JPH0827667B2 - Load drive circuit - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a load drive circuit, and more particularly to a load drive circuit in which a base current of a load drive transistor is changed according to a load condition.

(2) Conventional Technology and Problems Conventionally, when a motor is taken as an example of a load, in a circuit that drives a load on and off by a transistor, the current supplied to the base of the drive transistor is constant regardless of the load condition of the motor. . An example of such a conventional load drive circuit is shown in FIG. In the drive circuit of FIG. 1, the base current of the transistor Q ₂ that drives the motor M is the transistor Q ₁ , the diode D ₃ and the current source.
Supplied by a constant current source composed of I ₁ . Since this base current is constant regardless of the operating current of the motor M, it needs to be set with respect to the maximum operating current of the transistor Q ₂ . Therefore, for example, if the maximum drive current is 1 A and the current amplification factor β of the transistor Q ₂ is 20, it is necessary to flow 50 mA as the base current. By the way, when the motor is driven at 100 mA in the above example, the base current of 5 mA is sufficient, and 45 mA is wasted.

(3) Object of the Invention The main object of the present invention is to reduce the useless base current by changing the base current of the drive transistor according to the load condition in view of the above-mentioned problems of the conventional type.
It is to provide a load drive circuit capable of reducing power consumption. Another object of the present invention is to facilitate heat radiation design by reducing power consumption and heat generation due to reduction of power consumption when the circuit is integrated into an IC.

(4) Configuration of the Invention According to the present invention, there is provided a load driving circuit which has a transistor for driving a load and a base current supply circuit for the transistor, and which respectively decreases or increases the base current according to the decrease or increase of the load current. The base current supply circuit includes a constant current supply unit that supplies a constant current to the base of the load driving transistor, and a supply current to the base of the load driving transistor according to a change in the load current. A constant current source, a first amplification unit that detects a change in load current and generates an output current according to the magnitude of the constant current source, A second amplifier that responds to a collector potential of the load driving transistor and controls the first amplifying unit to a non-operating state when the load driving transistor is in a reverse bias state. And a transistor for driving the load by a change in supply current determined by a difference current between a current from the constant current source and an output current from the first amplifying unit that changes according to a change in load current. While changing the output of
There is provided a load driving circuit characterized in that a constant base current is supplied by the constant current supply unit to ensure a minimum output of the load driving transistor.

In the above configuration, the constant current supply unit can supply a constant base current (that is, drive current) to the load driving transistor even if the output of the variable current supply unit becomes zero (0). It is possible to prevent the load from entering a non-operating state (for example, a stopped state in the case of a motor) when the output of the variable current supply unit becomes zero.

(5) Embodiment of the Invention FIG. 2 shows a load drive circuit as an embodiment of the present invention. The load drive circuit of FIG. 2 has a transistor Q ₃ ,
Q ₄ , diodes D ₁ and D ₂ , differential amplifiers A ₁ and A ₂ , constant current sources I ₂ and I
₃ , reference voltage sources V ₁ and V ₂ , resistors R ₂ , R ₃ and R _4, and a switch SW.

In the drive circuit of FIG. 2, the operating current I ₀ of the motor M,
Let I _{B be} the base current of the drive transistor Q ₄ . Operating current
The change in I ₀ is detected by the resistor R ₄ and input to one input of the amplifier A ₂ . The other input of amplifier A ₂ has a reference voltage source V ₁
Is input. If the operating current is small, A ₂
Has a large sinking current at the output of the constant current source I ₂
Most of the current is drawn into A ₂ , and diode D ₁ is off. The base current of the drive transistor Q _{4 in} this case is set as follows, where I ₄ is the collector current of the transistor Q ₃ .

(1) I _B = I ₃ + I ₄ (2) V _BE (D ₂ ) + R ₂ I ₃ = V _BE (Q ₃ ) + R ₃ I ₄ where V _BE is respectively Is determined by I ₃ and I ₄ .

When the operating current I ₀ increases, the sink current of the amplifier A ₂ decreases and a part of the current of the constant current source I ₂ flows to the diode D ₁ , so that the base current of the transistor Q ₃ increases, As a result, the collector current I ₄ of the transistor Q ₃ increases, and the base current I _B of the transistor Q ₄ increases. In this way, in the motor drive circuit of FIG. 2, a change in the operating current of the motor M is detected and the base current of the drive transistor is controlled accordingly.

In the motor drive circuit of FIG. 2, the collector potential of the transistor Q ₄ is also input to the amplifier A ₁ , and the counter electromotive force is generated by the L component of the motor when the motor is forcibly stopped and the collector of the transistor Q ₄ is collected. When the potential is negative, that is, when the transistor is reverse biased, the output of A ₁ turns off A ₂ . Therefore, in such a case, the base current of the transistor Q ₄ is set to the maximum point,
The L component energy of the motor is promptly released and the reverse bias of the transistor Q ₄ is released.

The change of I _B with respect to I ₀ in the motor drive circuit of FIG. 2 is shown in FIG.

In this embodiment, since the constant current supply unit (I ₃ , D ₂ , Q ₃ , R ₂ and R ₃ ) for supplying a constant current is provided, it is supplied to the base of the load driving transistor Q _4. Drive current
At least I ₃ + I ₄ is guaranteed for I _B. Generally, in the output control by feedback, the output current supplied to the load becomes zero (0) due to the response delay of the feedback system and the like, so that the load cannot be driven satisfactorily. According to this, since the minimum output is guaranteed as described above, there is an advantage that the minimum drive of the load (in this case, the motor M) can be guaranteed.

(6) Effects of the Invention According to the present invention, it is possible to reduce power consumption by reducing a useless base current in a drive transistor in a load drive circuit, and quickly release reverse bias of the drive transistor due to load conditions. it can. In particular, when the circuit is integrated into an IC, the power consumption can be reduced, so that the heat radiation design for reducing the heat generation can be facilitated and the destruction due to the reverse bias can be prevented.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a conventional motor drive circuit, FIG. 2 is a circuit diagram of a motor drive circuit according to an embodiment of the present invention, and FIG. 3 is an I to I ₀ in the circuit of FIG. It is a characteristic view which shows the change of _B. (Explanation of symbols) M: motor, Q ₁ , Q ₂ , Q ₃ , Q ₄ ... transistor, D ₁ , D ₂ , D
₃ ...... _{diode, R 2, R 3, R} 4 ...... resistance, A _1, A ₂ ...... differential amplifier, SW ...... switch.

Claims (1)

[Claims] 1. A load drive circuit having a load driving transistor and a base current supply circuit for the transistor, wherein the base current is reduced or increased in accordance with a decrease or increase in the load current. The circuit is a constant current supply unit that supplies a constant current to the base of the load driving transistor,
A variable current supply unit that changes the supply current to the base of the load driving transistor according to a change in the load current, wherein the variable current supply unit detects a change in the load current and a constant current source. And a first amplifier that generates an output current according to the magnitude of the load driving transistor, and the first amplifier when the load driving transistor is in a reverse bias state in response to the collector potential of the load driving transistor. A second amplifier that controls the amplifier to a non-operating state, and a difference between a current from the constant current source and an output current from the first amplifier that changes according to a change in load current. The output of the load driving transistor is changed by the change of the supply current determined by the current, and the minimum output of the load driving transistor is supplied by the constant base current supplied by the constant current supply unit. Load drive circuit and performs the warranty.