AN3151

Independent current control for a multi-string system

Doc ID 17051 Rev 1

15/34

A sort of “state machine”, implemented in the EOC ISR, manages the current control on four

strings while also looking after the dimming function.

Further details of the multi-string control algorithm follow.

Once the first channel conversion is completed the control checks which string is to be

controlled and updates the new value of duty cycle thanks to a PI standard controller.

Once the duty cycle is updated, the control sets the next channel, related to another string to

be controlled, which is converted during the next Update event on seven PWM counter

overflows.

In short, a single PI controller in the EOC ISR is performed.

The four PIDs are executed in 200 µs calculated in this way:

PWM period=10 µs

The Update event occurs every 5 x PWM period= 50 µs

And, multiplying by 4, the total timing to execute and update the four PI controllers is

obtained.

PIDs execution = 200 µs

Figure 9.

Basic flowchart

Note:

After initialization, wait until interrupt state is entered. The idea is to convert the current and

correct the duty cycle of the PWM in order to maintain the same average current level.

In Figure 10 the basic algorithm flowchart is shown:

●

Basic initialization of peripherals, timer synchronization with ADC, interrupts and I/O

pins.

●

Reading the LED currents the ADC EOC is served and the duty cycle updated. The

entire control algorithm is executed into the interrupt routine thanks to PI controllers

with anti wind-up in order to regulate the average current at target.