Fast PWM dimming apparatus and control method

A system comprising:

• a high-side switch and a low-side switch connected in series between an input voltage bus and ground;
• an inductor and a current sense resistor connected in series between a common node of the high-side switch and the low-side switch, and an output terminal of the step-down power converter;
• an output capacitor connected between the output terminal of the step-down power converter and ground;
• and
• a controller comprising:
• an on timer configured to determine an on time of the high-side switch of a step-down power converter;
• an offset current control stage configured to convert a voltage difference between a compensation signal and a predetermined reference into a current difference;
• a current comparison stage configured to amplify a sensed current signal;
• and
• a PWM comparator having inputs coupled to two outputs of the current comparison stage and two outputs of the offset current control stage, wherein during a load transient from a first load level to a second load level and in a first turn-on pulse of the high-side switch, the on timer is disabled so that a current flowing through the step-down power converter reaches the second load level within the first turn-on pulse of the high-side switch, and wherein during the load transient from the first load level to the second load level and in the first turn-on pulse of the high-side switch, the two outputs of the offset current control stage are disconnected from the two inputs of the PWM comparator so that the current flowing through the step-down power converter reaches the second load level within the first turn-on pulse of the high-side switch.