CR oscillation circuit

1. A CR oscillation circuit, comprising:

an oscillation unit which has first through third inverting circuits connected in series between a first node and a second node;

a capacitance element provided between the first node and an output terminal of the second inverting circuit;

a conducting resistor connected between the first node and the second node, the conducting resistor controlling a conducting resistance between the first node and the second node according to a level of a control voltage;

a constant current unit which outputs a constant voltage by producing a constant current according to a resistance value of an outside resistive element; and

a level conversion unit which converts a voltage level of the constant voltage and produces the control voltage.

2. The CR oscillation circuit according to claim 1 , wherein the constant current unit comprises:

a first current mirror circuit to which a first voltage is applied, and

a second current mirror circuit which is connected to the first current mirror circuit via a third node and to which a second voltage is applied via the outside resistive element.

3. The CR oscillation circuit according to claim 2 , wherein the first current mirror circuit comprises:

a first P channel MOS transistor having a source electrode to which the first voltage is applied, and gate and drain electrodes connected to the third node, and

a second P channel MOS transistor having a source electrode to which the first voltage is applied, and a gate electrode connected to the gate electrode of the first P channel MOS transistor, and

wherein the second current mirror circuit comprises:

a first N channel MOS transistor having a source electrode to which the second voltage is applied, a drain electrode connected to a drain electrode of the second P channel MOS transistor, and a gate electrode connected to the drain electrode thereof, and

a second N channel MOS transistor having a source electrode to which the second voltage is applied via the resistive element, a drain electrode connected to the third node, and a gate electrode connected to the gate electrode of the first N channel MOS transistor.

4. The CR oscillation circuit according to claim 1 , wherein the level conversion unit comprises:

a third P channel MOS transistor having a source electrode to which the first voltage is applied, a gate electrode connected to the third node, and a drain electrode connected to a fourth node, and

a third N channel MOS transistor having a source electrode to the second voltage is applied, and gate and drain electrodes connected to the fourth node.

5. The CR oscillation circuit according to claim 1 , wherein conducting resistor comprises:

a fourth P channel MOS transistor having a source electrode connected to the first node or the second node, a drain electrode connected to the second node or the first node, and a gate electrode to which the control voltage is applied, and

a fourth N channel MOS transistor having a source electrode connected to the first node or the second node, a drain electrode connected to the second node or the first node, and a gate electrode to which the control voltage is applied.

6. A CR oscillation circuit, comprising:

an amplifier circuit connected between a first node and second nodes, for outputting a signal of a low level to the second node when a potential of the first node is less than or equal to a threshold voltage, and outputting a signal of a high level when the potential thereof exceeds the threshold voltage;

an inverting amplifier circuit for inverting a signal at the second node to thereby output an oscillation signal to a third node;

a capacitor connected between the first and second nodes;

transistors connected between the third and first nodes, said each transistor providing a conducting resistor having a value controlled by a control voltage;

a constant current circuit for allowing a constant current corresponding to a value of resistor connected to the outside to flow to thereby output a constant voltage; and

a level converting circuit for level-converting the constant voltage to produce the control voltage.

7. The CR oscillation circuit according to claim 6 , wherein the transistors comprise a first conductive type MOS transistor whose conducting resistor has a value controlled by the constant voltage, and a second conductive type MOS transistor whose conducting resistor has a value controlled by the control voltage.