Constant voltage generating apparatus with simple overcurrent/short-circuit protection circuit

1. A constant voltage generating apparatus, comprising:

an output circuit controlled in accordance with a control voltage;

a voltage detection signal generating circuit adapted to generate a voltage detection signal in accordance with a difference between an output voltage signal of said output circuit and a first reference signal;

a current detection signal generating circuit adapted to generate a current detection signal in accordance with a difference between an output current signal of said output circuit and a second reference signal;

a control current generating circuit adapted to generate a control current in accordance with said voltage detection signal and said current detection signal; and

a control current-to-control voltage converting circuit adapted to convert said control current into said control voltage,

wherein said control current generating circuit comprises:

a first current mirror circuit adapted to generate a first current in accordance with said voltage detection signal; and

a second current mirror circuit adapted to generate a second current in accordance with said current detection signal, said first and second current mirror circuits being combined with each other so that said control current is determined in accordance with a difference between said first and second currents.

2. The constant voltage generating apparatus as set forth in claim 1 , wherein said voltage detection signal generating circuit comprises:

a first detection voltage generating circuit adapted to generate a first detection voltage in accordance with the output voltage signal of said output circuit; and

an error amplifier adapted to amplify a difference between said first detection voltage and said first reference signal to generate said voltage detection signal in accordance with said difference.

3. The constant voltage generating apparatus as set forth in claim 2 , wherein said voltage detection signal generating circuit further comprises a voltage divider adapted to divide the output voltage signal of said output circuit to generate said first detection voltage.

4. The constant voltage generating apparatus as set forth in claim 2 , wherein said voltage detection signal generating circuit further comprises a reference voltage source adapted to generate a definite voltage as said first reference signal.

5. The constant voltage generating apparatus as set forth in claim 1 , wherein said current detection signal generating circuit comprises:

a second detection voltage generating circuit adapted to generate a second detection voltage in accordance with the output current signal of said output circuit; and

a comparator adapted to compare said second detection voltage with said second reference signal to generate said current detection signal.

6. The constant voltage generating apparatus as set forth in claim 5 , wherein said second detection voltage generating circuit comprises:

a circuit that forms a current mirror circuit with said output circuit to generate a detection current in response to the output current signal of said output circuit; and

a resistor adapted to convert said detection current into said second detection voltage.

7. The constant voltage generating apparatus as set forth in claim 5 , wherein said current detection signal generating circuit further comprises a reference voltage generating circuit adapted to generate said second reference signal.

8. The constant voltage generating apparatus as set forth in claim 7 , wherein said reference voltage generating circuit generates one of a definite voltage, and first and second variable voltages, said definite voltage corresponding to a limit current of the output current signal of said output circuit, said first variable voltage being decreased immediately after the output voltage signal of said output circuit is decreased from a limit voltage, said second variable voltage being decreased after the output voltage signal of said output circuit is decreased from a voltage lower than said limit voltage.

9. The constant voltage generating apparatus as set forth in claim 7 , wherein the reference voltage generating circuit comprises:

a constant current source;

a diode circuit including first, second, and third diodes connected in series between an input terminal and a ground terminal; and

a fourth diode connected between a node of the constant current source and the diode circuit and an output terminal.

10. The constant voltage generating apparatus as set forth in claim 9 , further comprising a first switch connected between the anode of the third diode a cathode of the third diode and a second switch,

wherein a cathode of the first diode is connected to the ground terminal,

wherein a cathode of the second diode is connected to an anode of the first diode,

wherein the cathode of the third diode is connected to an anode of the second diode,

wherein an anode of the fourth diode is connected to the switch node, and

wherein a cathode of the fourth diode is connected to the second switch connected between the switch node and the anode of the fourth diode.

11. The constant voltage generating apparatus as set forth in claim 10 , wherein when the first switch and the second switch are both turned OFF, the reference voltage is fixed by the first, second, and third diodes, and

wherein, when the first switch is turned OFF and the second switch is turned ON, the reference voltage depends upon the output voltage, and

wherein, when the first and second switches are both turned ON, the reference voltage depends upon the output voltage.

12. The constant voltage generating apparatus as set forth in claim 1 , wherein said control current-to-control voltage converting circuit comprises a resistor.

13. A constant voltage generating apparatus, comprising:

first and second power supply terminals;

an output terminal;

an output transistor connected between said first power supply terminal and said output terminal;

a voltage divider connected between said output terminal and said second power supply terminal and adapted to generate a divided voltage;

an error amplifier connected to said voltage divider and adapted to amplify an error between said divided voltage and a definite reference voltage to generate a voltage detection signal;

a first current detection transistor connected to said first power supply terminal and forming a current mirror circuit with said output transistor;

a first current-to-voltage converting resistor connected to said first current detection transistor and adapted to generate a current detection voltage;

a variable reference voltage generating circuit adapted to generate a variable reference voltage depending upon an output voltage at said output terminal;

a comparator with a first input terminal connected to said first current-to-voltage converting resistor and a second input terminal connected to said variable reference voltage generating circuit, and adapted to compare said current detection voltage with said variable reference voltage to generate a current detection signal;

a second current detection transistor connected between an output of said error amplifier and said second power supply terminal;

a third current detection transistor connected between a control terminal of said output transistor and said second power supply terminal and forming a current mirror circuit with said second current detection transistor;

a fourth current detection transistor connected between an output of said comparator and said second power supply terminal;

a fifth current detection transistor connected between the output of said comparator and said second power supply terminal and forming a current mirror circuit with said fourth current detection transistor; and

a second current-to-voltage converting resistor connected between said first power supply terminal and the control terminal of said output transistor.

14. The constant voltage generating apparatus as set forth in claim 13 , wherein said definite reference voltage corresponds to a limit voltage at said output terminal.

15. The constant voltage generating apparatus as set forth in claim 13 , wherein said variable reference voltage generating circuit comprises:

a constant current source connected between said first power supply terminal and the second input of said comparator;

a first diode circuit connected between the second input of said comparator and said second power supply terminal; and

a second diode circuit connected between said output terminal and the second input of said comparator.

16. The constant voltage generating apparatus as set forth in claim 15 , wherein said first and second diode circuits are determined so that said variable reference voltage is decreased immediately after an output voltage at said output terminal is decreased from a limit voltage at said output terminal.

17. The constant voltage generating apparatus as set forth in claim 15 , wherein said first and second diode circuits are determined so that said variable reference voltage is decreased after an output voltage at said output terminal is decreased from a voltage smaller than a limit voltage at said output terminal.

18. A constant voltage generating apparatus comprising:

first and second power supply terminals;

an output terminal;

an output transistor connected between said first power supply terminal and said output terminal;

a voltage divider connected between said output terminal and said second power supply terminal and adapted to generate a divided voltage;

an error amplifier connected to said voltage divider and adapted to amplify an error between said divided voltage and a first definite reference voltage to generate a voltage detection signal;

a first current detection transistor connected to said first power supply terminal and forming a current mirror circuit with said output transistor;

a first current-to-voltage converting resistor connected to said first current detection transistor and adapted to generate a current detection voltage;

a comparator connected to said first current-to-voltage converting resistor and adapted to compare said current detection voltage with a second definite voltage to generate a current detection signal;

a second current detection transistor connected between an output of said error amplifier and said second power supply terminal;

a third current detection transistor connected between a control terminal of said output transistor and said second power supply terminal and forming a current mirror circuit with said second current detection transistor;

a fourth current detection transistor connected between an output of said comparator and said second power supply terminal;

a fifth current detection transistor connected between the output of said comparator and said second power supply terminal and forming a current mirror circuit with said fourth current detection transistor; and

a second current-to-voltage converting resistor connected between said first power supply terminal and the control terminal of said output transistor.

19. The constant voltage generating apparatus as set forth in claim 18 , wherein said first and second definite reference voltages correspond to a limit voltage and a limit current, respectively, at said output terminal.