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AN2983 Datasheet(PDF) 16 Page - STMicroelectronics |
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AN2983 Datasheet(HTML) 16 Page - STMicroelectronics |
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16 / 32 page  Circuit description AN2983 16/32 Doc ID 15679 Rev 1 The drawback of this configuration is that since the circuit is now more sensitive to input voltage variations, Vin has to be more tightly regulated. The following equations demonstrate what has been previously asserted. Equation 33 Equation 34 Where: I_Led_avg is the LEDs’ average current, I_Led_pk is the LEDs’ peak current and Va = (Vin - VLed) is the LEDs’ cathode voltage. Equation 35 Equation 36 To make I_Led_avg independent of VLed, the VLed coefficient has to be reduced to zero. Equation 37 That is to say, setting: Equation 38 reduces Equation 36 to: Equation 39 In other words, connecting Ra to the cathode of the LEDs’ string, the average LED current depends only on Vth and Vin. VLed * L toff 2 1 pk _ Led _ I avg _ Led _ I ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = Ra Rb * Va Ra Rb Ra * Vth * Rs 1 pk _ Led _ I () VLed * L toff 2 1 Ra Rb * VLed Vin Ra Rb Ra * Vth * Rs 1 avg _ Led _ I ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + + ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = L toff 2 1 Ra Rb * Rs 1 * VLed Ra Rb * Vin Ra Rb Ra * Vth * Rs 1 avg _ Led _ I 0 L toff 2 1 Ra Rb * Rs 1 = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ 0 Rs / L toff 2 1 Ra Rb = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ () ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = Ra Rb * Vin Ra Rb Ra * Vth * Rs 1 avg _ Led _ I |
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