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A1667 Datasheet(PDF) 12 Page - Allegro MicroSystems |
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A1667 Datasheet(HTML) 12 Page - Allegro MicroSystems |
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12 / 15 page  True Zero-Speed, High Accuracy, Ring Magnet Sensor IC A1667 12 Allegro MicroSystems, LLC 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com POWER DERATING The device must be operated below the maximum junction temperature of the device, TJ(max).Undercertaincombinationsof peak conditions, reliable operation may require derating supplied power or improving the heat dissipation properties of the appli- cation. This section presents a procedure for correlating factors affecting operating TJ. (Thermal data is also available on the Allegro website.) ThePackageThermalResistance,RθJA, is a figure of merit sum- marizing the ability of the application and the device to dissipate heat from the junction (die), through all paths to the ambient air. ItsprimarycomponentistheEffectiveThermalConductivity,K, of the printed circuit board, including adjacent devices and traces. Radiation from the die through the device case, RθJC, is relatively small component of RθJA. 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,atPD. PD = VIN × IIN (1) ∆ T = PD × RθJA (2) TJ = TA + ΔT (3) Forexample,givencommonconditionssuchas:TA= 25°C, VCC = 12V, ICC = 7.5 mA, and RθJA =177°C/W,then: PD = VCC × ICC = 12 V × 7.5 mA = 90 mW ∆ T = PD × RθJA = 90 mW × 177°C/W = 11.3°C TJ = TA + ∆T = 25°C + 11.3°C = 36.3°C A worst-case estimate, PD(max), represents the maximum allow- able power level (VCC(max), ICC(max)), without exceeding TJ(max), at a selected RθJA and TA. Example :ReliabilityforVCC at TA=150°C,packageK,usinga single-layerPCB. Observetheworst-caseratingsforthedevice,specifically: RθJA=177°C/W,TJ(max)=165°C,VCC(max) =24 V,and ICC(max) = 12 mA. Calculate the maximum allowable power level, PD(max).First, invert equation 3: ∆ 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) = ∆Tmax÷RθJA =15°C÷177°C/W=84.7 mW Finally,invertequation1withrespecttovoltage: VCC(est)=PD(max) ÷ ICC(max)=119mW÷12 mA=7.1V The result indicates that, at TA, the application and device can dissipateadequateamountsofheatatvoltages≤VCC(est). CompareVCC(est)toVCC(max).IfVCC(est)≤VCC(max), then reli- ableoperationbetweenVCC(est)andVCC(max) requires enhanced RθJA.IfVCC(est)≥VCC(max),thenoperationbetweenVCC(est) and VCC(max) is reliable under these conditions. |
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