In so-called DALI-compatible ballasts, to monitoring of the base state it is also possible to add monitoring of the load, for example for detecting a failure of the lamp, implemented by detecting periodically (for example, at intervals of less than 30 s) the variations in current for each load of the dimmer of the lamp down to low dimming levels (e.g., 5%).

The variations of load are monitored by a current-detection circuit, which, to be able to make an accurate and repeatable measurement, requires a minimum pulse ON time.

At high regulation frequencies (>2 kHz) and low dimming levels (<10%) this measurement (as exemplified in U.S. Pat. No. 9,986,608 B2) may become critical on account of the excessively short ON time.

For a solid-state source such as a LED, the light pulse emitted is proportional to the injected charge (i.e., the time integral of the forward current through the LED) during each pulse of the PWM; consequently, for a fixed current level, the light emitted increases as the duty-cycle of the pulse increases.

As is known, by “duty-cycle D of a PWM signal” is meant the ratio between the duration of the ON time t_ON and the period of the PWM pulse, the latter being given by the sum of the duration of the ON time t_ON and the duration of the OFF time t_OFF, i.e.,
D=t_ON/(t_ON+t_OFF)