JPH0654867B2 - Load control device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an improvement of a load control device having an overvoltage detection function and an output monitor signal transmission function.

[Conventional technology]

The output circuit for controlling the on / off of the output current flowing through the load in accordance with the input signal H (high) and L (low) is conventionally the third type.
As shown in the figure, the overvoltage detection circuit 1 may be provided to prevent the circuit from being destroyed when the power supply Vcc rises abnormally.
To explain the operation without the circuit 1, the input signal 1
N is amplified by the buffer 2 and drives the transistor Q ₁ . Since the transistor Q ₁ is a pnp type, it turns off when 1N = H and turns on when IN = L. When the transistor Q ₁ is turned on, current flows from the power source Vcc to the load 3. On the contrary, if the transistor Q ₁ is off, no current flows in the load 3. OUT is an output voltage applied to the load 3, and is compared with the reference voltage 5 by the comparator 4. The output of the comparator 4 becomes the monitor signal MON, and the state of the load 3 can be determined by comparing this with the signal IN at that time.
The logic is as follows.

The load 3 has a normal state and a fault state of short circuit or open. Since no voltage is generated in the load 3 in the short-circuited state, the monitor signal MON is L regardless of H or L of the input IN. On the other hand, no current flows in the load 3 in the open state. Therefore, a small current (several hundred μ
If A) is flown and flows into the comparator 4 without flowing to the load 3, the monitor signal MON becomes H. Also in this case, it does not depend on the input H or L. Therefore, the input IN is H,
When the monitor signal MON changes to L and H of opposite phase when changing to L, it can be judged that the load is normal, and when the monitor signal MON similar to the input IN is obtained (when IN is L, MO
If N is L, and MON is H when IN is H, then it can be determined that the load has failed.

[Problems to be solved by the invention]

However, when the overvoltage detection circuit 1 is provided, the transistor Q ₁ is turned off regardless of whether the input IN is H or L when Vcc rises abnormally. For this reason, the monitor signal when the load is normal remains at L level, so when the input IN is L, the monitor signal M
When ON is L, it becomes the same state as the load short circuit. When the system receiving the monitor signal MON detects a load short circuit, it stores this, and fixes the signal IN at H to make the transistor Q
Lock so that ₁ cannot be turned on. Therefore, if this happens during overvoltage detection, the output circuit will not operate even if Vcc returns to normal thereafter. The present invention seeks to improve this point.

[Means for solving problems]

The present invention monitors a switching element that controls on / off of a current flowing through a load according to an input signal, and a power supply voltage of the switching element, and forcibly turns off the switching element when it becomes an overvoltage. To turn on the switching element by means of an overvoltage detection circuit kept at, a comparator that compares the voltage applied to the load with a reference voltage to create an output monitor signal, and the logic of the output monitor signal and the input signal of the switching element. If a monitor signal indicating that the voltage applied to the load is less than or equal to the reference voltage is generated for the input signal, it is determined that the load is short-circuited, and the load for the input signal that turns off the switching element. If a monitor signal indicating that the voltage applied to is higher than the reference voltage is generated, it is determined that the load is open, and the switching element is A monitor signal indicating that the voltage applied to the load is higher than the reference voltage is generated for the input signal to turn on, and a monitor signal indicating that the voltage is lower than the reference voltage for the input signal that turns off the switching element. If it occurs, it is determined that the load is normal, and when it is determined that the load is short-circuited, the overvoltage detection circuit detects an overvoltage in the load control device including means for forcibly keeping the switching element in the off state. In this case, a control circuit is provided which uses, as an output monitor signal, a signal when the load is normal according to the input signal instead of the output monitor signal from the comparator.

FIG. 1 is a block diagram of its principle, and 7 is a control circuit added by the present invention.

[Action]

The control circuit 7 receives the output of the overvoltage detection circuit 1,
If this is a normal value, the output of the comparator 4 is output as it is as the monitor signal MON, but if it is abnormal, the output (input IN) of the buffer 2 is inverted to become the monitor signal MON.

That is, when the overvoltage is detected, the monitor signal MON is normally set to the normal load regardless of the load state (normal, short circuit, open). With this, there is no erroneous detection such as a load short circuit although the load is normal. The load status is detected when the power supply voltage is normal.

Therefore, when the power supply is cut off by the overvoltage detection circuit 1, it is not erroneously determined that the load is short-circuited, and the switching element is not locked.

〔Example〕

FIG. 2 is a circuit diagram showing an embodiment of the present invention, in which the overvoltage detection circuit 1 and the control circuit 7 are shown in detail. The operation will be described. When Vcc rises abnormally, the Zener diode ZD becomes conductive, turning on the transistor Q _{2 and} turning off the transistor Q ₃ . At this time, the collector voltage b of the transistor Q ₃ becomes H level, forcing the output of the NOR gate G ₁ to L. As a result, the output of the inverter I ₁ becomes H and the transistor Q ₁ is turned off regardless of the level of the input IN. The buffer 2 is composed of a comparator 21, a reference voltage 22 and an inverter I _3.
3 output is in phase with input IN, comparator 21 output a
Are in the opposite phase.

The control circuit 7 includes an inverter I ₂ and a NOR gate G ₂
Consists ~G _5, 3 inputs a, b, with respect to c, produce a · b + a · + · ...... (1) comprising logic output MON. In the above equation, when b = 0 (when Vcc is normal) a · + = (2), the monitor signal MON is the comparator 4 (+ and − of this input are opposite to those in FIG. 1). Is the inverted value of the output c. On the other hand, when b = 1 (when Vcc is abnormal), a + a. = A (3), so that the monitor signal MON is the inverted value a = of the input IN.
It becomes ▲ ▼.

As a result, IN = L, M even if an overvoltage is detected when the load is normal.
Since ON = H, it is not necessary to make an erroneous determination as short circuit detection. Therefore, even if the transistor Q ₁ is temporarily fixed to off due to overvoltage detection, normal operation can be resumed if Vcc returns to normal.

〔The invention's effect〕

As described above, according to the present invention, the output circuit having the overvoltage detection function does not output the monitor signal which is erroneously determined to be the load short circuit when the overvoltage is detected, so that the switching element can be prevented from locking.

[Brief description of drawings]

FIG. 1 is a block diagram showing the principle of the present invention, FIG. 2 is a circuit diagram showing an embodiment of the present invention, and FIG. 3 is a block diagram showing an example of a conventional output circuit. In the figure, 1 is an overvoltage detection circuit, 3 is a load, 4 is a comparator, 5 is a reference voltage, 6 is a constant current source, 7 is a control circuit, G _{2 to} G ₅ are NOR gates, and I ₂ is an inverter.

Claims (1)

[Claims] 1. A current flowing through a load is turned on according to an input signal,
A switching element that is turned off, an overvoltage detection circuit that monitors the power supply voltage of the switching element and forcibly keeps the switching element in an off state when it becomes an overvoltage, and a voltage applied to the load as a reference voltage. The voltage applied to the load is less than or equal to the reference voltage with respect to the input signal for turning on the switching element, according to the comparator for comparing and generating the output monitor signal and the logic of the output monitor signal and the input signal of the switching element. If a monitor signal indicating is generated, it is determined that the load is short-circuited, and a monitor signal indicating that the voltage applied to the load is greater than or equal to the reference voltage with respect to the input signal that turns off the switching element is generated. If so, it is determined that the load is open, and the voltage applied to the load is greater than the input signal that turns on the switching element. If a monitor signal indicating that the voltage is above the quasi voltage is generated and a monitor signal that indicates that the voltage is below the reference voltage is generated for the input signal that turns off the switching element, it is determined that the load is normal, and the load In the load control device having means for forcibly keeping the switching element in the off state when it is determined that the output voltage is short-circuited, when the overvoltage detection circuit detects an overvoltage, the output monitor signal from the comparator is input instead. A load control device comprising a control circuit that uses a signal when a load is normal according to the signal as an output monitor signal.