abstract:
A temperature detector and method of detecting a shifted temperature provides multiple detected temperature points using a single branch. The temperature detector generates multiple detected temperature points in response to temperature control signals sequentially generated in a single branch. Since a shifted temperature for the single branch is found and a trimming operation in response to the shifted temperature is carried out, the test time is reduced. Various refresh periods can be set in response to various trip point temperatures and thus power consumption of a DRAM can be decreased.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application claims priority under 35 U.S.C. §119 from Korean Patent Application No. 2004-43484, filed on Jun. 14, 2004, the contents of which are hereby incorporated by reference in their entirety for all purposes as if fully set forth herein.  
       BACKGROUND AND SUMMARY  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to a temperature detector of a semiconductor circuit and, more particularly, to a temperature detector providing multiple detected temperature points using a single branch and a method of detecting a shifted temperature.  
         [0004]     2. Description of the Related Art  
         [0005]     One operating characteristic of a semiconductor device is its temperature characteristic. In the case of a CMOS device, the access time t ACCESS  increases as temperature is increased (A) (and thus the operating speed of the device decreases), and current consumption IDD increases as the temperature is decreased (B), as shown in  FIG. 1 . The temperature characteristic is important to devices that require a refresh operation, such as a dynamic random access memory (DRAM). The DRAM is a volatile memory and requires a refresh operation. In a DRAM cell, leakage current is increased when the temperature is increased and thus the data retention characteristic is deteriorated and the data refresh period t ST  is reduced.  
         [0006]     Developments in electronics technology enable the design and cost-effective manufacturing of portable electronic devices, including pagers, cellular phones, audio players, calculators, lap-top computers, PDAs and so on. The portable electronic devices generally need DC power and thus at least one battery is used as an energy source for providing the DC power. In a battery-operated system, such as a portable electronic device, it is critically important to reduce power consumption. Particularly, in a sleep mode for saving power, circuit components included in the system are turned off. However, any DRAM included in the system should refresh DRAM cell data in order to continuously preserve the data.  
         [0007]     One technique to reduce power required for a DRAM to operate is to vary the refresh period in response to temperature. Specifically, a temperature range is divided into multiple regions and the refresh period is increased in a low temperature region, that is, a refresh clock frequency is decreased, so as to reduce power consumption. Accordingly, a temperature detector is required to detect an internal temperature of the DRAM.  
         [0008]     FIGS.  2 A-B illustrate a conventional temperature detector. Referring to FIGS.  2 A-B, a temperature detector  100  using a band gap reference circuit includes a plurality of branches  110 ,  120  and  130 , PMOS transistors, and NMOS transistors. The temperature detector  100  further includes comparators  210 ,  220  and  230 , which respectively compare temperatures OT 1  through OTn, detected by the multiple branches  110 ,  120  and  130 , with a reference temperature ORef.  
         [0009]     The temperature detector  100  provides detected temperature points set to multiple specific temperatures. For instance, the first branch  110  may provide a detection point (or trip point) of 45° C. while the third branch  130  provides a detection point (or trip point) of 85° C.  
         [0010]     The temperature detector  100  is very sensitive to variations in the semiconductor device manufacturing process. Thus, a temperature tuning operation for tuning a changed detection temperature point to a designed detection temperature point should be carried out for each DRAM chip at the wafer level. To perform temperature trimming during the temperature tuning operation, an operation of detecting a shifted temperature due to a variation in the manufacturing process must be carried out in advance.  
         [0011]      FIG. 3  illustrates the distribution of shifted temperature detected from each chip in a lot, or batch, of chips. Referring to  FIG. 3 , when a refresh period, which is set when the temperature of a DRAM chip is in the range of 45° C. through 85° C., is 64 ms (X=64 ms), for example, the refresh period of the DRAM chip is set to half of 64 ms when the temperature is higher than 85° C. and three times 64 ms when the temperature is lower than 45° C.  
         [0012]     However, when the temperature detector  100  employs the multiple branches  110 ,  120  and  130 , the trip point of 45° C. of the first branch  110  may be shifted to a maximum of 50° C. and the trip point of 85° C. of the third branch  130  may be shifted to a minimum of 70° C. after the DRAM chip is manufactured. To tune the trip points, shifted to 50° C. and 70° C., to the desired set trip points, a separate trimming operation should be carried out for each branch. Accordingly, the temperature detector  100  requires a long period of time to detect a shifted temperature for each branch and to perform a trimming operation for the detected shifted temperature. Furthermore, the refresh period is varied by more than a factor of six, from three times the set refresh period to half the set refresh period in the range of 50° C. through 70° C.  
         [0013]     Accordingly, it would be desirable to provide a temperature detector providing multiple trip points, or detection temperature points, using a single branch.  
         [0014]     It would also be desirable to provide a method of detecting a shifted temperature using the temperature detector.  
         [0015]     According to one aspect of the present invention, there is provided a temperature detector detecting a temperature shifted from a set target temperature, comprising an automatic pulse generator sequentially generating temperature control signals in response to a temperature detection signal; a comparator comparing detected temperatures with a predetermined reference temperature in response to the temperature control signals; a trip temperature increasing part comprising first short-circuiting switching transistors that selectively short-circuit a plurality of serially connected first binary weighted resistors in response to first test input signals and increasing the detected temperature when the shifted temperature is lower than the target temperature, the trip temperature increasing part being connected to a single branch; a trip temperature decreasing part comprising second short-circuiting switching transistors that selectively short-circuit a plurality of serially connected second binary weighted resistors in response to second test input signals and decreasing the detected temperature when the shifted temperature is higher than the target temperature, the trip temperature decreasing part being connected to the single branch; and a temperature detection controller selectively short-circuiting a plurality of serially connected resistors using third switching transistors in response to the temperature control signals to provide the detected temperatures, the temperature detection controller being connected to the single branch.  
         [0016]     According to another aspect of the present invention, there is provided a method of detecting a shifted temperature that is changed from a set, target temperature, comprising sequentially generating temperature control signals in response to a temperature detection signal; selectively short-circuiting a plurality of serially connected resistors using switching transistors in response to the temperature control signals to provide detected temperatures, the switching transistors are being connected to a single branch; and comparing the detected temperatures with a predetermined reference temperature in response to the temperature control signals to search the shifted temperature.  
         [0017]     Preferably, the shifted temperature detecting method further comprises increasing the detected temperatures using short-circuiting switching transistors that selectively short-circuit a plurality of serially connected binary weighted resistors in response to first test input signals when the shifted temperature is lower than the target temperature, and carrying out a trimming operation of short-circuiting the binary weighted resistors in response to the first test input signals obtained by binary weighted approximation. The short-circuiting switching transistors are connected to the single branch.  
         [0018]     The shifted temperature detecting method further comprises decreasing the detected temperatures using short-circuiting switching transistors that selectively short-circuit a plurality of serially connected binary weighted resistors in response to second test input signals when the shifted temperature is higher than the target temperature, and carrying out a trimming operation of short-circuiting the binary weighted resistors in response to the second test input signals obtained by binary weighted approximation. The short-circuiting switching transistors being connected to the single branch.  
         [0019]     According to the temperature detector of the present invention, the temperature detection controller connected to the single branch provides multiple trip point temperatures in response to the temperature control signals sequentially generated by the automatic pulse generator. Since a shifted temperature for the single branch is found and a trimming operation in response to the shifted temperature is carried out, test time is reduced. Furthermore, various refresh periods can be set in response to various trip point temperatures and thus consumption power of a DRAM can be decreased. The temperature detector of the present invention requires a layout area smaller than the layout area of the conventional temperature detector using multiple branches because the temperature detector of the present invention uses a single branch. Moreover, temperatures are shifted in the same direction such that 85° C. is shifted to 90° C. when 45° C. is shifted to 50° C. according to the temperature detector of the present invention. Thus, a stable refresh period is maintained even if the temperature is shifted. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0021]      FIG. 1  illustrates temperature characteristics of a CMOS device;  
         [0022]     FIGS.  2 A-B illustrate a conventional temperature detector;  
         [0023]      FIG. 3  illustrates the distribution of a shifted temperature detected from a chip;  
         [0024]     FIGS.  4 A-D illustrate a temperature detector according to one or more aspects of the present invention; and  
         [0025]      FIG. 5  is a graph for explaining a single detected temperature point set by the temperature detector of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION  
       [0026]     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. Throughout the drawings, like reference numerals refer to like elements.  
         [0027]     FIGS.  4 A-D illustrate a temperature detector. The temperature detector includes a temperature detection unit  400  of  FIG. 4A , an automatic pulse generator  500  of  FIG. 4B , a comparator  600  of  FIG. 4C , and registers  710 ,  720  and  730  of  FIG. 4D .  
         [0028]     Referring to  FIG. 4A , the temperature detection unit  400  includes first, second and third PMOS transistors MP 1 , MP 2  and MP 3 , first, second and third NMOS transistors MN 1 , MN 2  and MN 3 , first and second diodes D 1  and D 2 , a trip temperature increasing part  410 , a trip temperature decreasing part  420 , and a temperature detection controller  430 .  
         [0029]     The first, second and third PMOS transistors MP 1 , MP 2  and MP 3  have the same size as each other. That is, the first, second and third PMOS transistors MP 1 , MP 2  and MP 3  have the same channel length and width. The first, second and third NMOS transistors MN 1 , MN 2  and MN 3  also have the same size as each other. The ratio of the size of the first diode D 1  to the size of the second diode D 2  is 1:M.  
         [0030]     The current lo and the current Ir are identical to each other according to current mirror of the first and second PMOS transistors MP 1  and MP 2  and the first and second NMOS transistors MN 1  and MN 2 . That is, lo:lr=1:1.  
         [0031]     In the meantime, the turn-on current ID of a diode is as follows: 
 
 ID=Is ×( e   VD/VT −1)÷ Is ×( e   VD/VT )   [Equation 1]
 
         [0032]     Here, Is is the reverse saturation current of the diode, VD is the diode voltage, and VT is a temperature voltage represented by dT/q. Thus, the current lo flowing through the first diode D 1  is as follows: 
 
 Io=Is   1 ×( e   VD1/VT )   [Equation 2]
 
         [0033]     In other words, the first diode voltage VD 1  is represented by the following equation: 
 
 VD   1 = VT ×ln( Io/Is   1 )   [Equation 3]
 
         [0034]     In addition, the second diode voltage VD 2  is represented by the following equation: 
 
 VD   2 = VT ×ln( Ir/Is   2 )= VT ×ln( Io /( M*Is   1 ))   [Equation 4]
 
         [0035]     Since the current lo is identical to the current Ir, the voltage VNA of the node NA is identical to the voltage VNB of the node NB. Thus, the following relationship is obtained: 
 
 VNA=VNB=VD   1 = VD   2 + Ir×R [Equation  5]
 
         [0036]     When Equation 5 is replaced by Equations 3 and 4, the following equation is obtained: 
 
 VT ×ln( Io/Is   1 )= VT ×ln( Io /( M*Is   1 ))+ Ir×R    [Equation 6]
 
         [0037]     Thus, the current Ir is represented as follows: 
 
 Ir=VT ×ln( M )/ R    [Equation 7]
 
         [0038]     Accordingly, the current Ir increases in proportion to temperature.  
         [0039]     When the current I 1  of the node NC is identical to the current lo, the voltage VNC of the node NC is identical to the voltage VNB of the node NB as follows: 
 
 VNC=VD   1 = VT ×ln( Io/Is   1 )   [Equation 8]
 
         [0040]     Here, as the temperature increases, the reverse saturation current Is 1  increases much more than the temperature voltage VT. Thus, the voltage of the node NC decreases as the temperature is increased. Accordingly, the current I 1  decreases as the temperature increases.  
         [0041]     Therefore, the temperature detector  400  sets a specific temperature T 1  at which the current Ir and the current I 1  cross each other, shown in  FIG. 5 , as a trip point. In this embodiment, a single trip point is set to 45° C.  
         [0042]     The trip temperature increasing part  410  includes first short-circuiting switching transistors  411  through  416 , which selectively short-circuit a plurality of first binary weighted resistors RU 0  through RU 5  serially connected between nodes N 410  and N 420  in response to first test input signals AU 0  through AU 5 , respectively. When the first test input signals AU 0  through AU 5  are in a normal state, AU 5 , AU 4 , AU 3 , AU 2 , AU 1 , AU 0 =0, 0, 0, 0, 0, 0 are input to the short-circuiting switching transistors  411  through  416  and thus the short-circuiting switching transistors  411  through  416  are turned off. Accordingly, all the binary weighted resistors RU 0  through RU 5  of the trip temperature increasing part  410  function as resistors. Subsequently, the first test input signals AU 0  through AU 5  are selectively changed to a logic high level to search for and set a trip point temperature.  
         [0043]     The trip temperature decreasing part  420  includes second short-circuiting switching transistors  421  through  426 , which selectively short-circuit a plurality of second binary weighted resistors RD 0  through RD 5  serially connected between the node N 420  and a node N 430  in response to second test input signals AD 0  through AD 5 , respectively. When the second test input signals AD 0  through AD 5  are in a normal state, AD 5 , AD 4 , AD 3 , AD 2 , AD 1 , AD 0 =1, 1, 1, 1, 1, 1 are input to the short-circuiting switching transistors  421  through  426  and thus the short-circuiting switching transistors  421  through  426  are turned on. Accordingly, all the binary weighted resistors RD 0  through RD 5  of the trip temperature decreasing part  420  are short-circuited and do not function as resistors. Subsequently, the second test input signals AD 0  through AD 5  are selectively changed to a logic low level to search for and set a trip point temperature.  
         [0044]     Beneficially, the binary weighted resistors RU 0  through RU 5  of the trip temperature increasing part  410  can have resistance values Ra, 2Ra, 4Ra, 8Ra, 16Ra and 32Ra, respectively, while the binary weighted resistors RD 0  through RD 5  of the trip temperature decreasing part  420  can also have resistance values Ra, 2Ra, 4Ra, 8Ra, 16Ra and 32Ra, respectively.  
         [0045]     The temperature detection controller  430  includes switching transistors  431 ,  432  and  433 , which selectively short-circuit a plurality of resistors R 1  through Rn serially connected between the node N 430  and ground voltage VSS in response to temperature control signals C 1  through Cn. The temperature control signals C 1  through Cn are sequentially generated by the automatic pulse generator  500  of  FIG. 4B  as explained below. The temperature control signals C 1  through Cn are initially at a logic low level and then changed to a logic high level, or initially at a logic high level and then changed to a logic low level. The resistors R 1  through Rn function as resistors when the respective temperature control signals C 1  through Cn are at a logic low level, and the resistors R 1  through Rn do not function as resistors when the respective temperature control signals C 1  through Cn are at a logic high level. The resistors R 1  through Rn can have resistance values Ra, 2Ra, 4Ra, 8Ra, . . . , nRa, respectively.  
         [0046]     The temperature detection unit  400  is connected to the comparator  600  of  FIG. 4C , which compares a temperature OT 1  detected by the trip temperature increasing part  410 , trip temperature decreasing part  420  and temperature detection controller  430 , with the reference temperature ORef. The comparator  600  compares the detected temperature OT 1  with the reference temperature ORef selectively in response to the temperature control signals C 1  through Cn and outputs the comparison result OUTi (I=1, 2, . . . , n). The output signals OUTi of the comparator  600  are respectively stored in the registers  710 ,  720  and  730  of  FIG. 4D .  
         [0047]     The operation of the temperature detector of FIGS.  4 A-D will now be explained.  
         [0048]     The temperature detection controller  430  is operated after the trip temperature increasing part  410  and trip temperature decreasing part  420  are operated. Here, the comparator  600  of  FIG. 4C  is enabled.  
         [0049]     The operation of the trip temperature increasing part  410  will now be described on the assumption that a test temperature is set to a fixed temperature 85° C. (ORef), a target trip point of the temperature detector is 45° C. and the trip point is shifted to 50° C. due to an error of 5° C. generated caused by a variation in manufacturing processes.  
         [0050]     The comparator  600  compares the detected temperature OT 1 , 50° C., with the reference temperature ORef, 85° C., in response to AU 5 , AU 4 , AU 3 , AU 2 , AU 1 , AU 0 =0,0,0,0,0,0, which are input to the trip temperature increasing part  410  in the normal state, and outputs a logic high level signal. When the signal AU 5  is changed such that AU 5 , AU 4 , AU 3 , AU 2 , AU 1 , AU 0 =1,0,0,0,0,0 are input to the trip temperature increasing part  410 , the comparator  600  compares a detected temperature OR 1  of 82° C. with the reference temperature ORef of 85° C. and outputs a logic high level signal. When the signal AU 4  is additionally changed such that AU 5 , AU 4 , AU 3 , AU 2 , AU 1 , AU 0 =1,1,0,0,0,0 are input to the trip temperature increasing part  410 , the comparator  600  compares a detected temperature OR 1  of 98° C. with the reference temperature ORef of 85° C. and outputs a logic low level signal.  
         [0051]     Then, AU 5 , AU 4 , AU 3 , AU 2 , AU 1 , AU 0 =1,0,1,0,0,0 are input to the trip temperature increasing part  410 , the comparator  600  compares a detected temperature OR 1  of 90° C. with the reference temperature ORef of 85° C. and outputs a logic low level signal. When AU 5 , AU 4 , AU 3 , AU 2 , AU 1 , AU 0 =1,0,0,1,0,0 are input to the trip temperature increasing part  410 , the comparator  600  compares a detected temperature OR 1  of 86° C. with the reference temperature ORef of 85° C. and outputs a logic low level signal. When AU 5 , AU 4 , AU 3 , AU 2 , AU 1 , AU 0 =1,0,0,0,1,0 are input to the trip temperature increasing part  410 , the comparator  600  compares a detected temperature OR 1  of 84° C. with the reference temperature ORef of 85° C. and outputs a logic high level signal.  
         [0052]     When AU 5 , AU 4 , AU 3 , AU 2 , AU 1 , AU 0 =1,0,0,0,1,1 are input to the trip temperature increasing part  410 , the comparator  600  compares a detected temperature OR 1  of 85° C. with the reference temperature ORef of 85° C. and outputs a signal vibrating between a logic high level and a logic low level. The finally changed values AU 5 , AU 4 , AU 3 , AU 2 , AU 1 , AU 0 =1,0,0,0,1,1 are stored in registers (not shown) included in a test apparatus. The values 1,0,0,0,1,1 stored in the registers correspond to the decimal number  35 . When 35° C. is subtracted from 85° C., 50° C. is obtained. Consequently, the shifted temperature of the temperature detector becomes 85° C.−35° C.=50° C. because the test temperature is 85° C. and the first test input signals AU 0  through AU 5 , which are input to the trip temperature increasing part when the output signal of the comparator  600  vibrates, correspond to  35 .  
         [0053]     Next, the operation of the trip temperature decreasing part  420  to find the shifted temperature of 50° C. when the test temperature is set to a fixed temperature −5° C. will now be explained.  
         [0054]     The comparator  600  compares a detected temperature OT 1  of 50° C. with the reference temperature ORef of −5° C. in response to the second test input signals AD 5 , AD 4 , AD 3 , AD 2 , AD 1 , AD 0 =1,1,1,1,1,1, which are input to the trip temperature decreasing part  420  in the normal state, and outputs a logic low level signal. When the signal AD 5  is changed to  0  such that AD 5 , AD 4 , AD 3 , AD 2 , AD 1 , AD 0 =0,1,1,1,1,1 are input to the trip temperature decreasing part  420 , the comparator  600  compares a detected temperature OT 1  of 18° C. with the reference temperature ORef of −5° C. and outputs a logic low level signal. When AD 5 , AD 4 , AD 3 , AD 2 , AD 1 , AD 0 =0,0,1,1,1,1 are input to the trip temperature decreasing part  420 , the comparator  600  compares a detected temperature OT 1  of 2° C. (=18−16) with the reference temperature ORef of −5° C. and outputs a logic low level signal. When AD 5 , AD 4 , AD 3 , AD 2 , AD 1 , AD 0 =0,0,0,1,1,1 are input to the trip temperature decreasing part  420 , the comparator  600  compares a detected temperature OT 1  of −6° C. (=2−8) with the reference temperature ORef of −5° C. and outputs a logic high level signal.  
         [0055]     When AD 5 , AD 4 , AD 3 , AD 2 , AD 1 , AD 0 =0,0,1,0,1,1 are input to the trip temperature decreasing part  420 , the comparator  600  compares a detected temperature OT 1  of −2° C. (=2−4) with the reference temperature ORef of −5° C. and outputs a logic low level signal. When AD 5 , AD 4 , AD 3 , AD 2 , AD 1 , AD 0 =0,0,1,0,0,1 are input to the trip temperature decreasing part  420 , the comparator  600  compares a detected temperature OT 1  of −4° C. (=−2−2) with the reference temperature ORef of −5° C. and outputs a logic low level signal.  
         [0056]     When AD 5 , AD 4 , AD 3 , AD 2 , AD 1 , AD 0 =0,0,1,0,0,0 are input to the trip temperature decreasing part  420 , the comparator  600  compares a detected temperature OT 1  of −5° C. (=−4−1) with the reference temperature ORef of −5° C. and outputs a signal vibrating between a logic low level and a logic low high signal. The finally changed values AD 5 , AD 4 , AD 3 , AD 2 , AD 1 , AD 0 =0,0,1,0,0,0 are inverted and the inverted values 1,1,0,1,1,1 are stored in registers (not shown) included in the test apparatus. The values 1,1,0,1,1,1 stored in the registers correspond to the decimal number  55 . Thus, 55° C. is added to −5° C. to obtain 50° C. Consequently, the shifted temperature of the temperature detector is −5° C.+55° C.=50° C. because the test temperature is −5° C. and the second test input signal AD 0  through AD 5 , which are input to the trip temperature decreasing part when the output signal of the comparator  600  vibrates, correspond to  55 .  
         [0057]     The shifted temperature detected by the trip temperature increasing part  410  or trip temperature decreasing part  420  allows a temperature trimming part (not shown) to selectively short-circuit the first binary weighted resistors RU 0  through RU 5  and the second binary weighted resistors RD 0  through RD 5 . Accordingly, the temperature detector is operated at the originally designed trip point temperature, 45° C., in the normal state.  
         [0058]     As described above, the temperature detector is basically operated at the set trip point temperature of 45° C. according to the operations of the trip temperature increasing part  410  and trip temperature decreasing part  420 . A temperature detection signal TEMP_DET is periodically activated to enable the automatic pulse generator  500  as shown in  FIG. 4B . In response to the temperature detection signal TEMP_DET, the automatic pulse generator  500  sequentially generates the temperature control signals C 1  through Cn. The temperature detection controller  430  provides detected temperature OT 1  in response to the temperature control signals C 1  through Cn. The comparator  600  generates trip point temperatures T 1  through Tn by comparing the detected temperature OT 1  and the reference temperature ORef in response to the temperature control signals C 1  through Cn, and stores the trip point temperatures in the registers  710 ,  720 , . . . through  730 .  
         [0059]     The multiple trip point temperatures T 1  through Tn provided in response to the temperature control signals C 1  through Cn are more useful when the originally set temperature is not found due to various reasons, even when a temperature detection test is finished. That is, even if the temperature detector is initially set to 45° C./85° C., for example, the trip temperature of the devices has a Gaussian distribution with 45° C./85° C. in the center when the trip temperature is measured after packaging the devices, because characteristics of resistors and transistors are changed due to various tests or the power supply voltage is varied. In this case, the conventional temperature detector has the problem that the refresh period is varied by a ratio of more than 6:1, from three times the set refresh period to half of the set refresh period across the range from 50° C. through 70° C., as shown in  FIG. 3 . In the temperature detector of FIGS.  4 A-D, however, the refresh period is changed from three times the set refresh period to half of the set refresh period across the range from 45° C. through 85° C., because temperatures are shifted in the same direction such that 85° C. is shifted to 90° C. when 45° C. is shifted to 50° C. This is because the temperature detector of FIGS.  4 A-D uses a single branch. Accordingly, a stable refresh period is maintained even if the temperature is shifted.  
         [0060]     Therefore, the temperature detector provides the multiple trip point temperatures T 1  through Tn using the trip temperature increasing part  410 , trip temperature decreasing part  420  and temperature detection controller  430 , which are connected in a single branch.  
         [0061]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.