Level shift circuit

1. A level shift circuit adapted to convert a signal of a first power supply voltage of a first supply terminal, which is supplied to an input terminal, into a signal of a second power supply voltage of a second supply terminal and outputs the converted signal to an output terminal, the level shift circuit comprising:

a control circuit configured to detect whether the first power supply voltage is above a predetermined voltage,

a first transistor configured to set a voltage of the output terminal to the second power supply voltage when the signal supplied to the input terminal is below a threshold;

a first switch connected in parallel to the first transistor;

a second transistor configured to set the voltage of the output terminal to the ground voltage when the signal supplied to the input terminal is above the threshold; and

a second switch connected between the second transistor and the output terminal,

wherein the first switch and the second switch are controlled by a detection signal of the control circuit, and wherein a voltage of the output terminal of the level shift circuit is fixed to one of the second power supply voltage or a ground voltage according to the detection signal of the control circuit.

2. The level shift circuit according to claim 1 , wherein the first transistor and the second transistor correspond to NMOS transistors.

3. The level shift circuit according to claim 2 , wherein the control circuit comprises:

an third NMOS transistor having a gate thereof connected to the first supply terminal, a source thereof connected to a ground terminal, and a drain thereof connected to a first output terminal of the control circuit;

a resistive element provided between the second supply terminal and the first output terminal of the control circuit; and

an inverter provided between the drain of the third NMOS transistor and a second output terminal of the control circuit,

wherein the first switch is controlled by a signal of the first output terminal, and the second switch is controlled by a signal of the second output terminal.

4. A level shift circuit mounted on a semiconductor device, comprising:

a signal processing circuit and a control circuit, wherein the signal processing circuit comprises:

a first NMOS transistor having a source thereof connected to a ground terminal and a drain thereof connected to a first internal node;

a second NMOS transistor having a source thereof connected to a ground terminal and a drain thereof connected to a second internal node;

a first inverter having an input terminal thereof connected to an input terminal of the signal processing circuit and a gate of the second NMOS transistor, an output terminal thereof connected to a gate of the first NMOS transistor, and a supply terminal thereof connected to a first supply terminal;

a first switch which is controlled by a signal of a first control signal terminal of the signal processing circuit and which is provided between the first internal node and a ground terminal;

a second switch which is controlled by a signal of a second control signal terminal of the signal processing circuit and which is provided between an output terminal of the signal processing circuit and the second internal node;

a first PMOS transistor having a gate thereof connected to the output terminal of the signal processing circuit, a source thereof connected to a second supply terminal, and a drain thereof connected to the first internal node; and

a second PMOS transistor having a gate thereof connected to the first internal node, a source thereof connected to the second supply terminal, and a drain thereof connected to the output terminal of the signal processing circuit; and

the control circuit comprises:

a voltage detector circuit which has an input terminal thereof connected to the first supply terminal and which detects when a first power supply voltage becomes a voltage of the total of a minimum operating power supply voltage and a predetermined voltage; and

a second inverter having an input terminal thereof connected to an output terminal of the voltage detector circuit and the first control signal terminal, an output terminal thereof connected to the second control signal terminal, and a supply terminal thereof connected to the second supply terminal.

5. The level shift circuit according to claim 4 , wherein the voltage detector circuit comprises:

a third NMOS transistor having a gate thereof connected to the first supply terminal, a source thereof connected to a ground terminal, and a drain thereof connected to the output terminal of the voltage detector circuit; and

a resistive element provided between the second supply terminal and the output terminal of the voltage detector circuit.

6. The level shift circuit according to claim 5 , wherein a threshold voltage of the voltage detector circuit is adjusted to be the total voltage.