When the light source module 13 is not connected to the DC-to-DC converting module 12 and the resettable over-current protection circuit SRP, the gate voltage of the second transistor Q2 can be expressed by Equation (1) given below:
Vg2=(r3+r6)/(r1+r3+r6)*Vzd??(1)

In Equation (1), Vg2 stands for the gate voltage of the second transistor Q2; r1 stands for the resistance of the first resistor R1; r3 stands for the resistance of the third resistor R3; r6 stands for the resistance of the sixth resistor R6; Vzd stands for the voltage across the diode.

When the light source module 13 is connected to the DC-to-DC converting module 12 and the resettable over-current protection circuit SRP, the driving current Cd passes through the first transistor Q1 and the third resistor R3 to reach the power source negative electrode V? of the power supplying module 11. At this time, a potential difference is generated across the two ends of the third resistor R3, causing an increase in the gate voltage of the second transistor Q2.

When the driving current Cd continues to increase, the potential difference across the two ends of the third resistor R3 also continues to increase, which makes the gate voltage of the second transistor Q2 continuously increase. When the gate voltage of the second transistor Q2 continues to increase to the gate threshold voltage, the second transistor Q2 is turned on (the maximal load current can be calculated from the above mechanism). At the same time, the gate voltage of the first transistor Q1 decreases, which makes the first transistor Q1 be turned off.