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A1125EUA-T Datasheet(PDF) 11 Page - Allegro MicroSystems |
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A1125EUA-T Datasheet(HTML) 11 Page - Allegro MicroSystems |
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11 / 13 page  Chopper Stabilized Precision Hall Effect Switches A1120, A1121 and A1125 11 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 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 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 MicroSystems website.) The Package Thermal Resistance, RJA, 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, RJC, is relatively small component of RJA. 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) T = PD × RJA (2) TJ = TA + ΔT (3) For example, given common conditions such as: TA= 25°C, VCC = 12 V, ICC = 1.6 mA, and RJA = 165 °C/W, then: PD = VCC × ICC = 12 V × 1.6 mA = 19 mW T = PD × RJA = 19 mW × 165 °C/W = 3°C TJ = TA + T = 25°C + 3°C = 28°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 RJA and TA. Example: Reliability for VCC at TA=150°C, package LH, using a minimum-K PCB. Observe the worst-case ratings for the device, specifically: RJA=228°C/W, TJ(max) =165°C, VCC(max) = 24 V, and ICC(max) = 4 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÷RJA =15°C÷228 °C/W=66mW Finally, invert equation 1 with respect to voltage: VCC(est) = PD(max) ÷ ICC(max)= 66mW÷4mA=16.5 V 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 reli- able operation between VCC(est) and VCC(max) requires enhanced RJA. If VCC(est) ≥ VCC(max), then operation between VCC(est) and VCC(max) is reliable under these conditions. |
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