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SP6686ER-L Datasheet(PDF) 8 Page - Exar Corporation |
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SP6686ER-L Datasheet(HTML) 8 Page - Exar Corporation |
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8 / 12 page  S SP P6 66 68 86 6 4 40 00 0m mA A B Bu ucck 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 8/12 Rev. 2.0.0 THEORY OF OPERATION The SP6686 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 SP6686 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 milliseconds 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 SP6686 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 200μsec, and then SP6686 goes through a soft-start mode designed to reduce inrush current. The SP6686 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 SP6686 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 SP6686 automatically switches to the 2x mode. The SP6686 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 SP6686, 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. In the Torch mode, (Flash = GND) the Flash pin is set to logic low and the SP6686 FB pin regulates to 50mV output: 50 When in Flash mode, (Flash = VIN), the FB regulation voltage is set by the resistor RSET connected between the RSET pin and SGND and the equation: 1.26 11.2Ω Where 1.26V is the internal bandgap reference voltage and 11.2kΩ is an internal resistance used to scale the RSET current. Typical values of RSET are 40kΩ to 180kΩ for a range of VFB = 300mV to 75mV in Flash mode. The output current is then set in either Flash or Torch mode by the equation: OVER TEMPERATURE PROTECTION When the temperature of the SP6686 rises above 145°C, the over temperature protection circuitry turns off the output switches to prevent damage to the device. If the temperature drops back down below 135°C, the part automatically recovers and executes a soft start cycle. OVER VOLTAGE PROTECTION The SP6686 has over voltage protection. If the output voltage rises above the 5.5V threshold, the over voltage protection shuts off all of the output switches to prevent the output voltage from rising further. When the output decreases below 5.5V, the device resumes normal operation OVER CURRENT PROTECTION The over current protection circuitry monitors the average current out of the VOUT=50mV (Torch Mode) pin. If the average output current exceeds approximately 1Amp, then the over current protection circuitry shuts off the output switches to protect the chip. COMPONENT SELECTION The SP6686 charge pump circuit requires 3 capacitors: 4.7μF input, 1μF output and |
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