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## RT8511B Datasheet(PDF) 9 Page - Richtek Technology Corporation

 Part # RT8511B Description 43V Asynchronous Boost WLED Driver Download 11 Pages Scroll/Zoom 100% Manufacturer RICHTEK [Richtek Technology Corporation] Direct Link http://www.richtek.com Logo

## RT8511B Datasheet(HTML) 9 Page - Richtek Technology Corporation

 9 / 11 pageRT8511B9DS8511B-05February 2015www.richtek.com©Copyright2015 Richtek Technology Corporation. All rights reserved.is a registered trademark of Richtek Technology Corporation.OUT, OVPOVPR2V= V1+R1where R1 and R2 make up the voltage divider connectedto the OVP pin.Over Temperature ProtectionThe RT8511B has an Over Temperature Protection (OTP)function to prevent overheating caused by excessive powerdissipation from overheating the device. The OTP will shutdown switching operation if the junction temperatureexceeds 160°C. The boost converter will start switchingagain when the junction temperature is cooled down byapproximately 30°C.Inductor SelectionThe 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 selectedas an example, the inductor ripple current can becalculated according to the following equation :OUTOUTIN(MAX)(MIN)IN(MIN)RIPPLEIN(MAX)VII=VI= 0.4 Iwhereη is the efficiency of the boost converter, IIN(MAX) isthe maximum input current, IOUT is the total current fromall LED strings, and IRIPPLE is the inductor ripple current.The input peak current can be calculated by maximuminput current plus half of inductor ripple current shown asfollowing equation :IPEAK = 1.2 x IIN(MAX)Note that the saturated current of the inductor must begreater than IPEAK. The inductance can eventually bedetermined according to the following equation :Over Voltage ProtectionThe RT8511B equips Over Voltage Protection (OVP)function. When the voltage at the OVP pin reaches athreshold of approximately 1.2V, the MOSFET drive outputwill turn off. The MOSFET drive output will turn on againonce the voltage at the OVP pin drops below the threshold.Thus, the output voltage can be clamped at a certainvoltage level, as shown in the following equation :)2INOUTIN2OUTOUTOSCV(VVL =0.4VIf where fOSC is the switching frequency. For better efficiency,it is suggested to choose an inductor with small seriesresistance.Diode SelectionThe Schottky diode is a good choice for an asynchronousboost converter due to its small forward voltage. However,when selecting a Schottky diode, important parameterssuch as power dissipation, reverse voltage rating, andpulsating peak current must all be taken intoconsideration. A suitable Schottky diode's reverse voltagerating must be greater than the maximum output voltage,and its average current rating must exceed the averageoutput current.Capacitor SelectionTwo 1μF ceramic input capacitors and two 1μF ceramicoutput capacitors are recommended for driving 10 WLEDsin series. For better voltage filtering, ceramic capacitorswith low ESR are recommended. Note that the X5R andX7R types are suitable because of their wide voltage andtemperature ranges.Thermal ConsiderationsFor continuous operation, do not exceed absolutemaximum junction temperature. The maximum powerdissipation depends on the thermal resistance of the ICpackage, PCB layout, rate of surrounding airflow, anddifference between junction and ambient temperature. Themaximum power dissipation can be calculated by thefollowing formula :PD(MAX) = (TJ(MAX)− TA) / θJAwhere TJ(MAX) is the maximum junction temperature, TA isthe ambient temperature, andθJAis the junction to ambientthermal resistance.For recommended operating condition specifications, themaximum junction temperature is 125°C. The junction toambient thermal resistance,θJA, is layout dependent. ForWDFN-8L 2x2 package, the thermal resistance,θJA, is120°C/W on a standard JEDEC 51-7 four-layer thermaltest board. The maximum power dissipation at TA = 25°Ccan be calculated by the following formulas :

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