To stabilize the operating frequency of a step-down DC/DC converter that drives a light-emitting element.

A current detection circuit 10 generates an off-signal S_OFF asserted when a current I_M1 flowing through a switching transistor M1 reaches a predetermined peak current. A pulse generation circuit 30 generates a pulse signal S2 having a level that changes every time when an on-signal S_ON and the off-signal S_OFF are asserted. A current source 24 charges a first capacitor 22 by a charge current according to an output voltage V_OUT of a DC/DC converter 6. An arithmetic circuit 50 generates a threshold voltage V_C4 according to an input voltage V_IN and the output voltage V_OUT of the DC/DC converter 6 on the basis of V_C4=(V_IN-V_OUT)×V_OUT/V_IN×m (m is a constant value). A first comparator 28 generates the on-signal S_ON asserted when a voltage of the first capacitor 22 reaches the threshold value V_C4.