Re: Why does the L3M3421 LED Driver datasheet call it "predictive off time"?
According to the data sheet, standard constant current switched mode controllers use a fixed frequency and a variable duty cycle that is a function of the current measurement. This is strictly feedback control, and the data sheet says this feedback control loop can be difficult to stabilize, especially when the duty cycle goes above 50%. The "predictive off-time control" described in the data sheet implies that they are not using a fixed frequency. They use an on time which is a function of the current measurement, just like in the traditional method. But then they use a calculated off time that is based on the input voltage. This would be feed-forward instead of feedback. It is predictive because it takes corrective action to correct a current error before the current error is actually measured. It is "predicting" that the increase in the input voltage will cause an increase in the average LED current at the present on and off times. So they lengthen the off-time immediately. Taking corrective action sooner rather than later helps to stabilize control loops.