Constant current circuit

1. A constant current circuit, comprising:

a first current mirror including a first transistor formed on a first current path and a second transistor formed on a second current path;

a second current mirror including a third transistor formed on the first current path and a fourth transistor formed on the second current path;

a first diode formed on the first current path;

a second diode formed on the second current path;

a resistor formed on the second current path;

a variable resistance element directly connected with the third transistor and with the resistor; and

a feedback unit to control a resistance value of the variable resistance element based on a current flowing through the second current path.

2. The constant current circuit according to claim 1 , wherein the variable resistance element includes a transistor.

3. The constant current circuit according to claim 2 , wherein the feedback unit includes:

a fifth transistor connected with the second transistor to form a current mirror; and

a first load, and

the feedback unit generates a voltage to be fed back to the variable resistance element based on a voltage drop in the first load.

4. The constant current circuit according to claim 2 , further comprising:

a first level shifter to shift a level of a voltage at a prescribed node on the second current path and output a level-shifted voltage to the feedback unit.

5. The constant current circuit according to claim 2 , wherein the feedback unit includes:

a sixth transistor connected with the second transistor to form a current mirror;

a seventh transistor to receive a signal based on a voltage at a prescribed node on the first current path;

an eighth transistor to receive a signal based on a voltage at a prescribed node on the second current path; and

a second load connected with the eighth transistor, and

the feedback unit generates a voltage to be fed back to the variable resistance element based on a voltage drop in the second load.

6. The constant current circuit according to claim 2 , wherein the transistor includes a first node connected to the third transistor and a second node connected to the resistor.

7. The constant current circuit according to claim 1 , wherein the feedback unit includes:

a fifth transistor connected with the second transistor to form a current mirror; and

a first load, and

the feedback unit generates a voltage to be fed back to the variable resistance element based on a voltage drop in the first load.

8. The constant current circuit according to claim 7 , further comprising:

a first level shifter to shift a level of a voltage at a prescribed node on the second current path and output a level-shifted voltage to the feedback unit.

9. The constant current circuit according to claim 1 , further comprising:

a first level shifter to shift a level of a voltage at a prescribed node on the second current path and output a level-shifted voltage to the feedback unit.

10. The constant current circuit according to claim 1 , wherein the feedback unit includes:

a sixth transistor connected with the second transistor to form a current mirror;

a seventh transistor to receive a signal based on a voltage at a prescribed node on the first current path;

an eighth transistor to receive a signal based on a voltage at a prescribed node on the second current path; and

a second load connected with the eighth transistor, and

the feedback unit generates a voltage to be fed back to the variable resistance element based on a voltage drop in the second load.

11. The constant current circuit according to claim 10 , further comprising:

a second level shifter to shift a level of a voltage at the prescribed node on the first current path and output a level-shifted voltage to the feedback unit; and

a third level shifter to shift a level of a voltage at the prescribed node on the second current path and output a level-shifted voltage to the feedback unit.

12. A constant current circuit, comprising:

a first diode;

a second diode;

a resistor coupled to a first terminal of the second diode;

first and second transistors of a first conductivity type, control gates of the first and second transistors being connected to each other, the first transistor being coupled to the first diode, the second transistor being coupled to a second terminal of the resistor;

third and fourth transistors of a second conductivity type, control gates of the third and fourth transistors being connected to each other, the third transistor being coupled to the first transistor, the fourth transistor being coupled to the second transistor;

a variable resistance element coupled between a first crossing node of the first diode and the first transistor and a second crossing node of the resistor and the second diode; and

a feedback unit which controls to reduce a resistance value of the variable resistance element when a current obtained by the third and fourth transistors becomes larger, in order to reduce a voltage potential difference at the first crossing node and the second crossing node.

13. The constant current circuit according to claim 12 , wherein the variable resistance element includes a transistor which has a first node connected to the first crossing node, a second node connected to the second crossing node, and a control gate controlled by a signal from the feedback unit.

14. The constant current circuit according to claim 12 , wherein

the first and second transistors are connected in a current mirror structure, and

the third and fourth transistors are connected in a current mirror structure.