JP3587056B2 - Overvoltage protection circuit - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an overvoltage protection circuit for an integrated circuit.
[0002]
[Prior art]
Various protection functions, including overvoltage protection, in integrated circuits require a certain amount of time (between the time constants determined by externally connected capacitors, etc.) before the protection function operates and the output stops to prevent malfunction due to noise. What you need is common. However, while securing a delay time for preventing malfunction due to noise, when the power supply voltage becomes higher than the rated voltage, if the output is not cut off instantaneously, there is a possibility that equipment connected to the output may be destroyed.
[0003]
If the method of shutting off the output between the overvoltage protection function and the other protection functions is changed, it is easy to instantaneously shut off the output only when the overvoltage protection function is activated.
[0004]
[Problems to be solved by the invention]
When the method of shutting off the output between the overvoltage protection function and the other protection functions is changed, an extra circuit is required as compared with the case where the output shutoff method is one.
It is an object of the present invention to provide an economical overvoltage protection circuit that is resistant to malfunction due to noise and instantaneously shuts off the output only when the overvoltage protection function is activated.
[0005]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, in the present invention, a first current source, a capacitor connected in series to the first current source, and a constant voltage diode connected in parallel to the capacitor are provided at normal time. Is an overvoltage protection circuit comprising: a series circuit with a first switch that is on ; an overvoltage detection circuit; and a comparator that compares a signal at a connection point between the first current source and the capacitor with a reference voltage. , A series circuit connected in parallel with the first current source, the second circuit being composed of a second current source and a second switch that is normally off when the overvoltage is detected. a first switch and a turning on-off Toshikatsu second switch by a signal from the detection circuit.
[0006]
Further, in the above overvoltage protection circuit, the second current source supplies a larger current than the first current source.
In the above overvoltage protection circuit, an OR circuit having an input of an output of the overvoltage detection circuit and an output of another protection detection circuit is provided, and the output of the OR circuit turns on and off the first switch. I decided.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, 1 and 5 are current sources, 2 is a capacitor, 3 is a constant voltage diode, 4 is a comparator input, 6 is a comparator, 7 is a reference voltage, 8 is an OR circuit, and 9 and 10 are switches.
To the OR circuit 8, in addition to the output of the overvoltage detection circuit (not shown), the output of each detection circuit (none is shown) such as overheat detection, overload detection, and low voltage detection is led. When any of the circuits detects an abnormality, it outputs a logic high signal. When any one of the detection circuits detects an abnormality, the switch 9 is turned off by the output of the OR circuit 8, and charging of the capacitor 2 is started by the current supplied from the current source 1.
[0008]
The capacitor 2 is normally charged up to the constant voltage of the constant voltage diode 3 connected in parallel with the capacitor 2, and when the switch 9 is turned off due to abnormality detection, the terminal voltage of the capacitor 2 (the voltage of the comparator input 4) ) Starts increasing more linearly from the constant voltage of the constant voltage diode 3. Eventually, when the voltage of the comparator input 4 reaches the reference voltage defined by the reference voltage 7, the comparator 6 outputs a cutoff signal to an output cutoff circuit (not shown).
[0009]
The time required from the abnormality detection to the output of the cutoff signal is determined by the magnitude of the current supplied from the current source 1, the capacitance of the capacitor 2, the constant voltage of the constant voltage diode 3, and the reference voltage of the reference voltage 7. However, in consideration of the noise resistance and the allowable operation delay time of the protection circuit, an appropriate value that ensures a certain delay time is selected.
[0010]
The output of the overvoltage detection circuit is also guided to a switch 10 in addition to the OR circuit 8. When an overvoltage is detected, the switch 10 is turned on, and the capacitor 2 is supplied to the capacitor 2 in addition to the current from the current source 1. The current from the current source 5 also flows in. Since the time constant of charging the capacitor is inversely proportional to the charging current, the charging time of the capacitor 2 is reduced by adding the current from the current source 5. Therefore, when overvoltage is detected, the charging time of the capacitor 2 is shorter than when another abnormality is detected in which the charging current is supplied only from the current source 1, and a shutoff signal from the comparator 6 to the output shutoff circuit is quickly output. .
[0011]
Here, the magnitude of the current supplied from the current source 5 may be approximately the same as the magnitude of the current supplied from the current source 1. However, in order to quickly output a cutoff signal when an overvoltage is detected, a capacitor is required. The current supplied from the current source 5 should be larger than the current supplied from the current source 1 (for example, about 4 to 5 times) because the current supplied to the current source 2 should be increased to shorten the charging time. ) It is preferable to set.
[0012]
【Example】
FIG. 2 is a diagram showing a specific embodiment of the present invention. 2, MOS-FETs 101 and 105 correspond to the current sources 1 and 5 in FIG. 1, and a common gate bias signal is applied to the MOS-FETs 101 and 105. The MOS-FETs 109 and 110 correspond to the switches 9 and 10, and the MOS-FET 103 corresponds to the constant voltage diode 3. The other components that are the same as those in FIG.
[0013]
The inverted output of the OR circuit 8 is led to the gate of the MOS-FET 109, and normally 5 V is applied to the gate to turn on the MOS-FET 109. And the capacitor 2 is charged to about 2.3V. Here, if the reference voltage 7 is set to 3 V, the comparator 6 does not operate.
[0014]
When an overvoltage is detected by the overvoltage detection circuit, the inverted output of the OR circuit becomes 0 V, the MOS-FET 109 turns off, and the MOS-FET 110 turns on. Then, the current flows through the route of VCC → MOS-FET101 → capacitor 2, and also flows through the route of VCC → MOS-FET105 → MOS-FET110 → capacitor 2. The capacitor 2 is charged, and the voltage of the comparator input 4 is rapidly increased. To rise. When the voltage exceeds the set voltage (3 V) of the reference voltage 7, the comparator 6 outputs a cutoff signal to the output cutoff circuit.
[0015]
Note that the input signal from the output cutoff circuit is provided so that once the output cutoff circuit operates, it cannot be restored unless the power supply VCC is turned off.
A common gate bias signal is applied to the MOS-FETs 101 and 105. Since the gate bias signal is common, if the same element is selected as the MOS-FETs 101 and 105, a current of the same magnitude can be supplied as a current source, and the current amplification factor of the MOS-FET 105 is larger than that of the MOS-FET 101. If an element is selected, the current supplied from the current source 5 can be increased as described above.
[0016]
【The invention's effect】
According to the present invention, a circuit for instantaneously shutting off the output only when the overvoltage protection is activated can be realized by adding a small number of circuits as compared with a system in which the output is not shut off instantaneously. The chip area hardly changes, and there is almost no increase in cost due to the use of this method.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an embodiment of the present invention.
FIG. 2 is a diagram showing a specific embodiment of the present invention.
[Explanation of symbols]
1, 5, 101, 105 current source, 2 capacitor, 3, 103 constant voltage diode, 4 comparator input, 6 comparator, 7 reference voltage, 8 OR circuit, 9, 10, 109, 110 ... Switch.

Claims (3)

A first current source, a capacitor connected in series with the first current source, and a series connected in parallel with the capacitor, a constant voltage diode and a first switch that is normally on. A circuit, an overvoltage detection circuit, and an overvoltage protection circuit including a comparator for comparing a signal at a connection point between the first current source and the capacitor with a reference voltage, wherein the overvoltage protection circuit is connected in parallel to the first current source; normal and the second current source comprises a series circuit composed of a second switch is off, when an overvoltage is detected and off the first switch by a signal from the overvoltage detecting circuit and an overvoltage protection circuit which is characterized in that turning on the second switch. 2. The overvoltage protection circuit according to claim 1, wherein the second current source supplies a larger current than the first current source. 3. The overvoltage protection circuit according to claim 1, further comprising: an OR circuit that receives an output of the overvoltage detection circuit and an output of another protection detection circuit as inputs, and controls the first switch by an output of the OR circuit. 4. An overvoltage protection circuit characterized by turning on and off.

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