JPH0827668B2 - Constant voltage power supply protection circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a protection circuit for protecting an output transistor of a constant voltage power supply from being destroyed by an excessive current.

(B) Prior Art "Voltage Regulator Handbook", pages 1-8, published by Seibundo Shinkosha on September 10, 1982, describes a constant voltage power supply having a current limiting circuit. As shown in FIG. 2, the constant voltage power supply includes an output transistor (4) having a collector connected to a power supply terminal (1) and an emitter connected to an output terminal (2) through an overcurrent detection resistor (3), and First and second resistors (5) and (6) connected in series between the output terminal (2) and ground;
And a comparison circuit (8) for comparing the voltage obtained at the connection point of the second resistors (5) and (6) with the voltage from the reference power source (7), and the voltage drop of the overcurrent detection resistor (3). And a protection transistor (9) that operates accordingly. Since the output transistor (4) is controlled by the output signal of the comparison circuit (8), a constant voltage can be obtained at the output terminal (2) and the output transistor (4) When an excessive current flows, the protection transistor (9) can be activated to protect the output transistor (4) from damage.

(C) Problems to be Solved by the Invention However, in the circuit of FIG. 2, the overcurrent detection resistor (3) is inserted between the emitter of the output transistor (4) and the output terminal (2), and Transistor (4)
Since it is an NPN type, there is a problem that a voltage drop between the power supply terminal (1) and the output terminal (2) becomes large and a low loss constant voltage power supply cannot be produced. Moreover, since the protection transistor (9) is simply activated by detecting an excessive current, there is a problem that the detection sensitivity changes according to variations in elements and temperature changes.

(D) Means for Solving the Problems The present invention has been made in view of the above-mentioned points, and is based on a current detection transistor whose base and collector are commonly connected to an output transistor and an emitter current of the detection transistor. A constant resistance driving type first level shift circuit for level shifting the terminal voltage of the resistance, a constant current driving type second level shift circuit for generating a reference voltage, And a comparison circuit for comparing the outputs of the second level shift circuit and a limiting transistor for limiting the current of the output transistor according to the output of the comparison circuit.

(E) Operation According to the present invention, the current flowing in the current detection transistor is caused to flow through the detection resistor, and the terminal voltage of the detection resistor is level-shifted by the current-driven first level shift circuit to be used as the input of the comparison circuit. In the comparison circuit, since the reference voltage obtained by the resistor is compared with the voltage level-shifted by the constant current drive type second level shift circuit, the operation start voltage of the protection circuit is changed by the resistance ratio and the current. Can be set. Therefore, reliable protection of the output transistor can be achieved.

(F) Embodiment FIG. 1 shows an embodiment of the present invention. In (10), the emitter is the power supply terminal (11) and the collector is the output terminal (12).
PNP type output transistors, each connected to (1
3) and (14) are first and second voltage dividing resistors that divide the output voltage V _OUT obtained at the output terminal (12), and (15) is the first and second voltage dividing resistors (13) and ( The first comparison circuit for comparing the voltage obtained at the voltage dividing point of 14) with the voltage of the reference power source (16) and generating an output signal according to the difference between the two voltages, (17) and (18) are The first and second drive transistors controlled by the output signal of the first comparison circuit (15), (19) a current detection transistor whose base and collector are commonly connected to the output transistor (10), and (20) the detection A detection resistor that causes a voltage drop according to the emitter current of the transistor (19), and a base of the (21) is the detection transistor (1
A first level shift transistor, which is connected to the emitter of 9) and constitutes a first level shift circuit, (22) and (2)
3) is the first which is differentially connected and constitutes the second comparison circuit ( 24 ).
And a second transistor, (25) is a first constant current transistor whose collector is connected to the emitter of the first level shift transistor (21), and (26) and (27) are a power supply terminal (11) and the second transistor. A resistor-diode-connected second level shift transistor that is connected in series with the base of (23) to form a second level shift circuit,
(28) is a second constant current transistor whose collector is connected to the emitter of the second level shift transistor, and (29) is a limiting transistor which shuts off the output signal of the first comparison circuit (15).

The output voltage V _OUT obtained at the output terminal (12) is divided by the first and second voltage dividing resistors (13) and (14) and compared with the reference voltage in the first comparison circuit (15). First
An output signal corresponding to the difference between the two voltages is generated at the output terminal of the comparison circuit (15), and the output signal is transmitted to the first drive transistor (1
Applied to the base of 7). In that case, if the output voltage V
_{When OUT} decreases, the output signal of the first comparison circuit (15) becomes large and the first and second drive transistors (17) and (18)
The collector current of the output transistor becomes large, and the output transistor (10)
Increase the collector current of and increase the output voltage V _OUT . Further, if the output voltage V _OUT rises, the output signal of the first comparison circuit (15) becomes small and the collector currents of the first and second drive transistors (17) and (18) become small,
It reduces the collector current of the output transistor (10) and lowers the output voltage V _OUT . Therefore, a constant output voltage can be obtained at the output terminal (12).

Output transistor (10) and current detection transistor (1
If the emitter area ratio with 9) is set to N: 1, the emitter current I _E1 of the output transistor (10) becomes And the emitter current I of the current detection transistor (19)
_E2 is Becomes Then, from the equations (1) and (2), I
The relationship between _E1 and I _E2 is Becomes

By the way, since the emitter current I _E2 of the current detection transistor (19) flows through the detection resistor (20), the base voltage V 2 of the first transistor (22) of the second comparison circuit ( 24 ).
_B1 is V _B1 = V _IN −R _A · _{IE 2} −V _BE3 …… (4) Becomes The base voltage V _B2 of the second transistor (23) of the second comparison circuit ( 24 ) is V _B2 = V _IN −R _B · I ₀₂ −V _BE4 (5) Becomes Then, the first and second transistors (2
2) and (23) base voltages V _B1 and V _B2 become equal,
If the protection operation is started when the first transistor (22) is turned on, the fourth (5) and (5)
From the formula, Becomes Here, the operating currents of the first and second level shift transistors (21) and (27) are given by the first and second constant current transistors (25) and (28), and their values are I ₀₁ = (Vref-V _BE5 ) / R ₅ ...... (7) I ₀ = (Vref-V _BE6 ) / R ₆ ...... (8) _Therefore , if the values of the resistors (30) and (31) are equal to each other and V _BE5 = V _BE6 , then I ₀₁ = I ₀₂ and the first
Also, the operating currents of the second level shift transistors (21) and (27) can be made equal, and as a result, their base-emitter voltages can be made equal. Therefore, the equation (6) is Therefore, the emitter current I _E2 of the current detection transistor (19) at the start of the protection operation is uniquely determined. Then, the more the (3), once the said emitter current I _E1 of at protection operation starts, since the emitter current I _E1 is also determined protection operation start of the output transistor (10), after all, the protection according to the present invention The protection action of the circuit is the value of the sense resistor (20).
R ₄ and the value R _B of the resistor (26) and the collector current I ₀₂ of the second constant current transistor (28). In the integrated circuit, the resistance ratio can be set with high accuracy and the constant current can be set accurately, so that the start of the protection operation can be set accurately.

Therefore, with the circuit configuration as shown in FIG. 1, when the overcurrent I _E1 flows through the output transistor (10), the emitter current I _E2 flows through the current detection transistor (19), and the second current of the second comparison circuit ( 24 ) appears. Since the 1-transistor (22) is turned on and the limiting transistor (29) is turned on, the output of the first comparison circuit (15) is cut off, the current of the output transistor (10) is limited, and the output transistor (10) is turned on. ) Protection is achieved.

(G) Effect of the Invention As described above, according to the present invention, it is possible to obtain the protection circuit capable of surely protecting the output transistor of the constant voltage power supply from the destruction due to the excessive current. Further, according to the present invention, it is possible to obtain the protection circuit in which the start of the protection operation can be set accurately and easily. Further, according to the present invention, it is possible to provide a protection circuit suitable for integration into an integrated circuit.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional constant voltage power supply. Description of main figure numbers (10) …… Output transistor, (19) …… Current detection transistor, (20) …… Detection resistor, (21) …… First level shift transistor, ( 24 ) …… Second comparison Circuits, (26)
...... Resistance, (27) ...... Second level shift transistor,
(29) …… Limiting transistor.

Claims (1)

[Claims] 1. A protection circuit for a constant voltage source, comprising a current detection transistor whose base and collector are commonly connected to the base and collector of an output transistor, wherein the detection resistor causes a voltage drop according to a current flowing through the detection transistor. A constant current drive type first level shift circuit for level shifting the terminal voltage of the detection resistor, a resistor for generating a reference voltage, and a constant current drive type second level shift for level shifting the reference voltage. A circuit, and the first and second
A protection circuit for a constant voltage power supply, which comprises a comparison circuit for comparing the outputs of the level shift circuits and a transistor for limiting the current of the output transistor according to the output of the comparison circuit, and which protects the output transistor from being destroyed by an excessive current.