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SP6685 Datasheet(PDF) 10 Page - Exar Corporation |
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SP6685 Datasheet(HTML) 10 Page - Exar Corporation |
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10 / 14 page  S SP P6 66 68 85 5 7 70 00 0m mA A B Bu uc ck k/ /B Bo oo os stt C Ch ha arrg ge e P Pu um mp p L LE ED D D Drriiv ve err © 2009 Exar Corporation 10/14 Rev. 2.0.0 APPLICATION INFORMATION The SP6685 can be used with multiple LEDs in parallel as shown in figure 34. For best performance, the LEDs should be in a single package, preferably from a single die to have better matching for forward voltage Vf for a given forward current If. In practice, if the Vf of one LED is higher than the others, it will consume a larger If, which will raise its temperature which will then cause its Vf to reduce, correcting the imbalance. The overall current will be the sum of the individual currents, for example Itotal = 4*ILED. Fig. 34: Multiple LED Flash Circuit THEORY OF OPERATION The SP6685 is a charge pump regulator designed for converting a Li-Ion battery voltage of 2.7V to 4.2V to drive a white LED used in digital still camera Flash and Torch applications. The SP6685 has two modes of operation which are pin selectable for either Flash or Torch. Flash mode is usually used with a pulse of about 200 to 300 millisecond to generate a high intensity Flash. Torch can be used continuously at a lower output current than Flash and is often used for several seconds in a digital still camera “movie” mode. The SP6685 also has two modes of operation to control the output current: the 1X mode and 2X mode. Operation begins after the enable pin EN receives a logic high, the bandgap reference wakes up after about 200µsec, and then SP6685 goes through a soft-start mode designed to reduce inrush current. The SP6685 starts in the 1X mode, which acts like a linear regulator to control the output current by continuously monitoring the feedback pin FB. In 1X mode, if the SP6685 auto detects a dropout condition, which is when the FB pin is below the regulation point for more than 32 cycles of the internal clock, the SP6685 automatically switches to the 2X mode. The SP6685 remains in the 2X mode until one of four things happens: 1) the enable pin EN has been toggled, 2) the Flash pin has changed from High to Low, 3) VIN is cycled. 4) a thermal fault occurs. The 2X mode is the charge pump mode where the output can be pumped as high as two times the input voltage, provided the output does not exceed the maximum voltage for the SP6685, which is internally limited to about 5.5V. In the 2X mode, as in the 1X mode, the output current is regulated by the voltage at the FB pin. |
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