Abstract:
A process-independent thermal protection circuit for microelectronic circuits is disclosed, including a thermal ramp generator suitable to generate a first thermal ramp signal and a second thermal ramp signal, a differentiator suitable to determine the difference between the first and second thermal ramp signals in order to generate a difference voltage signal, and a comparator suitable to compare the difference voltage signal with a reference voltage signal in order to assert a thermal protection signal when the difference voltage signal drops below the reference voltage signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to a process-independent thermal protection circuit for microelectronic circuits. 
     2. Description of Related Art 
     It is known that microelectronic circuits are provided with thermal protection in order to protect their operation if the temperature rises above a predetermined threshold temperature. 
     These thermal protection circuits generally use the temperature-dependent variation of an implantation resistor, coupled to the variation of a voltage between the base and the emitter of a bipolar transistor. 
     The coupling can be direct, i.e., the resistor is connected between the base and the emitter of the bipolar transistor. FIG. 1 illustrates a direct coupling configuration, in which resistor RT is the resistor coupled between the base and the emitter of the bipolar transistor  1 . 
     FIG. 2 shows an example of an indirect coupling configuration, in which a comparator  2  detects the positive variation of the voltage on one side caused by the resistor RT, and the negative variation on the other side caused by one or more diodes  3  and  4 . 
     Usually, especially in processes with high production capacities, control of process-related characteristics is unreliable, resulting in the resistance of resistor RT varying considerably between different batches and throughout the years of product production. Statistical estimates confirm resistance variations on the order of plus or minus 10%. 
     Moreover, two or three tests with different contacts are usually performed in order to calibrate a thermal protection. These tests slow the development of the device, especially when this is done by using lines which are not dedicated to development. 
     SUMMARY OF THE PRESENT INVENTION 
     The aim of the present invention is to provide a thermal protection circuit for microelectronic circuits which is substantially process-independent. 
     Within the scope of this aim, a preferred embodiment the present invention is a thermal protection circuit for microelectronic circuits which substantially completely eliminates the uncertainty of process-dependent variations in a temperature sensing resistor. 
     The preferred embodiment of the present invention allows for the simple selection of a triggering temperature level. 
     The preferred embodiment of the present invention is highly reliable and relatively easy to produce at competitive costs. 
     This aim and others which will become apparent hereinafter are achieved by a process-independent thermal protection circuit for microelectronic circuits, characterized in part by including a thermal ramp generator suitable to generate a first thermal ramp signal and a second thermal ramp signal, a differentiator suitable to determine the difference between the first and second thermal ramp signals in order to emit a difference voltage signal, and a comparator suitable to compare the difference voltage signal with a reference voltage signal in order to activate a thermal protection signal when the difference voltage signal drops below the reference voltage signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further characteristics and advantages will become apparent from the following detailed description of a preferred but not exclusive embodiment of the circuit according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: 
     FIG. 1 is a circuit diagram of a conventional thermal protection circuit; 
     FIG. 2 is a circuit diagram of another conventional thermal protection circuit; 
     FIG. 3 is a circuit diagram of the thermal protection circuit according to an embodiment of the present invention; 
     FIG. 4 is a transistor diagram of the thermal protection circuit according to a variation of the embodiment of the present invention shown in FIG. 3; 
     FIG. 5 is a plot of various voltage signals of the thermal protection circuit of FIG. 3 over temperature; and 
     FIG. 6 is a circuit diagram of a portion of the thermal protection circuit according to a second embodiment of the present invention. 
    
    
     In all the Figures, identical reference numerals designate identical elements. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Since FIGS. 1 and 2 have already been described, they are not discussed further and accordingly reference is made only to FIG. 3, which illustrates the circuit according to an embodiment of the present invention. 
     The circuit includes a thermal ramp generator for generating thermal ramp signals, generally designated by the reference numeral  5 , having a first circuit branch with a diode  6  and a second circuit branch with a plurality of series-connected diodes. In the case of FIG. 3 there are four series-connected diodes  7 , but the number of diodes can vary according to the setting chosen for thermal intervention at which the present protection circuit is triggered or otherwise asserts a protection signal. Each diode  6  and  7  preferably includes a p-n junction. 
     It is understood that other active elements whose performance is affected by temperature may be used in place of diodes  6  and series-connected diodes  7 . For example, bipolar transistors that are configured as diodes may be used in place of diodes  6  and  7 . FIG. 4 illustrates a circuit diagram of the present circuit having diodes  6  and  7  configured as bipolar transistors. 
     The first and second circuit branches of the thermal ramp generator  5  are each supplied a reference current I ref  which is mirrored by a current mirror in the first and second branches of the thermal ramp generator  5 . 
     The current mirror circuit is formed by bipolar transistors  8 ,  9  and  10 , connected respectively between the reference current source I ref  and ground, between diode  6  and ground, and between the plurality of series-connected diodes  7  and ground. The current I ref  flowing through transistor  8  of the first stage of the current mirror is mirrored in transistors  9  and  10  of the second stages. 
     The first circuit branch, which includes diode  6 , generates a first thermal voltage signal VT 1 , while the second circuit branch, which includes the series-connected diodes  7 , generates a second thermal voltage signal VT 2 . Thermal voltage signals VT 1  and VT 2  are fed to a differentiator, generally designated by the reference numeral  15 , which includes an operational amplifier  16  that receives thermal voltage signals VT 2  and VT 1  respectively at the inverting and noninverting inputs thereof. 
     Two resistors R 1  and R 2  are respectively connected in parallel to the noninverting input of the operational amplifier  16 . The resistor R 1  is connected to the first branch of the thermal ramp generator  5  and particularly to the cathode of diode  6 . 
     Likewise, a second pair of resistors R 3  and R 4  is connected to the inverting input of the operational amplifier  16 . 
     The resistor R 3  is connected to the second circuit branch of the thermal ramp generator  5  and particularly to one end of series-connected diodes  7 . 
     The output Vdif of differentiator  15 , represented as the output voltage signal Vdif of the differentiator  15 , is fed to comparator  20 , which compares signal Vdif with a reference voltage Vref. In particular, the voltage signal Vdif is fed to the inverting input of operational amplifier  17 , and the reference voltage Vref is fed to the noninverting input thereof. 
     A bipolar transistor  18  is cascade-connected to the comparator  16  and is driven thereby. 
     The thermal ramp generator  5  utilizes the thermal variation of the voltage between the anode and the cathode of the diode  6  with respect to the much larger variation of the chain of four series-connected diodes  7  (the number of diodes  7 , as mentioned, can vary according to requirements). Whereas in the exemplary protection circuit of FIG. 3 the thermal variation of the voltage between the anode and the cathode of the diode  6  is approximately −1.8 mv per ° C., the chain of series-connected diodes  7  generates four times this variation, i.e., approximately 7.2 mv per ° C. 
     FIG. 5 is a graph illustrating the operation of the present invention. Specifically, FIG. 5 shows how voltage signals VT 1  and VT 2  increase with an increase in temperature, and that voltage signal VT 2  increases at a faster rate than voltage signal VT 1  as the temperature increases (i.e., voltage signal VT 2  has a greater slope than voltage signal VT 1  over temperature). As can be seen, the voltage signal Vdif steadily decreases as the temperature increases. When the voltage differential between voltage signals VT 1  and VT 2  approaches a predetermined value such that voltage signal Vdif falls below reference voltage Vref, in this case at approximately 155° C., the output of comparator  20  is driven to a high voltage level which turns on transistor  18  and causes Vterm to fall. 
     The slopes of the thermal ramp signals are positive because they are referenced to the supply voltage, which is set to 5v for exemplary purposes. However, it is advisable to connect the chain of series-connected diodes  7  to an internal reference between 3v (not shown) and 5v. The internal reference may be provided, for example, by a Zener diode (not shown). 
     The slope obtained as a difference, in the differentiator  15 , of the two above mentioned slopes can be employed for the intended thermal intervention by the present protection circuit. 
     It should be observed that it is also possible to trigger thermal intervention based upon techniques other than the difference between voltage drops along two sets of diodes. For example, an amplifier and/or multiplier may be employed which multiplies the voltage between the anode and the cathode of one or more diode  7  in the second circuit branch of the ramp generator  5 . FIG. 6 illustrates a circuit in which the second circuit branch includes resistors R H  and R L  formed as a voltage divider. In this configuration, the voltage across resistor R H  is added to the voltage across the base-emitter terminals of transistor T 1  and the voltage across diodes  7  to form signal VT 2 . The voltage across resistor R H  may be set based upon the resistance values selected for resistors R H  and R L  . 
     Alternatively, a number of diodes that is different from the number of series-connected diodes  7  shown in FIG. 3 may be utilized in the second circuit branch of ramp generator  5 , depending on how the thermal intervention is desired to be set. 
     A further refinement of the embodiment of the present circuit thermal protection circuit can be achieved by appropriately setting the currents in resistors R 1 -R 4 . Although resistors R 1 -R 4  can be affected by process variations, resistors R 1 -R 4  have no first-order temperature effect because Resistors R 1 -R 4  are configured as voltage dividers with matching and/or mutually identical resistances. 
     The signal obtained at the output of the differentiator  15  and the reference voltage Vref are applied to the comparator  20 . When the difference voltage Vdif drops below the reference voltage Vref, thermal intervention is triggered and thermal protection is enabled. 
     In practice it has been observed that the thermal protection circuit according to the present invention allows the thermal protection provided in microelectronic circuits to be substantially independent of the variation in the absolute value of the thermal sensing resistor normally used in conventional circuits. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.