Driving circuit for light emitting element

Abstract

A driving circuit for causing a light emitting element to emit light in response to a driving signal is provided. The driving circuit comprising a first current supply circuit that starts to supply a driving current to the light emitting element in response to the driving signal, a second current supply circuit that starts to supply a supplementary current to the light emitting element in response to the driving signal. The second current supply circuit stops to supply the supplementary current upon detecting that a voltage applied to the light emitting element has reached a threshold voltage.

1 . A driving circuit for causing a light emitting element to emit light in response to a driving signal, the driving circuit comprising:
 a first current supply circuit that starts to supply a driving current to the light emitting element in response to the driving signal;   a second current supply circuit that starts to supply a supplementary current to the light emitting element in response to the driving signal;   wherein the second current supply circuit stops to supply the supplementary current upon detecting that a voltage applied to the light emitting element has reached a threshold voltage.   
     
     
         2 . The driving circuit according to  claim 1 , wherein the threshold voltage is not greater than a light emission voltage of the light emitting element. 
     
     
         3 . The driving circuit according to  claim 1 , wherein:
 the first current supply circuit comprises a current mirror circuit that supplies to the light emitting element, as the driving current, a current that depends on a current supplied from a constant current source; and   the current mirror circuit takes on an operative state in response to the driving signal.   
     
     
         4 . The driving circuit according to  claim 3 , wherein:
 the current mirror circuit comprises a first MOS transistor and a second MOS transistor;   a first main electrode of the first MOS transistor and a first main electrode of the second MOS transistor are connected to a constant voltage source;   a second main electrode of the first MOS transistor is connected to the constant current source;   a second main electrode of the second MOS transistor is connected to the light emitting element.   
     
     
         5 . The driving circuit according to  claim 1 , wherein the second current supply circuit comprises:
 a MOS transistor whose source is connected to the light emitting element, a constant voltage being applied to the drain of the MOS transistor; and   a diode fixing a potential of the gate of the MOS transistor in response to the driving signal;   wherein the second current supply circuit stops supplying the supplementary current as the MOS transistor stops operating as a source follower when the voltage applied to the light emitting element reaches the threshold voltage.   
     
     
         6 . The driving circuit according to  claim 1 , wherein the second current supply circuit comprises:
 a MOS transistor whose source is connected to the light emitting element, a constant voltage being applied to the drain of the MOS transistor; and   an inverter whose input terminal is connected to the light emitting element;   wherein the inverter changes the level of a signal output therefrom when the voltage applied to the light emitting element has reached the threshold voltage; and   the second current supply circuit stops supplying the supplementary current as the potential of the gate of the MOS transistor changes in accordance with the output of the inverter.   
     
     
         7 . The driving circuit according to  claim 1 , wherein the second current supply circuit comprises:
 a MOS transistor whose source is connected to the light emitting element, a constant voltage being applied to the drain of the MOS transistor; and   a comparator whose first input terminal is connected to the light emitting element, a reference voltage being applied to a second input terminal of the comparator;   wherein the comparator changes the level of a signal output therefrom when the voltage applied to the light emitting element has reached the threshold voltage; and   the second current supply circuit stops supplying the supplementary current as the potential of the gate of the MOS transistor changes in accordance with the output of the comparator.   
     
     
         8 . The driving circuit according to  claim 1 , further comprising a third current supply circuit that supplies a preliminary current to the light emitting element prior to starting supply of the driving current.