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SC619EVB Datasheet(PDF) 9 Page - Semtech Corporation |
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SC619EVB Datasheet(HTML) 9 Page - Semtech Corporation |
9 / 13 page SC619 9 General Operation The SC619 is a powerful dual-mode charge pump designed to drive a high-intensity white LED. A constant current spotlight mode or a pulsed current with higher intensity is used for camera flash. The device operates in 2x mode while operating in flash mode. In spotlight mode it can operate in either 2x or 1x mode depending on the load current, load voltage, and input voltage levels. In flash mode the SC619 output can drive 500mA at 4.25V into an LED for a minimum duration of 500ms under all temperature and input conditions. The FLASH pin is used to trigger this mode (active high). In spotlight mode the SC619 regulates the output current to 20% of the flash mode current setting, when the CTRL pin is pulled high and the FLASH pin is low. Flash mode has priority over spotlight mode, i.e., the FLASH pin triggers flash mode regardless of the state of the CTRL pin. Output current is regulated by attaching the ISET pin to the cathode(s) of the LED package and a low resistance sense resistor (typically 0.5Ω). The ISET pin monitors the voltage at the cathode and signals the charge pump to increase or decrease the output current until the ISET voltage reaches the programmed setting. The resistor value is chosen to set the current through the LED based on this reference voltage. The resistor can be chosen to set either the flash mode current or spotlight mode current, but it should not be less than 0.35Ω due to output current limitations of the device. Flash Mode Flash mode is enabled whenever the FLASH pin is pulled high and remains active until the FLASH pin is released. This mode has higher priority than spotlight mode, so the state of the CTRL pin is overridden whenever the FLASH pin is activated. While in flash mode the reference voltage on the ISET pin is set to 250mV. The duration of the pulse is limited by the amount of power that can be dissipated in the SC619 before it exceeds the maximum operating temperature. If an output current less than the rated 500mA is desired, the duration of the pulse can increase proportionally. Extending flash mode beyond the 500ms pulse limit could adversely affect the long term reliability of the SC619. Flash duration must be derated at higher currents. For example, 700mA derates to a maximum of 350ms. In flash mode, I OUT = 250mV/R ISET . Figure 1 shows maximum flash mode current for extended duration times. 0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 900 1000 Flash Duration [ms] Figure 1 — Maximum Flash Duration Flash Mode Safety Timer The SC619 provides a time-out protection function when in flash mode. The output is disabled if the FLASH pin is held high for longer than 1 second to protect the LED. Once this time-out has occurred, the output will stay dis- abled until the FLASH pin has been pulled low and then re-enabled. Spotlight Mode Spotlight mode is enabled by setting the CTRL pin high and keeping the FLASH pin low. When in spotlight mode, the SC619 can maintain a constant current indefinitely to drive an LED or bank of LEDs. The ISET reference voltage is set to 50mV so that the output current is maintained at 100mA when a 0.5Ω resistor is used. Spotlight current is always 20% of flash current for any given value of R ISET . In spotlight mode, I OUT = 50mV/R ISET . To maximize power efficiency, the SC619 has been designed to operate in 1x mode when conditions do not require the charge pump to be active. When first enabled the SC619 attempts to regulate the voltage on the ISET pin in 1x mode. If the voltage on the ISET pin cannot be maintained and this condition persists for longer than 32ms, then the charge pump will change to 2x mode operation. In a typical application, the SC619 will operate in 1x mode until the supply voltage decreases sufficiently Applications Information |
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