ATS17501 datasheet(21/23 Pages) ALLEGRO

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Dual Output Differential Speed and Direction Sensor IC

ATS17501

Allegro MicroSystems

955 Perimeter Road

Manchester, NH 03103-3353 U.S.A.

www.allegromicro.com

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 MicroSystems 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 a rela-

tively 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 or 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(avg) = 8.5 mA, and RθJA = 126°C/W, then:

PD = VCC

× ICC(avg) = 12 V × 8.5 mA = 102 mW

T = PD

× RθJA = 102 mW × 126°C/W = 12.9°C

TJ = TA + ΔT = 25°C + 12.9°C = 37.9°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.

For example, calculating reliability of VCC given observed worst-

case ratings, specifically:

TA = 160°C, RθJA=126°C/W, TJ(max) =175°C, VCC(max)= 24 V,

and ICC(max) = 15 mA.

Calculation of the maximum allowable power, PD(max), can be

done by first inverting equation 3 and calculating the maximum

allowable increase to TJ:

Tmax = TJ(max) – TA = 175°C–160°C = 15°C

Then, maximum allowable power can be calculated by:

PD(max) = ΔTmax÷RθJA =15°C÷126°C/W=119mW

Finally, invert equation 1 with respect to voltage:

VCC(est) = PD(max) ÷ ICC(max)= 119mW÷15mA=7.9 V

The results indicate that, at TA, the application and ATS17501 can

dissipate adequate amounts of heat at voltages less than or equal

to 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

RθJA. If VCC(est) ≥ VCC(max), then operation between VCC(est) and

VCC(max) is reliable under these conditions.

POWER DERATING

Advance Information Datasheet

Subject to Change Without Notice

March 19, 2020

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