Abstract:
Methods and apparatus are disclosed for protecting circuits from damages caused by elevated temperatures. Presented embodiments illustrate IC thermal protection circuits that shut down power delivery circuits when the circuit temperature reaches a predefined upper threshold and restart the circuit when the circuit cools down to a predefined lower threshold. Other embodiments provide soft shutdown and soft restart, where not only the temperature range between the shutdown and the restart is predetermined, but also the time between the start of a shutdown process and the complete shutdown is controllable.

Description:
TECHNICAL FIELD 
   The embodiments described below relate to thermal protection circuits and, in particular, to limiting the charger IC die temperature by soft shutdown and soft restart within a predetermined temperature range. 
   BACKGROUND 
   Thermal protection is a vital requirement for power delivery circuits and prevents permanent damage due to prolonged operation at excessive temperatures. In integrated circuits (ICs), especially in power ICs, power dissipation can cause relatively high temperatures. To avoid degradations phenomena when the circuit temperature rises, or in some cases destructive failures of ICs as a result of excessive temperature, it is usually critical to incorporate a dedicated protection circuit to switch off, at least, the power output portion of the integrated circuit and temporarily disable the primarily source of power dissipation. 
   A thermal protection circuit limits the maximum operation temperature of the power delivery circuit through a temporary thermal shutdown. It provides safeguard by sensing a temperature of the power delivery circuit and automatically shutting down the power delivery circuit, whenever the circuit temperature exceeds a predetermined threshold. A thermal protection circuit subsequently turns the circuit back on after the circuit cools off to a predetermined lower temperature. 
   The power delivery circuit may oscillate by being turned on and off through the thermal shutdown circuit; however, the frequency of such oscillation is reduced by incorporation of hysteresis in the form of a temperature “range,” which separates the switch-off and the switch-on temperatures. 
   While the task of thermal protection circuits that are, for example, used in integrated power circuits is to switch off circuit components having a high dissipation power when a defined temperature threshold is exceeded, the abrupt on-off operations of the protection circuits can cause other damages to sensitive circuits or merely not be desirable for other reasons. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic circuit diagram of a thermal protection circuit, in accordance with an embodiment of the invention. 
       FIG. 2  is a schematic circuit diagram of a thermal protection circuit with soft shutdown and soft restart, in accordance with another embodiment of the invention. 
       FIG. 3  illustrate an embodiment where switches are appropriately biased transistors. 
       FIG. 4  illustrates a temperature independent current source using a current mirror configuration in conjunction with one of the switches. 
   

   DETAILED DESCRIPTION 
   Various embodiments of the invention will now be described. The following description provides specific details for a thorough understanding and enabling description of these embodiments. One skilled in the art will understand, however, that the invention may be practiced without many of these details. Additionally, some well-known structures or functions may not be shown or described in detail, so as to avoid unnecessarily obscuring the relevant description of the various embodiments. 
   The terminology used in the description presented below is intended to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain specific embodiments of the invention. Certain terms may even be emphasized below; however, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this Detailed Description section. 
   A linear battery charger IC can be overheated due to limited power dissipation through the package. The overheating is readily observed when a full charge current is supplied to a drained battery or when the input voltage of the charger is too high. To prevent damaging the charger, some sort of thermal protection is required. The embodiments described in this detailed description employ simple and effective methods and apparatus to limit die temperature when the IC consumes a lot of power. These embodiments illustrate thermal control mechanisms that limit the die temperature and prevent it from exceeding tolerable values. 
   The power dissipated in a charger IC can be expressed as:
 
I chg (V in −V batt ),
 
where V in  is the input voltage to the charger, V batt  is the momentary battery voltage during the charging process, I chg  is the charge current, which is usually set to a constant proportional to a reference voltage, V ref .
 
   The die temperature increases when power increase is faster than the heat dissipation. Since heat dissipation is a function of ambient temperature, geometrical conduits for heat, component placement, component properties, etc, component die temperature will be uniquely defined. Therefore, where the rise in temperature directly affects a circuit, there is a need for the circuit to be able to adjust automatically, based on local parameters, and maintain a certain temperature range while continuing to perform its designed function. 
   In the disclosed embodiments, when a die temperature increases to a preset threshold, for example 120° C., the charge current I chg  is turned off, allowing the IC to cool down. Subsequent to a shutdown and a cooling period, and after the IC temperature reaches a preset lower threshold, for example 110° C., the charge current is turned back on. By repeating these processes, even when (V in −V batt ) is too high, the die temperature will be limited to a range of 110° C. to 120° C. 
     FIG. 1  shows a simple circuit realization of the above mentioned method, wherein a reference voltage V ref  controls the output current I chg  of a voltage regulated current source  108  to charge a battery  110 . In one embodiment, buffer  112  eliminates the loading effect of the circuit from the reference voltage source. In the following description, it is assumed that the voltage input and the voltage output of buffer  112  are the same, V ref ; however, they may be different in alternative embodiments without departing from the inventive aspects of other disclosed embodiments. It should also be noted that a linear voltage regulated current source is an idealization, and that the actual behavior of a voltage regulated current source is an approximation of such ideal voltage-current relationship. 
   Temperature is an analog quantity but digital systems often use temperature to implement measurement, control, and protection functions. Reading temperature with a microcontroller (μC) is simple in concept. The μC reads the output code of an analog-to-digital converter (ADC) driven by a thermistor-resistor voltage divider, analog-output temperature sensor, or other analog temperature sensors. However, when a sensor output voltage range is significantly smaller than the ADC input voltage range, such as when the number of μC I/O pins is limited or the ADC has insufficient inputs available, there is a need for linear temperature-to-code transfer function. In such cases altering the thermistor is not a practical solution option. 
   Another possible solution is to transmit temperature data directly to the μC. The sensors measure their die temperatures, and because die temperature closely tracks lead temperature, each sensor should be placed so that its leads assume the temperature of the component being monitored. In some cases, however, a temperature may not be tightly coupled to a sensor whose die is much hotter than the surrounding board. An internal temperature sensor may enable the ASIC to shut itself down in response to a temperature fault; however, this capability alone lacks accuracy and seldom warns the system of an impending thermal overload. By adding an externally accessible p-n junction to the ASIC die, it is possible to measure die temperature directly by forcing two or more different forward currents through the sensing junction and measuring the resulting voltages. The difference between the two voltages is proportional to the absolute die temperature: 
               V   ⁢           ⁢   2     -     V   ⁢           ⁢   1       =       kT   q     ⁢     (     1   ⁢           ⁢   n   ⁢       I   2       I   1         )             
where I 1  and I 2  are the two currents forced through the p-n junction, V1 and V2 are the resulting forward voltages across the junction, k is Boltzmann&#39;s constant, T is the absolute temperature of the junction in degrees Kelvin, and q is the electron charge.
 
   This measurement, of course, requires precision circuitry for generating the accurate current ratios and measuring very small voltage differences while rejecting the noise produced by large transients on the power ASIC die. Some solutions require a digital interface, therefore, adding complexity and cost to obtain the accuracy. These also require programming to adjust the necessary parameters for cooling. There is a need for a simple built-in circuit to control the local temperature and to regulate the function based on local thermal conditions and parameters, without undue ADC or μC costs. 
   In the exemplary embodiment illustrated in  FIG. 1 , I chg  is proportional to the reference voltage V ref . In an alternative embodiment the I chg  and V ref  may have a different relationship. When the temperature of the IC  100  reaches, for example, 120° C. the temperature sensor  102 , using a switch control signal  104 , opens switch S 1  and, using a switch control signal  106 , closes switch S 2 , and brings V ref  down to 0 volt and, as a result, brings I chg  also down to 0 Amp. 
   In a bipolar switch, there are typically two current paths through a transistor. The small base current controls the larger collector current. When the switch is closed, a small current flows into the base of the transistor. This causes a much larger current to flow through the emitter. The transistor amplifies this small current to allow a larger current to flow through from its collector (C) to its emitter (E). When the switch is open no base current flows, so the transistor switches off the collector current. 
   Since there are many different transistors, switches and switch characteristics can vary widely with the transistor characteristics. Moreover, other circuits and components can act as switches and are not precluded here. When a transistor is used as a switch it must be either off or fully on. In the fully on state the voltage V CE  across the transistor is almost zero and the transistor is said to be saturated because it cannot pass any more collector current I C . The output device switched by the transistor is usually called the “load.” The important ratings in switching circuits are the maximum collector current I C  (max) and the minimum current gain h FE  (min). The transistor voltage ratings may be ignored unless using a supply voltage of more than about 15V. Transistors cannot switch AC or high voltages and they are not usually a good choice for switching large currents (&gt;5A). In the embodiments of this invention, the currents contemplated meet these criteria. More recent solutions use MOSFET, CMOS, NPN, PNP, and/or other types of transistors. 
   By switching the charge current I chg  off, the IC temperature starts to decrease towards the ambient temperature, which is assumed to be lower than the IC temperature. When the IC temperature reaches, for example 110° C., the temperature sensor  102 , using the switch control signal  104 , closes switch S 1  and, using the switch control signal  106 , opens switch S 2 , and as a result V ref  and I chg  resume their maximum/original values. In an alternative embodiment, a single control line may operate both switches S 1  and S 2 . 
   As mentioned above, in an alternative embodiment S 1  and S 2  may be transistors. Such an embodiment is shown in  FIG. 3 , where the switches are appropriately biased transistors. In this embodiment, the reference current at V d  is compared to the reference voltage V ref , by comparator  310 , to send a signal and control the switches when die temperature exceeds a set value. The components hysterisis characteristics can be utilized to form other embodiments and to control the switching signals. 
   In applications where an abrupt change in the charge current I chg  is not desirable, the control mechanism can be modified to incorporate soft shutdown and soft restart. In one embodiment, illustrated in  FIG. 2 , reference voltage to the voltage regulated current source  108  is coupled with a capacitor  114 , and current sources  116  and  118  are added in series with switches S 1  and S 2 , respectively. In this embodiment  200  the rising and falling rate of V ref  is controlled by the combination of the current sources  116  and  118  and capacitor  114 , wherein:
 
 d ( I   chg ) dt∝d ( V   ref )/ dt=I/C. 
 
   In embodiment  200 , when the IC temperature reaches, for example 110° C., the temperature sensor  102 , using the switch control signal  104 , closes switch S 1  and, using the switch control signal  106 , opens switch S 2 . As a result the output of the current source  116  becomes connected to the input of the voltage regulated current source  108 ; however, the reference voltage at the input of the voltage regulated current source  108 , momentarily, is equal to the voltage of capacitor  114  which keeps rising as it continues storing the current from the current source  116 . 
   Popular current mirrors are widely used as current sources. An ideal current source has infinite output impedance. That is, the output current does not change, even for large swings in output voltage, or in other words, ΔI/ΔV=0. A simple bipolar current mirror has two identical transistors, where the second transistor mirrors the current in the first. The current voltage relationship for a bipolar transistor is:
 
 Ic=Is*e   Vbe/Vt 
 
where the saturation current I s  is a constant. V be  is the base emitter voltage and V t  is the thermal voltage. KT/q=25.8 mV at room temperature.
 
   Identical transistors have the same I s . In a simple current mirror, both transistors have the same Vbe, therefore, both transistors will have the same I c . If base currents are ignored, I ref =I o . Therefore, while the voltage of capacitor  114  rises so does the reference voltage to the voltage regulated current source  108  and, consequently, so does I chg , until the reference voltage approaches its maximum potential. As mentioned above, the rise time of V ref  and I chg  is controllable and provides a soft restart. 
   In embodiment  200 , when the die temperature decreases to a preset threshold, for example 120° C., the temperature sensor  102 , using switch control signal  104 , opens switch S 1  and, using a switch control signal  106 , closes switch S 2  and, in a controlled time period, brings V ref  down to 0 volt and, as a result brings I chg  also down to 0 Amp. Once S 1  is opened and S 2  is closed, the reference voltage at the input of the voltage regulated current source  108  will follow the voltage of capacitor  114 , which has risen to V ref  during the charging process, while the temperature has been under 120° C. 
   It will take some time for the capacitor  114  to drain its charge through current limiting source  118 , and to lower the reference voltage of the voltage regulated current source  108 . Therefore, as the voltage of capacitor  114  drops so does the reference voltage to the voltage regulated current source  108  and, consequently, so does I chg , until the reference voltage approaches 0 volt and I chg  approaches 0 Amp. Therefore, as mentioned above, the fall time of V ref  and I chg  is controllable and provides a soft shutdown. 
   In this configuration the IC temperature remains between the two predetermined temperatures, for example 110° C. and 120° C., without abrupt on-off switching. Also, in embodiment  200 , buffer  112  eliminates the loading effect of the circuit from the reference voltage source. In an alternative embodiment, a single control line may operate both switches S 1  and S 2 . Again, in some IC embodiments of the circuit, S 1  and S 2  may be transistors. 
     FIG. 4  illustrates an embodiment with two temperature independent current sources that use two current mirror configurations  120  and  122  in conjunction with S 2  and S 1 , respectively. In an alternative embodiment only one of the current sources may be implemented by using a current mirror that employs bipolar FETs. 
   CONCLUSION 
   Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” As used herein, the terms “connected,” “coupled,” or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling of connection between the elements can be physical, logical, or a combination thereof. 
   Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or,” in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. 
   The above detailed description of embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed above. While specific embodiments of, and examples for, the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
   The teachings of the invention provided herein can be applied to other systems, not necessarily the system described above. The elements and acts of the various embodiments described above can be combined to provide further embodiments. 
   Changes can be made to the invention in light of the above Detailed Description. While the above description describes certain embodiments of the invention, and describes the best mode contemplated, no matter how detailed the above appears in text, the invention can be practiced in many ways. Details of the compensation system described above may vary considerably in its implementation details, while still being encompassed by the invention disclosed herein. 
   As noted above, particular terminology used when describing certain features or aspects of the invention should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the invention with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification, unless the above Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the invention encompasses not only the disclosed embodiments, but also all equivalent ways of practicing or implementing the invention under the claims. 
   While certain aspects of the invention are presented below in certain claim forms, the inventors contemplate the various aspects of the invention in any number of claim forms. Accordingly, the inventors reserve the right to add additional claims after filing the application to pursue such additional claim forms for other aspects of the invention.