A1250(2012) 查看數據表（PDF） - Allegro MicroSystems

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A1250
(Rev.:2012)

Hall Effect Latch / Bipolar Switch

Allegro MicroSystems 

A1250

Hall Effect Latch / Bipolar Switch

Power Derating

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, RJA, 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, RJC, is

relatively small component of RJA. 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 × RJA

(2)

TJ = TA + ΔT

(3)

For example, given common conditions such as: TA= 25°C,

VIN = 12 V, IIN = 4 mA, and RJA = 140 °C/W, then:

PD = VIN × IIN = 12 V × 4 mA = 48 mW



T = PD × RJA = 48 mW × 140 °C/W = 7°C

TJ = TA + T = 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 RJA

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:

RJA=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:

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 ÷ RJA = 15°C ÷ 165 °C/W = 91 mW

Finally, invert equation 1 with respect to voltage:

VCC(est) = PD(max) ÷ ICC(max) = 91 mW ÷ 4 mA = 23 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

reliable operation between VCC(est) and VCC(max) requires

enhanced RJA. If VCC(est) ≥ VCC(max) , then operation

between VCC(est) and VCC(max) is reliable under these condi-

tions.

Allegro MicroSystems, Inc.

9

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

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