ATS672LSB datasheet(12/15 Pages) ALLEGRO

12 / 15 page

30Dec03, Rev. 1.42

Worcester, Massachusetts 01615-0036 (508) 853-5000

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Allegro MicroSystems, Inc.

Self-Calibrating Gear Tooth Sensor with 9-Bit Signal Capture

ATS672LSB

Temperature Compensation

The device should be operated at or below the maximum junc-

tion temperature of the device, TJ(max) (see the Absolute Maxi-

mum Ratings section, on page 1). The actual operating TJ of the

device is affected by several factors. Under certain combinations

of peak conditions (corresponding to the gray area of the Power

De-Rating Curve chart), operation may require power de-rating

or heat sinking.

The relevent factors are characteristic of the device and pack-

age, as well as the application, including the effect of adjacent

external sources of heat. The Package Thermal Resistance, RθJA,

indicates the resitance to heat transfer from the heat-generating

portions of the die (the “junction”) through all paths to the ambi-

ent air. This includes heat sinking through the PCB, as well as

direct radiation from the die through the package, RθCA. Thermal

information on packages is available on the Allegro Web site.

The Allowable Power Dissipation, PD, represents the amount

of power that can be applied to the device at a given RθCA and

ambient temperature, TA, without causing the temperature of the

die to exceed , TJ(max). This section presents a procedure for cor-

relating these factors with operating voltage, VCC, and operating

current, ICC, to estimate their effect on TJ.

If the estimated operating TJ exceeds TJ(max), then power levels

can be reduced or external heat sinking can be applied. Typi-

cally, VCC is the factor reduced, to accommodate the required

TA. A power de-rating curve can be constructed, representing the

maximum allowable VCC per TA.

If the calculated VCC exceeds the power de-rating curve at the

required TA, and heat sinking is not preferred, estimate the

amount to reduce VCC. This can be estimated through calculat-

ing PD(max), the maximum allowable PD for the given device and

package. PD(max) is related to RθJA and TA. The following formu-

las represent the fundamental relationships used to calculate the

VCC adjustment, based on the temperature effect, ∆T.

TJ = TA + ∆T

(1)

where

∆T denotes the increase in TJ due to power dissipation

within the device.

∆T = PD

× RθJA

(2)

PD = VCC

× ICC

(3)

Examples for estimating VCC are provided on the next page.

These formulas and results can also be used to estimate TJ.

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

VCC = 5 V, ICC(on) = 6.5 mA, and RθJA = 150 °C/W then:

PD = VCC

× ICC(on) = 5 V × 6.5 mA = 32.5 mW

∆T = PD

× RθJA = 32.5 mW × 150 °C/W = 4.9°C

TJ = TA + ∆T = 25°C + 4.9°C = 29.9°C

0.0

2.0

4.0

6.0

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14.0

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26.0

28.0

100

120

140

160

180

TA (ºC)

Allowable operating range

without additional heat sinking

Some combinations of peak

ratings may require heat sinking

when operating in shaded area

VCC(max) = 26.5 V; TJ(max) = 170ºC

Power De-Rating Curve

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