ATS684LSN datasheet(14/16 Pages) ALLEGRO

14 / 16 page

Two-Wire, Zero-Speed

Differential Gear Tooth Sensor IC

ATS684LSN

Allegro MicroSystems, LLC

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 tem-

perature 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 website.)

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)

T = PD

× RθJA

(2)

TJ = TA + ΔT

(3)

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

VCC = 12 V, ICC = 6 mA, and RθJA = 150°C/W, then:

PD = VCC

× ICC = 12 V × 6 mA = 72 mW

T = PD

× RθJA = 72 mW × 150°C/W = 10.8°C

TJ = TA + ΔT = 25°C + 10.8°C = 35.8°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: Reliability for VCC at TA=150°C, package SN, using a

single-layer PCB.

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

RθJA =150°C/W, TJ(max) =165°C, and

ICC(max)= 16 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) = ΔTmax÷RθJA =15°C÷150°C/W=100 mW

Finally, invert equation 1 with respect to voltage:

VCC(est) = PD(max) ÷ ICC(max)= 100 mW÷16 mA= 6.3 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 RθJA. If VCC(est) ≥ VCC(max), then operation between

VCC(est) and VCC(max) is reliable under these conditions.

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