Patent ID: 7973525

Claim:
A constant current circuit for supplying a constant current, comprising: a second PMOS transistor; a first PMOS transistor through which a drain current flows based on a drain current of the second PMOS transistor; a first NMOS transistor through which a drain current equal to the drain current of the first PMOS transistor flows when a voltage based on a drain voltage of the first PMOS transistor is applied to a gate of the first NMOS transistor; a second NMOS transistor through which a drain current equal to the drain current of the second PMOS transistor flows when a voltage based on a gate voltage of the first NMOS transistor is applied to a gate of the second NMOS transistor, the second NMOS transistor being lower in threshold voltage than the first NMOS transistor; a first resistor provided between a source of the second NMOS transistor and a ground terminal, for generating a voltage based on a threshold voltage difference between the first NMOS transistor and the second NMOS transistor to supply the constant current; and an activating circuit coupled to the gate of the second NMOS transistor, wherein the activating circuit is configured to cause an activation current to flow from a power supply terminal to the gate of the second NMOS transistor when the constant current is smaller than a predetermined current.