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A1205ELHLT-T Datasheet(PDF) 10 Page - Allegro MicroSystems |
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A1205ELHLT-T Datasheet(HTML) 10 Page - Allegro MicroSystems |
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10 / 12 page  Continuous-Time Bipolar Switch A1205 10 Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 www.allegromicro.com Power Derating Power Derating The device must be operated below the maximum junction temperature of the device, TJ(max). Under certain combinations of peak conditions, reliable operation may require derating sup- plied power or improving the heat dissipation properties of the application. This section presents a procedure for correlating factors affecting operating TJ.(Thermaldataisalsoavailableon theAllegroMicroSystemsWebsite.) ThePackageThermalResistance,RθJA, is a figure of merit sum- marizing the ability of the application and the device to dissipate heatfromthejunction(die),throughallpathstotheambientair. ItsprimarycomponentistheEffectiveThermalConductivity, K, of the printed circuit board, including adjacent devices and traces.Radiationfromthediethroughthedevicecase,RθJC, is relativelysmallcomponentofRθJA. Ambient air temperature, TA, and air motion are significant external factors, damped by overmolding. Theeffectofvaryingpowerlevels(PowerDissipation,PD),can be estimated. The following formulas represent the fundamental relationships used to estimate TJ, at PD. PD = VIN ×IIN (1) Δ T = PD × RθJA(2) TJ = TA+ΔT (3) For example, given common conditions such as: TA= 25°C, VCC = 12 V,ICC = 4 mA, and RθJA =140°C/W,then: PD = VCC × ICC = 12 V × 4mA=48mW Δ T = PD × RθJA=48mW × 140°C/W=7°C TJ = TA + ΔT = 25°C + 7°C = 32°C A worst-case estimate, PD(max), represents the maximum allow- ablepowerlevel(VCC(max),ICC(max)),withoutexceedingTJ(max), ataselectedRθJA and TA. Example:ReliabilityforVCC at TA=150°C, package UA, using minimum-K PCB. Observe the worst-case ratings for the device, specifically: RθJA=165°C/W,TJ(max)=165°C, VCC(max)= 24 V, and ICC(max)= 7.5 mA. Calculate the maximum allowable power level, PD(max). First, invert equation 3: Δ Tmax = TJ(max) – TA = 165°C–150°C = 15°C This provides the allowable increase to TJ resulting from internal power dissipation. Then, invert equation 2: PD(max)= ΔTmax÷RθJA=15°C÷165°C/W=91mW Finally, invert equation 1 with respect to voltage: VCC(est) = PD(max)÷ICC(max)=91mW÷7.5mA=12.1 V The result indicates that, at TA, the application and device can dissipateadequateamountsofheatatvoltages≤VCC(est). Compare VCC(est) to VCC(max).IfVCC(est)≤VCC(max), then reli- able operation between VCC(est) and VCC(max) requires enhanced RθJA.IfVCC(est)≥VCC(max), then operation between VCC(est) and VCC(max) is reliable under these conditions. |
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