Datasheet ALT80600 (Allegro) - 10

LED Driver with Pre-Emptive Boost ALT80600 for Ultra-High Dimming Ratio and Low Output Ripple Powering Up: LED Detection Phase
VOUT VOUT The VIN pin has an undervoltage lockout (UVLO) function that prevents the ALT80600 from powering up until the UVLO thresh- Using LED Using all LED Channels 1-3 Channels old is reached. Once the VIN pin goes above UVLO and a high signal is present on the EN pin, the IC proceeds to power up. At LED1 LED1 this point, the ALT80600 is going to enable the disconnect switch LED2 LED2 LED3 LED3 and will try to check if any LED pins are shorted to GND and/or LED4 LED4 GND GND are not used. The LED detect phase starts when the GATE volt- 6.19 kΩ age of the input disconnect PMOS switch is pulled down to 3.3 V below VIN and PWM = H.
Figure 6: How to signal an unused LED channel during startup LED detection phase Table 2: LED Detection phase voltage threshold levels LED Pin Voltage Measured Interpretation Outcome
Cannot proceed with < 120 mV LED pin shorted to GND fault soft-start unless fault is removed LED channel is ~ 230 mV LED channel not in use removed from operation > 340 mV LED channel in use Proceed with soft-start
Figure 5: Startup showing EN+PWM, GATE, LED1, and ISET. Switching frequency = 2.15 MHz. Note that LED Detection Phase starts as soon as GATE pin is pulled down to 3.3 V below VIN.
Once the voltage threshold on VLED pins exceeds ~120 mV, a delay of 3584 clock cycles is used to determine the status of the pins. Therefore the duration of LED Detection phase depends on the switching frequency selected:
Table 1: Duration of LED Detection phase with respect to switching frequency Switching Frequency Approximate Detection Time
2.15 MHz 1.67 ms 1 MHz 3.6 ms
Figure 7: Normal startup showing all channels passed LED Detec-
500 kHz 7.2 ms
tion phase. Total LED current = 100 mA × 4 (only LED1 and LED2
250 kHz 14 ms
pin voltages are shown).
Unused LED pin should be terminated with a 6.19 kΩ resistor to GND. At the end of LED detection phase, any channel with pull down resistor is then disabled and will not contribute to the boost regulation loop. Allegro MicroSystems, LLC 10 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Document Outline Features and Benefits Description Applications Package Selection Guide Absolute Maximum Ratings Thermal Characteristics Typical Application – SEPIC Functional Block Diagram Pinout Diagram and Terminal List Electrical Characteristics Functional Description Enabling the IC Powering Up: LED Detection Phase Powering Up: Boost Output Undervoltage Soft Start Function Frequency Selection Synchronization Loss of External Sync Signal Switching Frequency Dithering Clock Out Function LED Current Setting PWM Dimming Pre-Emptive Boost (PEB) Analog Dimming with APWM Pin Extending LED Dimming Ratio Analog Dimming with External Voltage VDD Shutdown Fault Detection and Protection LED String Partial-Short Detect Boost Switch Overcurrent Protection Input Overcurrent Protection and Disconnect Switch Setting the Current Sense Resistor Input UVLO Fault Protection During Operation Fault Recovery Mechanism Package Outline Drawing Appendix A: Design Example