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A8509 Datasheet(PDF) 8 Page - Allegro MicroSystems |
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A8509 Datasheet(HTML) 8 Page - Allegro MicroSystems |
8 / 28 page Wide Input Voltage Range, High Efficiency 4-Channel Fault Tolerant LED Driver A8509 8 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com Functional Description Enabling the IC The IC turns on when a logic high signal is applied on the EN pin, and the input voltage present on the VIN pin is greater than the 8.5 V necessary to clear the UVLO (VUVLOrise) threshold. Before the LEDs are enabled, the A8509 driver goes through a system check to determine if there are any possible fault condi- tions that might prevent the system from functioning correctly. Powering up: LED pin short-to-GND check After the VIN pin goes above the UVLO threshold, and a high sig- nal is present on the EN pin, the IC proceeds to check if any LEDx pins are shorted to GND and/or are not used. Each unused pin should be connected to GND with a 2.4 kΩ pull-down resistor. After the voltage threshold on the LEDx pins exceeds 120 mV, a timer of 1536 clock cycles (2 ms at 800 kHz switching frequency, see figure 2) is applied during which the A8509 determines the status of the pins. Any unused pin connected to GND with the pull-down resistor will be taken out of regulation at this point and will not contribute to the boost regulation loop (see figure 3). A typical example is shown in figure 4. When a pin is connected to GND through a 2.4 kΩ resistor, the voltage on that LEDx pin during the LED detection period is about 200 mV. This is shown in figure 2. If an LEDx pin is shorted to ground such that LEDx pin voltage is < 100 mV, the A8509 will not proceed with soft start until the short is removed from the LEDx pin. This prevents the A8509 Figure 4. Channel select setup: (left) channel LED4 not used, (right) using all channels. GND 2.4 kΩ A8509 LED1 LED2 LED3 LED4 A8509 LED1 LED2 LED3 LED4 GND Figure 3. LED detect circuit operation for an LED pin that is not being used; shows VOUT (ch1, 10 V/div.), VEN (ch2, 5 V/div.), an unused LEDx with a 2.4 kΩ resistor from this pin to GND, VLEDa (ch3, 500 mV/div.), and a used LEDx, VLEDb (ch4, 500 mV/div.), t = 2 ms/div. Figure 2. LED detect circuit operation for two connected LEDs at fSW = 800 kHz; shows VOUT (ch1, 10 V/div.), VEN (ch2, 5 V/div.), an LEDx, VLEDa (ch3, 500 mV/div.), another LEDx, VLEDb (ch4, 500 mV/div.), t = 2 ms/div. t VLEDa(not used) VLEDb(used) VOUT VEN C1 C3 C4 C2 Figure 5. LED detect circuit operation: device powers-up after the short is removed from the LED pin; shows VOUT (ch1, 10 V/div.), VEN (ch2, 5 V/ div.), an LEDx with short, VLEDa (ch3, 500 mV/div.), and an LEDx without short, VLEDb (ch4, 500 mV/div.), t = 2 ms/div. t LED detection period Short removed VEN VOUT VLEDa (with temporary short) VLEDb (no short) C1 C3 C4 C2 t LED detection period VEN VLEDa VLEDb VOUT C1 C2 C3 C4 |
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