8.3.9.1 LED Fault Comparators and Adaptive Boost Control

Every LED current sink has 3 comparators for adaptive boost control and fault detection. Each comparator outputs is filtered. Filter control bits <BL_COMP_FILTER_SEL [3:0]> select how many PWM generator clock cycles (5 MHz if PLL disabled or PLL clock) high/mid comparator is filtered before it is used to detect shorted LEDs and boost voltage down-scaling. Usually 1 µs is sufficient; for 5-MHz frequency it means <BL_COMP_FILTER_SEL [3:0]> = 0000b, 10 MHz = 0001b, 20 MHz = 0010b, and 40 MHz = 0011b.

Adaptive boost-control function adjusts the boost output voltage to the minimum sufficient voltage for proper LED current sink operation. The output with the highest V_F LED string is detected and the boost output voltage adjusted accordingly. Current sink headroom can be adjusted with EEPROM bits <DRV_HEADR[2:0]>. Boost adaptive control voltage step size is 125 mV. Boost adaptive control operates similarly with and without PSPWM. Additionally, when faster boost response is needed in larger brightness steps, the "jump" command can be used. Jump allows increase of the boost voltage with greater steps. Jump is enabled with the <EN_JUMP> EEPROM bit. The threshold for the magnitude of brightness increase that requires use of jump can be set with the <JUMP_STEP_SIZE[1:0]> EEPROM bits. <BRIGHTNESS_JUMP_THRES[1:0]> EEPROM bits define when the jump command is activated.

Figure 27. Boost Voltage Adaptation

Figure 28. Output Voltage Comparators

Figure 29 shows different cases which cause boost voltage increase, decrease, or generate faults.

Figure 29. Protection and Boost Adaptation Algorithms