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SP7648 Datasheet(PDF) 8 Page - Sipex Corporation |
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SP7648 Datasheet(HTML) 8 Page - Sipex Corporation |
8 / 12 page 8 Date: 7/20/05 SP7648 Ultra-low Quiescent Current, High Efficiency Boost Regulator © Copyright 2005 Sipex Corporation current to avoid saturating the inductor, which would result in a loss in efficiency and could damage the inductor. The SP7648 evaluation board uses a RLIM value of 1KΩ for an IPEAK = 1.6A to allow the circuit to deliver up to 700mA for VIN = 3.3V and VOUT = 4.2V. Other values could be selected using the above relationships. _________________USING THE FLASH CONTROL PIN The SP7648 will regulate the output by the equations above depending on the state of the FLASH pin. When the FLASH pin is low (<0.4V), the internal reference voltage is defined as 0.8V. When the FLASH pin is high (>0.4V), the internal reference voltage is defined as 0.288V. This allows the use of smaller values for the sense resistor for current regulation mode. This improves effi- ciency and reduces the physical size of the sense resistor. An external MOSFET switch can be used to change the sense resistor when changing to the Flash Mode. ________________ HIGH BRIGHTNESS WHITE LED For the high brightness LumiLED white LED application, the SP7648 is generally pro- grammed as a current source. The bias resistors R1 and R2 are used to set the operating current of the white LED with the equation: R = VFB/IF where VFB is 0.8V in torch mode and 0.288V in flash mode, IF is the operating current of THEORY OF OPERATION the LED. To set the operating current to be about 200mA in torch mode, the flash pin is forced low, R2 is selected as 0.8V/ 0.2 = 4Ω, as shown in the typical application circuit. To set the operating current to 700mA in flash mode, the flash pin is forced high, R is selected as 0.288V/0.41Ω = 700mA. In real- ity R in Flash includes the series MOSFET RDSON and the parallel combination of R2 = 4Ω shown by the formula: R in Flash = {R1 X (R2 + Q1RDSON)} {R1 + R2 + Q1RDSON} If the SP7648 is powered up before the LED is plugged in, the circuit will bring the feed- back pin to ZeroV and the SP7648 has a feature to set the output voltage to be 5V. Once the LED is plugged in, the feedback pin will go up to 0.8V in torch mode or 0.288V in flash mode and begin to regulate. The output voltage will go from 5V to VF+VFB, where VF is the forward voltage of the LED. When the LED is open, the feedback pin voltage will go to ZeroV and the output voltage will go to 5V which will protect the part from overvoltage at the output. One approach to control LED brightness is to apply a PWM signal to the SHDN input of the SP7648. In this case, the output current will be equal to the product of VREF/R1 and the average duty cycle at the SHDN pin. An optional 10KΩ potentiometer may also be used for dimming the LED current by varying the potentiometer between low brightness and full brightness. If the FB pin is pulled below 150mV the output will default to 5V defined by an inter- nal resistor divider. |
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