JPH073943B2 - Overcurrent protection circuit - Google Patents

Description

The present invention relates to an overcurrent protection circuit, and more particularly to an overcurrent protection circuit for an integrated circuit that outputs a large current.

[Conventional technology]

Conventionally, this kind of overcurrent protection circuit detects the output current of the output transistor and turns off the output transistor when it determines that there is an abnormality.

FIG. 4 shows an example of a conventional circuit.

In the conventional circuit shown in FIG. 4, 1 is an input circuit, 3 is a driver circuit that drives the output transistor Q ₁ , and 4 is a current that detects the output current of the output transistor Q ₁ and outputs a current or voltage proportional to the value. The detection circuit 5 is a control circuit that outputs a high / low signal according to the output of the current detection circuit 4, and 2 is a logic circuit that outputs the logical sum of the output of the input circuit 1 and the output of the control circuit 5. Here, the control circuit 5 is configured such that when the output current of the output transistor Q ₁ exceeds a certain current value Is, its output level is inverted and a signal for turning off the output transistor Q ₁ is output.

Further, terminal A is an input terminal, terminal B is a power supply terminal, terminal C is an output terminal, and _RL is a load.

[Problems to be solved by the invention]

In such a configuration, the output current fault in the load (e.g. load becomes or load shorting reduced) is generated is larger than Ia, out signal for turning off the output transistor Q ₁ from the control circuit 5, the output transistor Q ₁ is turned off To do.

Therefore, the output current decreases, and when it becomes smaller than Is, the control circuit 5 outputs a signal for turning on the output transistor Q ₁ , so that the output transistor Q ₁ is turned on again. Then, the output current increases again, and as a result, the above operation is repeated. This state continues until the load abnormality is removed.

The above-mentioned repetition cycle and the on / off time of the output transistor Q ₁ are determined by the system delay and loop gain, and cannot be set to arbitrary values. Therefore, the output transistor may be damaged depending on the length of on-time and the value of on-time / off-time.

Further, as a control circuit of an overcurrent protection circuit, there has been proposed a circuit that includes a comparison circuit having a hysteresis characteristic, a resistor and a capacitor, and sets a conduction period and a cutoff period of an output transistor according to a charging / discharging time of the capacitor. (JP-A-57-210727).

However, in this circuit, if there is fluctuation or oscillation of the current flowing through the output transistor, the comparator circuit may be sensitive to the connection and disconnection of the output transistor to cause a malfunction.

An object of the present invention is to provide an overcurrent protection circuit that does not cause a malfunction even with such fluctuations and vibrations of the output current.

[Means for solving problems]

The overcurrent protection circuit of the present invention detects a current detection circuit for detecting an output current flowing through an output transistor, detects that the detection current of the current detection circuit exceeds a predetermined value, and controls the output transistor to be disconnected. In the overcurrent protection circuit including a control circuit for controlling the current, the control circuit inverts to a second output when the detection current of the current detection circuit exceeds a first predetermined value, and the first predetermined value A Schmitt circuit that becomes a first output when it becomes a lower second predetermined value or less, and a first constant current circuit that is controlled by the second output of the Schmitt circuit so as to flow out a constant current. And a capacitor connected to the output terminal of the first constant current circuit, and a second capacitor connected in parallel to the capacitor and controlled to sink a constant current at the first output of the Schmitt circuit. Constant current Characterized by comprising a circuit, a comparison circuit for comparing the terminal voltage and the reference voltage of the capacitor has a hysteresis outputs a control signal for the on-time and off-time of the output transistor.

〔Example〕

 Next, the present invention will be described with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, and the same numbers and symbols as in FIG. In FIG.
6 is a Schmitt circuit that outputs a high / low signal according to the output of the current detection circuit 4, 7 is a constant current circuit that is controlled by the output of the Schmitt circuit 6 to flow out a constant current, 8
Is a constant current circuit controlled to absorb a constant current by the output of the Schmitt circuit 6, 9 is a capacitor for charging / discharging, and 10 is a comparator circuit having hysteresis to compare the terminal voltage of the capacitor 9 with a threshold value. Here, the Schmitt circuit 6 is provided so as not to malfunction due to fluctuations and vibrations of the output current, and I _S1 and I _S2 (where I
_S1 > I _S2 ). On the other hand, constant current circuit 7
7 and 8 are configured so that 7 is off and 8 is on when the output current is I _S2 or less, and 7 is on and 8 is off when the output current is I _s1 . The threshold value of the comparison circuit 10 is V _{TH (H)} , V _{TH (L)} (where V _{TH (H)} > V
_{TH (L)} ).

The second timing chart of each part in the embodiment of FIG.
Shown in the figure. When the output current is normal, that is, when the output current is I _S1 or less (before to), the constant current circuit 7 is turned off, and the constant current circuit 8
Is on, the terminal voltage of the capacitor 9 becomes the ground potential, and the output of the comparison circuit 10 outputs a signal for turning on the output transistor Q ₁ . Next, when an abnormality occurs in the load at to and the output current exceeds I _S1 , the Schmitt circuit 6
Output is inverted, the constant current circuit 7 is turned on, and 8 is turned off. Therefore, the charging of the capacitor 9 is started, the terminal voltage of the capacitor 9 starts to rise, and reaches the threshold value V _{TH (H)} of the comparison circuit 10 at time t ₁ . Then the comparison circuit
The output of 10 is inverted, providing a signal that turns off output transistor Q ₁ . Therefore, the output transistor Q ₁ is turned off and the output current is reduced. When the output current is smaller than I _S2 , the output of the Schmitt circuit 6 is inverted and the constant current circuit 7 is turned off and 8 is turned on. Therefore, the capacitor 9 starts discharging, and starts decreasing the terminal voltage of the capacitor 9 and reaches the threshold value V _{TH (L)} of the comparison circuit 10 at time t ₂ . Then, the output of the comparison circuit 10 is inverted again, and a signal for turning on the output transistor Q ₁ is output. Therefore, the output transistor Q ₁ turns on and the output current increases. And this time the output current is greater than I _S1, the output constant current circuit 7 inverts again is on the Schmitt circuit 6, 8 is turned off, V _TH in the terminal voltage starts to rise time t ₃ of the capacitor 9 _{(H )} Is reached. Then, similarly to the operation at t ₁ , the output transistor Q ₁ is turned off, the terminal voltage of the capacitor 9 decreases, and V _{TH (L) at} time t ₄ .
The output of the comparison circuit 10 is inverted and the output transistor
Send a signal to turn on Q ₁ . This causes the output transistor Q ₁ to turn on and the output current to increase again. These series of operations are continued as long as the load is abnormal. But Figure 2
When the output current does not reach I _S1 as indicated by t ₅ in FIG. ₅ , that is, when the load abnormality disappears, the terminal voltage of the capacitor 9 is grounded because the constant current circuit 7 remains off and 8 remains on. The potential is reduced to the potential, and the output transistor Q ₁ becomes responsive to the input signal.

Here, the times (t ₂ −t ₁ ) and (t ₃ −t ₂ ) are called off time T _OFF and on time T _ON , respectively. These on time and off time can be arbitrarily set by the current values of the constant current circuits 7 and 8, the capacitance value of the capacitor 9 and the threshold values V _{TH (H)} and V _{TH (L)} of the comparison circuit 10.

However, it is necessary to set the on-time in consideration of the abnormal value of the output current from the safe operation area of the output transistor Q ₁ , and when the output current of about 7 A flows like this embodiment, it must be set to about 100 μs. On the other hand, the off-time must be determined from the on-time / off-time so as to suppress the power consumption of the output transistor Q ₁ . From the aspect of the circuit, the capacitance value of the capacitor 9 in the integrated circuit and the constant current circuits 7 and 8
And the threshold value V of the comparison circuit
_{Limited by TH (H)} and V _{TH (L)} .

From the above, in the present embodiment, the capacitance value C ₁ = 60 of the capacitor 9
p _F , set value of constant current circuit 7 I ₇ = 1.8 μA, set value of constant current circuit 8 I ₈ = 0.3 μA, V _{TH (H)} = 3.0 v, V _{TH (L)} = 1.0 v Time T _ON and off time T _OFF are Therefore, T _ON ≈ 70 μs and T _OFF ≈ 400 μs are set.
As a result, the power consumption is reduced to about 30% of the power consumption when the on time is equal to the off time in the conventional example.

Next, FIG. 3 shows another embodiment of the present invention. The same numbers and symbols as in FIG. 1 indicate the same. In FIG. 3, 4 in a dotted line is a current detection circuit, which includes resistors R ₁ , R ₂ , R ₃ and a diode D.
₁ , a transistor Q ₂ , and a constant current source I ₁ . Resistor provided between power supply terminal B and output transistor Q ₁
Detecting an output current by R _1, and generates a voltage proportional to the voltage across the R ₁ across the resistor R _3.

The operation of the circuit shown in FIG. 3 is exactly the same as the operation described in the embodiment of FIG.

〔The invention's effect〕

As described above, in the overcurrent protection circuit of the present invention, the operation of the time constant circuit that determines the on-time and off-time of the output transistor is controlled by the Schmitt circuit having hysteresis with respect to the current flowing through the output transistor. , Prevent frequent charge and discharge of the capacitor,
There is an effect that it is possible to prevent a malfunction.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG.
FIG. 3 is a timing chart of each part in the circuit shown in FIG. 3, and FIG. 3 is a circuit diagram showing another embodiment of the present invention. FIG. 4 is a diagram showing a conventional example. 1 ... Input circuit, 2 ... Logic circuit, 3 ... Driver circuit, 4 ... Current detection circuit, 5 ... Control circuit, 6 ... Schmidt circuit, 7 ... Constant current circuit, 8 ... Constant current circuit , 9
...... Capacitor, 10 …… Comparison circuit, Q _{1 to} Q ₂ …… Transistor, D ₁ …… Diode, R _{1 to} R ₃ …… Resistance, _RL …… Load, I ₁ …… Constant current source, A… … Input terminal, B… power supply terminal, C… output terminal.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-57-210727 (JP, A)

Claims (1)

[Claims] 1. A current detection circuit for detecting an output current flowing through an output transistor, and a first output in response to a detection output from the circuit indicating that the output current exceeds a first predetermined value. A Schmitt circuit for generating a second output in response to the detection output indicating that the output current is less than or equal to a second predetermined value that is lower than the first predetermined value. A first current circuit activated by the first output of the Schmitt circuit to charge the capacitor, and a second current activated by the second output of the Schmitt circuit to discharge the capacitor. A circuit and a first threshold and a second threshold below this threshold
A drive signal for shutting off the output transistor when the voltage across the capacitor exceeds the first threshold and for turning on the output transistor when the voltage across the capacitor becomes equal to or lower than the second threshold. An overcurrent protection circuit comprising: a comparison circuit that supplies the output transistor.