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A1250LUA-T Datasheet(PDF) 9 Page - Allegro MicroSystems |
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A1250LUA-T Datasheet(HTML) 9 Page - Allegro MicroSystems |
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9 / 12 page  Hall-Effect Latch / Bipolar Switch A1250 9 Allegro MicroSystems, LLC 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com The device must be operated below the maximum junction temperature of the device, TJ(max) . Under certain combina- tions of peak conditions, reliable operation may require derating supplied power or improving the heat dissipation properties of the application. This section presents a procedure for correlating factors affecting operating TJ. (Thermal data is also available on the Allegro MicroSystems Web site.) The Package Thermal Resistance, 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. Its primary component is the Effective Thermal Conductivity, 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. The effect of varying power levels (Power Dissipation, PD), can be estimated. The following formulas represent the fundamental relationships used to estimate TJ, at PD. PD = VIN × IIN (1) ΔT=PD × RθJA (2) TJ=TA+ΔT (3) For example, given common conditions such as: TA= 25°C, VIN = 12 V, IIN = 4 mA, and RθJA = 140 °C/W, then: PD = VIN × IIN = 12 V × 4 mA = 48 mW ΔT=PD × RθJA = 48 mW × 140 °C/W = 7°C TJ=TA+ΔT=25°C+7°C=32°C A worst-case estimate, PD(max) , represents the maximum allow- able power level, without exceeding TJ(max) , at a selected RθJA and TA. Example: Reliability for VCC at TA=150°C, package UA, using a single-layer PCB. Observe the worst-case ratings for the device, specifically: RθJA=165 °C/W, TJ(max) =165°C, VCC(max) =24V, and ICC(max) = 4 mA. Calculate the maximum allowable power level, PD(max) . First, invert equation 3: ΔTmax=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÷4mA= 23V The result indicates that, at TA, the application and device can dissipate adequate amounts of heat at voltages ≤VCC(est) . Compare VCC(est) to VCC(max) . If VCC(est) ≤ VCC(max) , then reliable operation between VCC(est) and VCC(max) requires enhanced RθJA. If VCC(est) ≥ VCC(max) , then operation between VCC(est) and VCC(max) is reliable under these condi- tions. POWER DERATING |
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