US 2008/0174253 A1Published application

Constant-current circuit and light-emitting diode drive device therewith

AssigneeRicoh Company, Ltd.

InventorIppei Noda

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Abstract

A constant-current circuit includes a first transistor for supplying a current based on a control signal input to a gate of the first transistor so as to serve as a current source, a second transistor for supplying a current to a load based on the control signal input to a gate of the second transistor, a voltage regulation unit for controlling a drain voltage of the first transistor according to a drain voltage of the second transistor, a current detector for detecting a value of a current flowing through the first transistor and output a current according to the detected value, and a controller for controlling each gate voltage of the first and second transistors according to the value detected by the current detector so that the current flowing through the first transistor becomes a predetermined value. The first and second transistors are MOS transistors having the same conductivity.

Claims20

Dependent claims

A constant-current circuit, comprising:

a first transistor formed of a MOS transistor and configured to supply a current based on a control signal input to a gate of the first transistor to serve as a current source;
a second transistor formed of a MOS transistor having the same conductivity as the first transistor and configured to supply a current to a load based on the control signal input to a gate of the second transistor that includes a source connected to ground, the gate connected to the gate of the first transistor, and a drain connected to the load;
a voltage regulation unit configured to control a drain voltage of the first transistor according to a drain voltage of the second transistor;
a current detector configured to detect a value of a current flowing through the first transistor and output a current according to a detected value;
and
a controller configured to control a gate voltage of the first transistor and a gate voltage of the second transistor according to the value detected by the current detector so that the current flowing through the first transistor becomes a predetermined value.

3 more elements

A light-emitting diode drive circuit comprising a constant-current circuit configured to generate a predetermined constant current and supply the constant current to a light-emitting diode, wherein the constant-current circuit includes:

a first transistor formed of a MOS transistor and configured to supply a current based on a control signal input to a gate of the first transistor to serve as a current source;
a second transistor formed of a MOS transistor having the same conductivity as the first transistor and configured to supply a current to a load based on the control signal input to a gate of the second transistor that includes a source connected to ground, the gate connected to the gate of the first transistor, and a drain connected to the load;
a voltage regulation unit configured to control a drain voltage of the first transistor according to a drain voltage of the second transistor;
a current detector configured to detect a value of a current flowing through the first transistor and output a current according to a detected value;
and
a controller configured to control a gate voltage of the first transistor and a gate voltage of the second transistor according to the value detected by the current detector so that the current flowing through the first transistor becomes a predetermined value.

3 more elements

Dependencies are inferred from the claim text.