JP2807700B2 - Output circuit having current limiting circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an output circuit having a current limiting circuit for protection.

[Conventional technology]

FIG. 3 shows a conventional current limiting circuit. This circuit is incorporated in the output circuit. First, the output circuit divides and detects the constant current source 1, the output transistor Q1, the load _RL supplied with the output current from the output transistor Q1 from the output terminal 2, and the voltage applied to the load _RL. Resistors R1 and R2,
The output voltage detected by the resistor R2 is negatively fed back to the error amplifier 3 to control the output voltage corresponding to the signal applied to the input terminal 4 to be supplied to the load _RL .

The current limiting circuit, the load current detecting resistor R3 connected to the emitter of the transistor Q1, consists transistor Q2 for detecting a voltage generated in the resistor R3, the voltage generated in the resistor R3 exceeds V _BE2 of the transistor Q2, the The transistor Q2 is turned on, and the current flowing from the output transistor Q1 to the load _RL is limited.

[Problems to be solved by the invention]

By the way, in this circuit, the limited current I _LIMIT is expressed by I _LIMIT = V _BE2 / R3, but the voltage V _BE2 has a negative temperature coefficient (−2mV /
° C) while resistor R3 has a positive temperature coefficient (eg, 2000pp
m / ° C.), the limiting current I _LIMIT has a negative temperature coefficient, and there is a problem that it is significantly affected by temperature.

The present invention has been made in view of such a point,
Its purpose is to make the limiting current insensitive to temperature.

[Means for solving the problem]

Therefore, the present invention provides an output transistor for amplifying the output of the error amplifier, a constant current source connected to the base of the output transistor, and a current connected between the emitter of the output transistor and the output terminal. And a current limiting circuit for limiting the current of the output transistor when the current detected by the current detection resistor exceeds a predetermined value. And a second transistor having an emitter area larger than that of the first transistor, connecting the current detection resistor to the emitter of the second transistor, and connecting the current detection resistor to the collectors of the first and second transistors. Connecting a current mirror circuit so that the first transistor and the second transistor have the same collector current. Ri, it was to perform the current limit.

A third transistor for detecting a collector potential of the first transistor; and a collector and an emitter connected between the output of the error amplifier and the output terminal, and the third transistor is turned on by the conduction of the third transistor to generate a current. A fourth transistor for limiting may be provided.

〔Example〕

Hereinafter, examples of the present invention will be described. FIG. 1 is a diagram showing an output circuit having a current limiting circuit of one embodiment. The same components as those in FIG. 3 are denoted by the same reference numerals.

In this embodiment, a transistor is used as the output transistor.
Using a transistor Q3 and a transistor Q4 having an emitter area N times larger than that of the transistor Q3, a current detecting resistor R3 is connected to the emitter side of the transistor Q4, and a current mirror circuit including transistors Q5 and Q6 is connected to the collector side. A transistor Q7 controlled by the collector potential of the transistor Q6 of the current mirror circuit is provided, and a current flows through the resistor R4 by the conduction of the transistor Q7 to control the current limiting transistor Q2.

Now, in this embodiment, the transistor Q4 is a transistor
Since the emitter area is N times larger than Q3, the output current Io
But, Io <a _{(V T l n N) /} R3 range, many people of the current flowing in the collector current I _C4 of the transistor Q4 than the collector current I _C3 of the transistors Q3, the transistor Q7 is turned off not to flow the base current Thus, the transistor Q2 is also off. Note that V _T = kT
In / q, k is the Boltzmann constant, T is the absolute temperature, and q is the electron charge.

On the other hand, the output current Io, if the _{Io ≧ (V T l n N} ) / R3, the more than the collector current I _C4 of the collector current I _C3 of the transistor Q3 transistors Q4, the transistor Q7 is flows the base current on Then, a voltage is generated in the resistor R4, the transistor Q2 is turned on, and the transistor Q3,
The base current of Q4 is cut, and the current is limited. The above state is shown in FIG.

Therefore, the current value I _LIMIT to be limited, I _LIMIT = is represented by _{(V T l n N) /} R3, the voltage V _T positive temperature coefficient (e.g., 3300 ppm /
° C) and the resistance R3 also has a positive temperature coefficient as described above, so that the temperature dependency is reduced.

〔The invention's effect〕

As described above, according to the present invention, there is an advantage that the value of the current to be limited is not greatly affected by the temperature, and therefore, even if the temperature changes, the current can be limited by a desired current value.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an output circuit having a current limiting circuit according to an embodiment of the present invention, FIG. 2 is an operation explanatory diagram, and FIG. 3 is a circuit diagram of an output circuit having a conventional current limiting circuit. 1 ... constant current source, 2 ... output terminal, 3 ... error amplifier,
4 Input terminal.

Claims (2)

(57) [Claims] An output transistor for amplifying an output of the error amplifier; a constant current source connected to a base of the output transistor; and a current detection connected between an emitter of the output transistor and the output terminal. And the resistance of
An output circuit having a current limiting circuit for limiting the current of the output transistor when the current detected by the current detection resistor exceeds a predetermined value, wherein the output transistor is a first transistor and the first transistor
The current detecting resistor is connected to the emitter of the second transistor, and the current mirror circuit is connected to the collectors of the first and second transistors. An output circuit having a current limiting circuit, wherein the first transistor and the second transistor have the same collector current to limit the current. 2. A third transistor for detecting a collector potential of the first transistor, and a collector and an emitter are connected between an output of the error amplifier and the output terminal, and the third transistor is turned on by conduction of the third transistor. 3. An output circuit having a current limiting circuit according to claim 1, further comprising a fourth transistor for limiting current.