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RT8471ZFP Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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RT8471ZFP Datasheet(HTML) 10 Page - Richtek Technology Corporation |
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10 / 15 page  10 DS8471-02 December 2013 www.richtek.com RT8471 © Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. The capacitor can be selected according to below equation : C = 1.5 x 10-6 x tSS where tss is the soft-start period. LED Current Ripple Reduction Higher LED current ripple will shorten the LED life time and increase heat accumulation of LED. There are two ways to reduce the LED current ripple. One way is by increasing the inductance to lower LED current ripple in absence of an output capacitor. The other way is by adding an output capacitor in parallel with the LED. This will then allow the use of a smaller inductor. Inductor Selection The inductance is determined by inductor current ripple, switching frequency, duty ratio, circuit specifications and component parameters, as expressed in the following equation : Diode selection To obtain better efficiency, the Schottky diode is recommended for its low reverse leakage current, low recovery time and low forward voltage. With its low power dissipation, the Schottky diode outperforms other silicon diodes and increase overall efficiency. Input Capacitor selection Input capacitor has to supply peak current to the inductor and flatten the current ripple on the input. The low ESR condition is required to avoid increasing power loss. The ceramic capacitor is recommended due to its excellent high frequency characteristic and low ESR, which are suitable for the RT8471. For maximum stability over the entire operating temperature range, capacitors with better dielectric are suggested. Thermal Protection A thermal protection feature is included to protect the RT8471 from excessive heat damage. When the junction temperature exceeds a threshold of 150 °C, the thermal protection will turn off the LX terminal. When the junction temperature drops below 120 °C, the RT8471 will turn back on the LX terminal and return to normal operations. where fSW is the switching frequency (Hz) RDS(ON) is the low side switch on-resistance of internal MOSFET ( = 0.35 Ω typical) D is the duty cycle determined by VOUT/VIN IOUT is the required LED current (A) ΔILis the inductor peak-peak ripple current (internally set to 0.3 x IOUT) VIN is the input supply voltage (V) VOUT is the total LED forward voltage (V) Besides, the selected inductance has also to satisfy the limit of the minimum switch on/off time. The calculated on time must be greater than 210ns of the minimum on time, and the off time must be greater than 170ns of the minimum off time. The following equation can be used to verify the suitability of the inductor value. where VD is the rectifier diode forward voltage (V) VSEN is the voltage cross current sense resistor (V) RL is the inductor DC resistance ( Ω) L is the inductance (H) The saturation current of the selected inductor must be higher than the peak output LED current, and the continuous current rating must be above the average output LED current. In general, the inductor saturation current should be 1.5 times the LED current. In order to reduce the output current ripple, a higher inductance is recommended at higher supply voltages. However, it could also cause a higher line resistance and result in a lower efficiency. IN OUT SEN DS(ON) OUT SW L D L > V V V R I fI L ON IN OUT OUT SEN L DS(ON) ON(MIN) LI t VV I R R R t (210ns typ.) L OFF OUT D SEN OUT L OFF(MIN) LI t VV V I R t (170ns typ.) |
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