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RT8511B Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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RT8511B Datasheet(HTML) 9 Page - Richtek Technology Corporation |
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9 / 11 page  RT8511B 9 DS8511B-05 February 2015 www.richtek.com © Copyright 2015 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. OUT, OVP OVP R2 V = V 1+ R1 where R1 and R2 make up the voltage divider connected to the OVP pin. Over Temperature Protection The RT8511B has an Over Temperature Protection (OTP) function to prevent overheating caused by excessive power dissipation from overheating the device. The OTP will shut down switching operation if the junction temperature exceeds 160 °C. The boost converter will start switching again when the junction temperature is cooled down by approximately 30 °C. Inductor Selection The inductance depends on the maximum input current. As a general rule, the inductor ripple current range is 20% to 40% of the maximum input current. If 40% is selected as an example, the inductor ripple current can be calculated according to the following equation : OUT OUT IN(MAX) (MIN) IN(MIN) RIPPLE IN(MAX) VI I = V I = 0.4 I where η is the efficiency of the boost converter, IIN(MAX) is the maximum input current, IOUT is the total current from all LED strings, and IRIPPLE is the inductor ripple current. The input peak current can be calculated by maximum input current plus half of inductor ripple current shown as following equation : IPEAK = 1.2 x IIN(MAX) Note that the saturated current of the inductor must be greater than IPEAK. The inductance can eventually be determined according to the following equation : Over Voltage Protection The RT8511B equips Over Voltage Protection (OVP) function. When the voltage at the OVP pin reaches a threshold of approximately 1.2V, the MOSFET drive output will turn off. The MOSFET drive output will turn on again once the voltage at the OVP pin drops below the threshold. Thus, the output voltage can be clamped at a certain voltage level, as shown in the following equation : ) 2 IN OUT IN 2 OUT OUT OSC V(V V L = 0.4 V I f where fOSC is the switching frequency. For better efficiency, it is suggested to choose an inductor with small series resistance. Diode Selection The Schottky diode is a good choice for an asynchronous boost converter due to its small forward voltage. However, when selecting a Schottky diode, important parameters such as power dissipation, reverse voltage rating, and pulsating peak current must all be taken into consideration. A suitable Schottky diode's reverse voltage rating must be greater than the maximum output voltage, and its average current rating must exceed the average output current. Capacitor Selection Two 1 μF ceramic input capacitors and two 1μF ceramic output capacitors are recommended for driving 10 WLEDs in series. For better voltage filtering, ceramic capacitors with low ESR are recommended. Note that the X5R and X7R types are suitable because of their wide voltage and temperature ranges. Thermal Considerations For continuous operation, do not exceed absolute maximum junction temperature. The maximum power dissipation depends on the thermal resistance of the IC package, PCB layout, rate of surrounding airflow, and difference between junction and ambient temperature. The maximum power dissipation can be calculated by the following formula : PD(MAX) = (TJ(MAX) − TA) / θJA where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and θJAis the junction to ambient thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 125 °C. The junction to ambient thermal resistance, θJA, is layout dependent. For WDFN-8L 2x2 package, the thermal resistance, θJA, is 120 °C/W on a standard JEDEC 51-7 four-layer thermal test board. The maximum power dissipation at TA = 25 °C can be calculated by the following formulas : |
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