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AN44 Arkusz danych(PDF) 3 Page - Zetex Semiconductors |
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AN44 Arkusz danych(HTML) 3 Page - Zetex Semiconductors |
3 / 6 page AN44 Issue 2 - July 2006 3 www.zetex.com © Zetex Semiconductors plc 2006 For continuous mode If the current does not reach zero after 1.7µs, but instead falls to a value of IMIN, then the device is said to be in ‘continuous’ mode. The LED current will ramp up and down between IMIN and IPEAK (probably at different di/dt rates) and the average LED current will therefore be the average of IPEAK and IMIN, as shown in Figure 3. Figure 3 Design example (Refer to Figure 1 and Table 1) Input = VIN = 12V LED forward drop = VLED = 9.6V VIN = VLED+VL Therefore VL = (12 - 9.6) = 2.4 The peak current = VSENSE / R1 (R1 is RSENSE) = 24mV/50mR = 480mA TON = IPEAK x L1/V(L1) These equations make the approximation that the LED forward drop is constant throughout the current ramp. In fact it will increase with current, but they still enable design calculations to be made within the tolerances of the components used in a practical circuit. Also, the difference between VIN and VLED is small compared to either of them, so the 6.2µs ramp time will be fairly dependent on these voltages. Note that, for an LED drop of 9.6V and a Schottky drop of 300mV, the time to ramp down from 680mA to zero would be: T ON 680mAx22 µH 2.4 --------------------------------------- 6.2 µs = TDIS 680mAx22 µH 9.6 0.3 + () --------------------------------------- 1.5 µs = |
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