A light-emitting device mounted within an information terminal device in which a first housing including a liquid crystal panel and a second housing are foldably connected with each other via a connection portion,
- the light-emitting device comprising:
- a backlight for the liquid crystal panel comprising a plurality of light-emitting diodes mounted within the first housing and, the diodes being connected together by an anode terminal of each diode;
- a current driving circuit mounted within the first housing, connected to a cathode terminal of each of the plurality of light-emitting diodes, and configured to control current that flows through the plurality of light-emitting diodes;
- a switching regulator mounted within the second housing and configured to generate a driving voltage to be supplied to the anode terminal of each diode of the plurality of light-emitting diodes connected together;
- and
- a control circuit mounted within the second housing and configured to control the switching regulator,
- wherein the current driving circuit comprises, integrated on a semiconductor chip:
- a plurality of constant-current circuits respectively connected to the cathode terminals of the plurality of light-emitting diodes;
- a minimum value circuit configured to produce an output of a lowest cathode terminal voltage from among the measurement of the voltage at each cathode terminal of the light-emitting diodes;
- and
- a feedback output terminal configured to feed an output of the minimum value circuit back to the control circuit of the switching regulator;
- and
- wherein the control circuit comprises:
- a first feedback input terminal configured to receive the lowest cathode terminal voltage as a first feedback signal from the feedback output terminal of the current driving circuit;
- a second feedback input terminal configured to receive, as a second feedback signal, a measurement of a second voltage from each anode terminal of the plurality of light-emitting diodes that are connected together;
- a pulse modulator configured to generate a switching signal for controlling an on/off state of the switching regulator so that the lowest cathode terminal voltage at the first feedback input terminal approaches a predetermined voltage;
- and
- a driver circuit configured to drive the switching regulator based on the switching signal and to control the on/off state of the switching regulator so that the second voltage at the second feedback input terminal does not exceed a predetermined threshold voltage;
- wherein the control circuit is integrated on a semiconductor chip which is separate from the semiconductor chip on which the current driving circuit is integrated, and
- the light-emitting device further comprises:
- a first wiring line provided between the first housing and the second housing via the connection portion and connecting an output of the switching regulator and the connected anode terminals of the plurality of light-emitting diodes;
- a second wiring line provided between the first housing and the second housing via the connection portion and connecting the feedback output terminal of the current driving circuit and the first feedback input terminal of the control circuit;
- and
- a third wiring line provided in the second housing and connecting the output of the switching regulator and the second feedback input terminal of the control circuit.