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SP7618ER-L/TR Datasheet(PDF) 6 Page - Sipex Corporation |
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SP7618ER-L/TR Datasheet(HTML) 6 Page - Sipex Corporation |
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6 / 10 page  6 Date: Dec 5-06 Rev E Constant Current LED Driver © 2006 Sipex Corporation Introduction The SP768 is a linear low-side, constant- current driver designed to drive high power LEDs from an input voltage rail. The design consists of a regulator reference voltage source, DAC, voltage-to-current converter, thermal shutdown, timeout shutdown, and outputdriver.Theprecisionreferencevoltage ensuresgoodperformanceovervoltageand temperature. The built-in thermal protection automatically stops LED current to prevent overheating. A timeout function serves as a failsafe, shutting down the output for cur- rents greater than 266mA after 4 seconds of continuous operation if the enable input is not cycled. Current Setting DAC The LED output current is defined by a cur- rent control DAC. The input to the DAC is the CTRL pin. The DAC outputs a unit current of 33.33mA with each applied pulse at the CTRL pin. The DAC has 5 bits, so the total number of steps that can be achieved is 25 or 32. The initial step of the DAC corresponds to zero, so the maximum current output is 3 times the unit current. Any multiple of the 33.33mA output current can be achieved from to 3 sequentially. The DAC CTRL input is positive-edge trig- gered and may be either at high or low logic state when the part is powered up. The EN- ABLE pin logic HIGH level may be applied with Vcc voltage. The CTRL input allows an increase in LED current only. If a lower Iled current value is required, the device should be sent into shutdown mode (EN =HIGH to LOW/LOW to HIGH) and a new pulse sequence should be applied to the CTRL pin. If the pulse number exceeds 3, all other pulses are ignored and 033 mA current is set. The SP768 can be placed into Sleep Mode by applying a logic LOW on the CTRL pin for longer than 00µs after any pulse sequence. In the Sleep Mode, the driver is disabled and Iled current is zero, but blocks associated with the DAC remain active, and Icc Sleep Mode Current is always higher than Icc current in Shutdown Mode. This function is used to output the Iled current for a speci- fied duration. If the Iledcurrent exceeds 266 mA(8 pulses) the maximum Time Duration is 4s. After that, the device is automatically sent into Sleep Mode. This function is intended to protect the LED if, for example, the device was left on in a high current condition for a long period of time. Appyling a new pulse sequence to the CTRL pinatSleepModewillwakeupthedevicebut increasetheIledcurrentat33.3mAmultiplied by the number of pulses.To return the device to the previous or lower Iled current value, use Shutdown (EN =HIGH to LOW/LOW to HIGH) and apply a new pulse sequence to the CTRL pin. The current control DAC accepts clock rates up to 4MHz. The code for the DAC loads whiletheoutputdrivertakes50µstorespond. This implies that the LED output current will reach its final value 50µs after the final pulse is applied to the CTRL pin. THEORY OF OPERATION |
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