Constant-voltage circuit device

1. A constant-voltage circuit device to convert a voltage input to an input terminal and output a predetermined constant voltage from an output terminal,

the constant-voltage circuit device comprising:

an output transistor to output an electrical current from the input terminal to the output terminal in response to a control signal;

a reference voltage circuit to generate a predetermined reference voltage;

a control circuit to adjust a voltage proportional to the output voltage output from the output terminal to the reference voltage output from the reference voltage circuit by controlling the output transistor; and

a soft start circuit including

a capacitor for soft start that is charged at start-up, and

a current control unit to control an electrical current supplied to the reference voltage circuit,

the current control unit adjusting the reference voltage to a voltage determined by the capacitor for soft start at the start-up until the reference voltage reaches a desired voltage.

2. The constant-voltage circuit device according to claim 1 , wherein the soft start circuit further comprises a differential amplifier circuit and a constant current source,

wherein the current control unit is a control transistor,

the differential amplifier circuit has a first input terminal connected to an output terminal of the reference voltage circuit and a second input terminal connected to a junction node between the constant current source and the capacitor for soft start, and

an output from the differential amplifier circuit is given to a control electrode of the control transistor.

3. The constant-voltage circuit device according to claim 2 , wherein the differential amplifier circuit comprises;

a pair of input transistors forming a differential pair; and

a current mirror circuit serving as a load to the differential pair,

wherein the input transistors are depression-type NMOS transistors.

4. The constant-voltage circuit device according to claim 3 , wherein the input transistors have unequal gate widths.

5. The constant-voltage circuit device according to claim 3 , wherein the input transistors have unequal gate lengths.

6. The constant-voltage circuit device according to claim 3 , wherein a tail current of the differential amplifier circuit is controlled by a depression-type NMOS transistor.

7. The constant-voltage circuit device according to claim 3 , further comprising a discharge unit to discharge both input terminals of the differential amplifier circuit when the constant-voltage circuit device is off.

8. A constant-voltage circuit device to convert a voltage input to an input terminal and output a predetermined constant voltage from an output terminal,

the constant-voltage circuit device comprising:

an output transistor to output an electrical current from the input terminal to the output terminal in response to a control signal;

a reference voltage circuit to generate a predetermined reference voltage;

a control circuit to adjust a voltage proportional to the output voltage output from the output terminal to the reference voltage output from the reference voltage circuit by controlling the output transistor;

a soft start circuit including a capacitor for soft start that is charged at start-up,

the soft start circuit raising the output voltage output from the output terminal according to a time period during which the capacitor for soft start is charged;

a current detection unit to detect an electrical current flowing to the output transistor; and

a charge-current control unit to control a charge given to the capacitor for soft start according to an electrical current detected by the current detection unit at the start-up.

9. The constant-voltage circuit device according to claim 8 , wherein the soft start circuit further comprises a differential amplifier circuit, a constant current source, and a control transistor to control an electrical current supplied to the reference voltage circuit,

wherein the differential amplifier circuit has a first input terminal connected to an output terminal of the reference voltage circuit and a second input terminal connected to a junction node between the charge-current control unit and the capacitor for soft start, and

an output from the differential amplifier circuit is given to a control electrode of the control transistor.

10. The constant-voltage circuit device according to claim 9 , wherein the differential amplifier circuit comprises:

a pair of input transistors forming a differential pair; and

a current mirror circuit serving as a load to the differential pair,

wherein the input transistors are depression-type NMOS transistors.

11. The constant-voltage circuit device according to claim 10 , wherein the input transistors have unequal gate widths or unequal gate lengths.

12. The constant-voltage circuit device according to claim 10 , wherein a tail current of the differential amplifier circuit is controlled by a depression-type NMOS transistor.

13. The constant-voltage circuit device according to claim 10 , further comprising a discharge unit to discharge both input terminals of the differential amplifier circuit when the constant-voltage circuit device is off.

14. A constant-voltage circuit device to convert a voltage input to an input terminal and output a predetermined constant voltage from an output terminal,

the constant-voltage circuit device comprising:

an output transistor to output an electrical current from the input terminal to the output terminal in response to a control signal;

a reference voltage circuit to generate a predetermined reference voltage;

a control circuit to adjust a voltage proportional to the output voltage output from the output terminal to the reference voltage output from the reference voltage circuit by controlling the output transistor;

a soft start circuit including a capacitor for soft start that is charged at start-up,

the soft start circuit raising the output voltage output from the output terminal according to a time period during which the capacitor for soft start is charged;

a voltage difference detection unit to detect a difference between the input voltage and the constant voltage output from the output terminal; and

a charge-current control unit to control a charge given to the capacitor for soft start according to the difference detected by the voltage difference detection unit at the start-up.

15. The constant-voltage circuit device according to claim 14 , wherein the soft start circuit further comprises a differential amplifier circuit, a constant current source, and a control transistor to control an electrical current supplied to the reference voltage circuit,

wherein the differential amplifier circuit has a first input terminal connected to an output terminal of the reference voltage circuit and second input terminal connected to a junction node between the charge-current control unit and the capacitor for soft start, and

an output from the differential amplifier circuit is given to a control electrode of the control transistor.

16. The constant-voltage circuit device according to claim 15 , wherein the differential amplifier circuit comprises:

a pair of input transistors forming a differential pair; and

a current mirror circuit serving as a load to the differential pair,

wherein the input transistors are depression-type NMOS transistors.

17. The constant-voltage circuit device according to claim 16 , wherein the input transistors have unequal gate widths or unequal gate lengths.

18. The constant-voltage circuit device according to claim 16 , wherein a tail current of the differential amplifier circuit is controlled by a depression-type NMOS transistor.

19. The constant-voltage circuit device according to claim 16 , further comprising a discharge unit to discharge both input terminals of the differential amplifier circuit when the constant-voltage circuit device is off.