Abstract:
In one embodiment, an integrated circuit is provided for detecting when a temperature reaches a specified value. The circuit includes a differential circuit block having first and second transistors. A control terminal of the first transistor is coupled to a first voltage source, and a control terminal of the second transistor is coupled to a second voltage source. The second transistor has an area larger than the first transistor. The differential circuit block compares a first current flowing into the first transistor and a second current flowing into the second transistor. The differential circuit block outputs a signal to indicate that the specified temperature has been reached when the first current equals the second current according to specified values of the first voltage source, the second voltage source, and the ratio of the areas of the first and second transistors. A single-ended circuit block amplifies the output signal of the differential circuit block to a predetermined amplitude.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention generally relates to the field of semiconductor integrated circuits (ICs), and more particularly, to a process-invariant temperature detection circuit. 
     2. Description of Related Art 
     An integrated circuit (IC) is a miniaturized electronic circuit consisting of a handful to millions of discrete electronic components fabricated on the surface of a thin substrate of semiconductor material. ICs are sometimes classified by the type of transistors used within the circuits. One type of transistor typically used in ICs is the bipolar junction transistor, which is an active semiconductor device formed by two p-n junctions. Another class of ICs are complementary metal-oxide-semiconductor (CMOS) devices. CMOS devices use metal-oxide-semiconductor field-effect transistors (MOSFETs), which are field-effect transistors comprised of a channel of p-type or n-type semiconductor material. An integrated circuit that uses both bipolar junction transistors and CMOS in a single device is known as a BiCMOS device. 
     A genre of circuits and systems require temperature sensing schemes to convert a temperature into a logic state. This logic state may be used to shut down the system or to raise a logic flag. For example, when a BiCMOS power integrated circuit is in operation, the transistors and other electronic components of the circuit dissipate heat. As the temperature approaches a certain level, the circuit may not operate as efficiently or may not perform as desired. In some situations, the circuit may even be damaged when it reaches a certain temperature. Therefore it is essential for many BiCMOS integrated circuits to include circuitry to detect a specific temperature in order to respond to these over temperature conditions. 
     Conventional BiCMOS temperature detection circuits are subject to variations in process parameters and considerable effort may be required to integrate the conventional temperature detection circuit into an IC. In addition, the conventional temperature detection circuit cannot be easily ported from one BiCMOS platform to another. When a different BiCMOS process is required, additional design and experimentation must be performed to integrate the temperature detection circuit into the new process. 
     SUMMARY OF THE INVENTION 
     In various embodiments, the present invention provides systems, circuitry, and methods for temperature detection independent of variations in process parameters. In one embodiment, a BiCMOS temperature detection circuit independent of variations in process parameters is provided. 
     In some embodiments, the detection circuit can be implemented with a current comparator having input voltages V 1  and V 2 . The difference (ΔV) between V 1  and V 2  will by design determine the target (or trip-point) temperature T T . When the devices comprising the differential pair follow an exponential current law, the relationship between ΔV and the trip-point temperature T T  is linear and independent of process parameters. Examples of devices which follow an exponential current law are a bipolar transistor operating in the active (non-saturated) current region and a MOSFET operating in the sub-threshold region. 
     In accordance with the embodiments of the present invention, the supply current can be well controlled and limited by the “tail” current IT. This can be an advantage for battery operated systems which require low-quiescent current operation. 
     In accordance with one embodiment of the present invention, an integrated circuit is provided for detecting when a temperature reaches a specified value. The circuit includes a first transistor and a second transistor, each of which has a respective base, emitter, and collector. The second transistor has an area greater than the first transistor. The base of the first transistor is coupled to a first voltage source. The base of the second transistor is coupled to a second voltage source. The emitter of the second transistor is coupled to the emitter of the first transistor. An active load circuit block comprises first and second active load transistors. The first active load transistor provides a first current into the collector of the first transistor. The second active load transistor provides a second current into the collector of the second transistor. The first current is equal to the second current at the specified temperature according to selected values of the first voltage source, the second voltage source, the ratio of the areas of the first and second transistors, and the ratio of the areas of the first and second active load transistors. 
     In accordance with another embodiment of the present invention, an integrated circuit is provided for detecting when a temperature reaches a specified value. The circuit includes a differential circuit block having first and second transistors. A control terminal of the first transistor is coupled to a first voltage source, and a control terminal of the second transistor is coupled to a second voltage source. The second transistor has an area larger than the first transistor. The differential circuit block compares a first current flowing into the first transistor and a second current flowing into the second transistor. The differential circuit block outputs a signal to indicate that the specified temperature has been reached when the first current equals the second current according to specified values of the first voltage source, the second voltage source, and the ratio of the areas of the first and second transistors. A single-ended circuit block amplifies the output signal of the differential circuit block to a predetermined amplitude. 
     Important technical advantages of the present invention are readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic block diagram of an exemplary implementation for a temperature detection circuit, according to an embodiment of the invention. 
         FIG. 2  is a schematic block diagram of another exemplary implementation for a temperature detection circuit, according to an embodiment of the invention. 
         FIG. 3  is a schematic block diagram of yet another exemplary implementation for a temperature detection circuit according to an embodiment of the invention. 
         FIG. 4  is a schematic block diagram of still yet another exemplary implementation for a temperature detection circuit, according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention and their advantages are best understood by referring to  FIGS. 1 through 4  of the drawings. Like numerals are used for like and corresponding parts of the various drawings. 
       FIG. 1  is a schematic block diagram of an exemplary implementation for a temperature detection circuit  100 , according to an embodiment of the invention. In one embodiment, temperature detection circuit  100  can be implemented in BiCMOS technology. 
     As shown, the temperature detection circuit  100  may include a first bipolar transistor (Q 1 )  210 , a second bipolar transistor (Q 2 )  220 , an active load circuit block  230 , a bias circuit block  240 , and a single-ended output buffer circuit block  300 . 
     Bipolar transistors  210  and  220  can be scaled such that the effective emitter area of transistor  220  is a magnitude m relative to that of transistor  210 . According to one embodiment, the effective emitter area of transistor  220  is a scale 16:1 to that of transistor  210 . The base of transistor  210  can be connected to a first voltage source V 1  and the base of the transistor  220  can be connected to a second voltage source V 2 . In one embodiment, the first and second voltage sources V 1  and V 2  may have fixed values. It can be important that the difference between V 1  and V 2  is a predictable constant greater than zero. In this way, the target (trip-point) or threshold temperature (T t ) becomes a predictable value independent of process variation. A current IC 1  flows into the collector of the transistor  210  and a current IC 2  flows into the collector of the transistor  220 . A current IE 1 , flows from the emitter of the transistor  210  and a current IE 2  flows into the emitter of the transistor  220 . A tail current IT is equal to the sum of the emitter currents IE 1  and IE 2 . Because the tail current IT limits the circuit bias, the quiescent current can be made low, by design. 
     The bias circuit block  240  may include a transistor (MN 1 )  242  to conveniently generate the tail current IT. According to an exemplary embodiment of the present invention, transistor  242  is implemented as an n-channel MOSFET. The emitters of transistors  210  and  220  can be connected to the drain of transistor  242 . The gate of the transistor  242  can be connected to a reference voltage V b  and the source of the transistor  242  can be connected to ground. 
     The active-load circuit block  230  for the differential comparator may comprise p-channel MOSFET (MP 1 )  232  and p-channel MOSFET (MP 2 )  234 . The sources of transistors  232  and  234  can be connected to a voltage supply rail V p . The gate and drain of transistor  232  can be connected to the gate of transistor  234 . In the operation of the active-load block circuit  230 , the scale of the current flowing from transistor  232  is a factor n relative to that of the current flowing from transistor  234 . According to the exemplary embodiment of the present invention in  FIG. 1 , the current flowing from transistor  232  has the same magnitude as the current flowing from transistor  234 . 
     The single-ended output buffer block circuit block  300  may include a p-channel MOSFET (MP 3 )  310  and an n-channel MOSFET (MN 2 )  320 . The gate of transistor  310  can be connected to the drain of transistor  234 , the source of transistor  310  can be connected to the voltage source V p , and the drain of transistor  310  can be connected to the output terminal OT. The gate of transistor  320  can be connected to a reference voltage source V b , the source of transistor  320  can be connected to ground, and the drain of transistor  320  can be connected to the output terminal OT and the drain of transistor  310 . 
     The threshold temperature T T , which can be set by a designer, is related to the voltage difference ΔV of the voltage sources V 1  and V 2  as defined by Equation (1) below, in which q is the amount of charge of electrons (1.602×10 −19  coulomb), k is the Boltzmann constant (1.38×10 −23  joule/kelvin), m is the scale of the emitter area of the second transistor  220  relative to the first transistor  210 , and n is the scale of the current of the first active load transistor  232  relative to the current of the second active load transistor  234 . 
     
       
         
           
             
               
                 
                   
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     The temperature detection circuit  100  operates such that the current flowing through transistor  210  is greater than the current flowing through transistor  220  (i.e., IC 1 &gt;IC 2 ) when the temperature is less than the threshold temperature T T , The current flowing through transistor  210  is less than the current flowing through transistor  220  (i.e., IC 1 &lt;IC 2 ) when the temperature is greater than the threshold temperature T T . In the example of  FIG. 1 , the current flowing through transistor  210  is equal to the current flowing through transistor  220  (i.e., IC 1 =IC 2 ) when the temperature of the circuit is equal to the threshold temperature T T . The criterion is more generally described by equation 1 which quantifies the following design condition: the current in transistor Q 2   220  will equal the current offered by transistor (MP 2 )  234  at the transition point. The current offered by transistor (MP 2 )  234 , in turn, is related to the current in (Q 1 )  210  through the active load. 
     That is, under a normal operating condition, when the temperature of an integrated circuit incorporating the temperature detection circuit  100  is less than the threshold temperature T T , the differential stage drives transistor  310  off (i.e., the gate-source voltage of transistor  310  is 0V. Accordingly, transistor  310  is in an off state and the output terminal OT is at a low level. 
     As the temperature in the integrated circuit incorporating the temperature detection circuit  100  rises, the demand for current in transistor  220  increases until it reaches the point where the current in transistor  234  will equal the current demanded from transistor  220 . This condition defines the transition or threshold temperature T T . 
     At the threshold temperature T T , the differential stage forces transistor  310  to conduct. By design, the gain of the single-ended output stage (having transistor  310  and bias transistor  320 ) can be large and will cause a change in logic state at node OT. This signals that the semiconductor junction temperature has reached temperature T T . 
     Thus, essentially, the output signal OT transitions from one voltage level (e.g., low) to another voltage level (e.g., high) when the temperature reaches the threshold temperature T T . 
       FIG. 2  is a schematic block diagram of another exemplary implementation for a temperature detection circuit  100 , according to an embodiment of the invention. This circuit  1100  uses a different area-scaling approach to reach the same result as the circuit  100  in  FIG. 1 . The implementation of circuit  1100  shown in  FIG. 2  uses the same bipolar transistors with area scaling  8  and using the active load with area scaling  2 . The resulting scale factor is again the number  16 . This further illustrates that equation (1) above, which is a function of ΔV and the factor (m×n), gives the designer the freedom to choose values for n, m, V 1 , V 2  (within common-mode ranges), and IT (tail current). Stated differently, once these values are selected, the comparator operating in its common-mode range will change state at the trip-point or threshold temperature T T . As an advantage, the quiescent is controlled by the tail current bias-level IT. 
       FIG. 3  is a schematic block diagram of yet another exemplary implementation for a temperature detection circuit  2100 , according to an embodiment of the invention. As shown, circuit  2100  is a complementary PNP implementation of circuit  1100  depicted in  FIG. 2 . In this case, input voltage V 2  will be greater than input voltage V 1 , and ΔV will be V 2  minus V 1 . As with the other implementations, at the threshold temperature T T , the differential stage forces a logic-state transition. 
       FIG. 4  is a schematic block diagram of still yet another exemplary implementation for a temperature detection circuit  3100 , according to an embodiment of the invention. In this embodiment, circuit  3100  using an NMOS differential pair to replace the NPN bipolar transistors. That is, the implementation of  FIG. 4  uses N-Channel MOSFETS  3210  and  3220  operating in the sub-threshold region (instead of bipolar transistors like that shown in  FIG. 1 ). Since in theory NMOS transistors may be designed to operate in the sub-threshold exponential region, this embodiment also attains the desired result of equation (1). In other words, the tail current IT and the differential NMOS devices  3210  and  3220  (labeled MN_exp 1  and MN_exp 2 , respectfully) are, by design, selected such that transistors  3210  and  3220  behave similar to bipolar transistors. 
     The temperature detection circuits  100 ,  1100 ,  2100 , and  3100  can be used in various applications. For example, these temperature detection circuits can be used for over-temperature protection so that a system in which any such circuit  100 ,  1100 ,  2100 , or  3100  shuts down before it overheats and causes permanent damage to the system. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims. That is, the discussion included in this application is intended to serve as a basic description. It should be understood that the specific discussion may not explicitly describe all embodiments possible; many alternatives are implicit. It also may not fully explain the generic nature of the invention and may not explicitly show how each feature or element can actually be representative of a broader function or of a great variety of alternative or equivalent elements. Again, these are implicitly included in this disclosure. Where the invention is described in device-oriented terminology, each element of the device implicitly performs a function. Neither the description nor the terminology is intended to limit the scope of the claims.