A1425LK datasheet(12/12 Pages) ALLEGRO

Part #	A1425LK

Description	High Accuracy Analog Speed Sensor with Integrated Filter Capacitor and Dual Zero-Crossing Output Signal

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Manufacturer	ALLEGRO [Allegro MicroSystems]

Direct Link	http://www.allegromicro.com

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High Accuracy Analog Speed Sensor IC with Integrated

Filter Capacitor and Dual Zero-Crossing Output Signal

A1425

12

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 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,

VCC = 5.0 V, ICC = 4.2 mA, and RJA = 177 °C/W, then:

PD = VCC

× ICC = 5.0 V × 4.2 mA = 21.0 mW

T = PD

× RJA = 21.0 mW × 177 °C/W = 3.7°C

TJ = TA + T = 25°C + 3.7°C = 28.7°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 RJA and TA.

Example

Reliability for VCC at TA=150°C, using minimum-K PCB

Observe the worst-case ratings for the device, speciﬁcally:

RJA=177°C/W, TJ(max) =165°C, VCC(max)=26.5V, and

ICC(max) = 7.0 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÷177 °C/W= 84mW

Finally, invert equation 1 with respect to voltage:

VCC(est) = PD(max) ÷ ICC(max)= 84mW÷7.0mA=12 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

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

and VCC(max) is reliable under these conditions.

For example, when a standard diode with a 0.7 V drop is used:

VS(max) = 12 V + 0.7 V = 12.7 V

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