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SP7682AER1-L Datasheet(PDF) 6 Page - Exar Corporation |
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SP7682AER1-L Datasheet(HTML) 6 Page - Exar Corporation |
6 / 19 page Sept 27-07 rev G SP7682: Backlight and Flash Driver © 2007 Exar Corporation Page 6 THEORY OF OPERATION The SP7682 and SP7682A are charge pump based backlight and flash LED drivers with I 2C and serial single wire interface (SWI) control respectively. The SP7682 family provides very accurate current drive capability for up-to four backlight LED channels and a flash LED. The backlight LED current can be set through a serial interface controlled 6-bit current DAC with 0.5mA steps, up to 31.5mA. Similarly, FLASH LED current is set thru a serial interface controlled 6-bit current DAC with 10mA steps, up to 500mA. Backlight and Flash current loops make sure that the corresponding output currents at each individual LED output are regulated to their DAC set current values. The inherent decision and control logic decides the operation mode of the "gear-box" charge pump to maximize overall efficiency. In order to decide on the most efficient operation mode for the charge pump, the decision and control logic observes the battery voltage, output voltage, load current, dropout voltage, over-voltage, over-current conditions and in-regulation feedback signals and forces the charge pump to operate in one of the 1x, 1.5x and 2x modes and dynamically switches between modes to maximize overall efficiency. The charge pump used in the heart of the design is a regulated charge pump and regulates the minimum of the LEDx outputs and FLASH output to be 350mV typical. Regulated output voltage depends on the forward voltage drop of the external LEDs used. The charge pump switches at a high frequency of 2.4MHz which allows tiny 1uF external capacitors to be used as flying capacitors. When any of the LED outputs or the FLASH output voltage drops below the "dropout" voltage (200mV typical), the charge pump decides to switch up to a less efficient mode, i.e. in 1x mode it switches to 1.5x mode, in 1.5x mode it switches to 2x mode. This switching to a less efficient mode guarantees the part to sustain LED currents in regulation. For the decision to switch back to a more efficient mode, the mode decision logic uses battery voltage, output voltage and load current information and relies on preset margins on the mode switching comparator thresholds. The FLASH channel can be used in torch mode as well as in Flash time-out mode. When used for a momentary Flash output, it is recommended to activate the I 2C control for timeout. Once the I 2C timeout is activated and for as long as the Flash is active, the SP7682 charge pump will operate in 2x mode in order to provide the continuous voltage and high current needed for Flash. For the SP7682A, which uses the single wire interface, the Flash channel will be in 2x mode whenever the Flash current is at least 230mA. The SP7682 and SP7682A are furnished with under- voltage lockout, current limit, thermal shutdown and over voltage protection features. |
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