SLVSH05 November 2023 LP8865U-Q1 , LP8865V-Q1 , LP8865W-Q1 , LP8865X-Q1 , LP8865Y-Q1 , LP8865Z-Q1
PRODUCTION DATA
For this design, the input voltage is 9 V to 16 V. The output is 8 white LEDs in series and the inductor current ripple by requirement is less than 40% of maximum LED current. To choose a proper peak-to-peak inductor current ripple, the low-side FET current limit should not be violated when the converter works in no-load condition. This requires half of the peak-to-peak inductor current ripple to be lower than that limit. Another consideration is to ensure reasonable inductor core loss and copper loss caused by the peak-to-peak current ripple. Once this peak-to-peak inductor current ripple is chosen, use Equation 28 to calculate the recommended value of the inductor L.
where
With the chosen inductor value, the user can calculate the actual inductor current ripple using Equation 14.
The design ratings of inductor RMS current and saturation current must be greater than those seen in the system requirement. This is to ensure no inductor overheat or saturation occurring. During power up, transient conditions or fault conditions, the inductor current may exceed its normal operating current and reach the current limit. Therefore, it is preferred to select a saturation current rating equal to or greater than the converter current limit. The peak-inductor-current and RMS current equations are shown in Equation 26 and Equation 27.
In this design, VIN(max) = 16V, VOUT = 15 V, ILED = 0.5 A, considering the efficiency as 0.8, IL(max) = 1.041 A, fSW = 400 kHz, choose KIND = 0.4, the calculated inductance is 46.45 µH. A 22-µH inductor is chosen. With this inductor, the ripple, peak, and rms currents of the inductor are 1.09 A, 1.587 A, 1.296 A, respectively.