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BW9910 Datasheet(PDF) 9 Page - Bruckewell Technology LTD |
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BW9910 Datasheet(HTML) 9 Page - Bruckewell Technology LTD |
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9 / 14 page  BW9910/A High Brightness LED Driver © 2012 Bruckewell Technology Corp., Ltd. 9 www.bruckewell-semi.com/ In a boost converter, when the external MOSFET is ON the energy is stored in the inductor which is then delivered to the output when the external MOSFET switches OFF. If the energy stored in the inductor is not fully depleted by the next switching cycle (continuous conduction mode) the DC conversion between input and output voltage is given by : (11) From the switching frequency, fOSC, the on-time of the MOSFET can be calculated : (12) From this the required inductor value can be determined by : (13) The boost topology LED driver requires an output capacitor to deliver current to the LED string during the time that the external MOSFET is on. In boost LED driver topologies if the LEDs should become open circuit, damage may occur to the power switch and so some form of detection should be present to provide over-voltage detection/protection. Figure 1. DC-DC Boost LED Driver Buck-Boost (Fly-Back) LED Driver Refer Figure 2, the buck-boost power conversion topology can be used when the forward voltage drop of the LED string is higher, equal or lower than the input supply voltage. For example, the buck-boost topology can be appropriate when input voltage is supplied by 24V system bus for trucks (voltage at supply battery is between 18V and 32V) and output string consists of six to nine HB LEDs, as the case may be for tail and break signal lights. In the buck-boost converter, the energy from the input source is first stored in the inductor or fly-back transformer when the switching transistor is ON. The energy is then delivered to the output during the OFF time of the transistor. When the energy stored in the fly- back inductor is not fully depleted by the next switching cycle (continuous conduction mode) the DC conversion between input and output voltage is given by : (14) or D (15) The output voltage can be either higher or lower than the input voltage, depending on duty ratio. Let us discuss the above example of 24V battery system LED driver that needs to drive six HB LEDs (VF 3.3V) at 350mA. Knowing the nominal input voltage VIN 24V, the nominal duty ratio can be determined as below : D 0.45 Then, given the switching frequency, in this example fOSC 50kHz, the required on-time of the MOSFET transistor can be calculated : 9µs |
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