1. Field of the Invention
The present invention relates to an over-temperature detecting circuit, and more particularly, to an over-temperature detecting circuit capable of accurately detecting the upper limit of temperature to trigger a thermal shutdown signal to shut relevant circuits for protection.
2. Description of the Prior Art
Please refer to
As illustrated in
VX1>VGSQ2+VBE1(T)=VGSQ2+VBE10+KT (1);
wherein VGSQ2 represents the threshold voltage of the transistor Q2. The values of the voltages VX1, VGSQ2 and VBE1(T) may be well designed such that the inequality (1) is true when the temperature meets the temperature upper limit TS1:
VX1>VGSQ2+VBE1+KTS1 (2).
Please refer to
However, the threshold voltage of the transistor may vary depending on the fabrication process. That is, in inequity (1), the threshold voltage VGSQ2 is not independent of the fabrication process but may drift due to different fabrication processes. Therefore, the inequity (2) may be true only under certain base-emitter voltage VBE1 (T) at other temperature as desired, such like TS3. And the inequity (2) then becomes: VX1>VGSQ2+VBE10+KTS3. Hence the temperature at which the over-temperature detecting circuit 100 determines to shut down the relevant circuit may drift from the predetermined temperature upper limit TS1, and the relevant circuit may not be shut down timely and consequently be the destroyed.
The present invention discloses an over-temperature detecting circuit. The over-temperature detecting circuit comprises a band-gap circuit for generating a temperature-drop-dependent voltage and a reference voltage not varying with the temperature, a transistor coupled to the band-gap circuit for generating a temperature-rise-dependent current according to the temperature-drop-dependent voltage, a resistor coupled to the transistor for generating a temperature-rise-dependent voltage according to the temperature-rise-dependent current, and a comparator coupled to the band-gap circuit and the resistor for generating a thermal shutdown signal according to the reference voltage and the temperature-rise-dependent voltage.
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.
Please refer to
The band-gap circuit 310 is utilized to provide a temperature-drop-dependent voltage VX2 and a reference voltage VBG. The temperature-drop-dependent voltage VX2 decreases along with the increase of the temperature, while the reference voltage VBG does not vary along with the temperature. The temperature-drop-dependent voltage VX2 may be represented by: VX2(T)=VX20−MT, wherein T represents the temperature, VX20 represents the initial value of the temperature-declining voltage VX2, and M represents a constant.
The control end, i.e. the gate, of the transistor Q7 is for receiving the temperature-drop-dependent voltage VX2, the first end, i.e. the source, of the transistor Q7 is for receiving the bias source VDD, and the second end, i.e. the drain, of the transistor Q7 is coupled to the resistor R3. The transistor Q7 generates the temperature-rise-dependent current IX2 according to the temperature-drop-dependent voltage VX2. For the temperature-drop-dependent voltage VX2 decreases along with the increase of the temperature, the gate-source voltage of the transistor Q7 increases by [VDD−VX2(T)=VDD−VX20+MT] in accordance. That is, the temperature-rise-dependent current IX2 increases along with the temperature. The temperature-rise-dependent current IX2 flows along the resistor R3 such that the temperature-rise-dependent voltage V3 crossing the resistor R3 increases along with the temperature as well.
The comparator CMP comprises a positive input, a negative input and an output. The positive input of the comparator CMP is coupled to the band-gap circuit 310 for receiving a reference voltage VBG. The negative input of the comparator CMP is coupled to the resistor R3 for receiving the temperature-rise-dependent voltage V3. The output of the comparator CMP is for outputting the thermal shutdown signal VTH2. When the voltage V3 is lower than the reference voltage VBG, the comparator CMP outputs the thermal shutdown signal VTH2 at the high voltage level (VH) to represent that the temperature has not reached the temperature upper limit TS2, and the relevant circuit coupled to the over-temperature detecting circuit 300 is able to remain normal operation and does not need to be shut down. On the contrary, when the temperature-rise-dependent voltage V3 exceeds the reference voltage VBG, the comparator CMP outputs the thermal shutdown signal VTH2 at the low voltage level (VL) to represent that the temperature has reached the temperature upper limit TS2 and the relevant circuit needs to be shut down.
The band-gap circuit 310 comprises four transistors Q3, Q4, Q5 and Q6, two resistors R1 and R2, and an operational amplifier OP. The transistors Q3 and Q4 are PNP bipolar junction transistors, and the transistors Q5 and Q6 are PMOS transistors. The structure of internal components of the band-gap circuit 310 is illustrated as follows.
The base of the transistor Q3 is coupled to the collector of the transistor Q3. The collector of the transistor Q3 is coupled to a bias source VSS (ground). The emitter of the transistor Q3 is coupled to the negative input of the operational amplifier OP and the resistor R1. The base of the transistor Q4 is coupled to the collector of the transistor Q4. The collector of the transistor Q4 is coupled to the bias source VSS (ground). The emitter of the transistor Q4 is coupled to resistor R2. The resistor R2 is coupled to the emitter of the transistor Q4, the positive input of the operational amplifier OP and the drain of the transistor Q6. The resistor R1 is coupled to the negative input of the operational amplifier OP, the drain of the transistor Q5 and the emitter of the transistor Q3. The source of the transistor Q5 is coupled to the bias source VDD. The gate of the transistor Q5 is coupled to the output of the operational amplifier OP. The drain of the transistor Q5 is coupled to the resistor R1. For the transistor Q6, the source is coupled to the bias source VDD, the gate is coupled to the output of the operational amplifier OP, and the drain is coupled to the resistor R2. The operation principle of the band-gap circuit 310 is well known to the people skilled in the art, and is not described here for conciseness. The band-gap circuit 310 takes the drain of the transistor Q5, or an end of the resistor R1, as an output for outputting the reference voltage VBG which does not vary with the temperature. The band-gap circuit 310 takes the output of the operational amplifier OP as another output for outputting the temperature-drop-dependent voltage VX2 which decreases along with the increase of the temperature.
Besides, in the band-gap circuit 310, the transistors Q5 and Q6 are PMOS transistors, while the transistors Q3 and Q4 are PNP bipolar junction transistors.
Dependence of the temperature-drop-dependent voltage VX2 on temperature variation does not change in different fabrication processes. Similarly, the dependence of the deduced temperature-rise-dependent voltage V3 does not change in different fabrication processes as well. Therefore, when the reference voltage VBG is stationary, the comparator CMP is able to accurately determine when the temperature reaches the temperature upper limit TS2 according to the temperature-rise-dependent voltage V3, so as to generate the thermal shutdown signal VTH2 at the low voltage level.
Besides, the bias voltage output from the bias source VDD is higher than the bias voltage output from the bias source VSS.
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To sum up, the high-precision over-temperature detecting circuit of the present invention may accurately determine when the temperature is over-high to output the thermal shutdown signal and shut down the relevant circuit, which decreases the damage of the relevant circuit caused by over-temperature, providing great convenience.
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.
Number | Date | Country | Kind |
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097136830 | Sep 2008 | TW | national |