ATS651LSH datasheet(11/12 Pages) ALLEGRO

11 / 12 page

Two-Wire Self-Calibrating Differential Speed and Direction Sensor with Vibration Immunity

ATS651LSH

651LSH-DS, Rev. 3

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

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Allegro MicroSystems, Inc.

Power Derating

The device must be operated below the maximum junction

temperature of the device, TJ(max). Under certain combinations of

peak conditions, reliable operation may require derating sup-

plied 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, 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)

ΔT = PD

× RθJA

(2)

TJ = TA + ΔT

(3)

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

VCC = 5 V, ICC = 14 mA, and RθJA = 126 °C/W, then:

PD = VCC

× ICC = 12 V × 4.0 mA = 70.0 mW

ΔT = PD

× RθJA = 70.0 mW × 126 °C/W = 8.8°C

TJ = TA + ΔT = 25°C + 8.8°C = 23.8°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 SH, using

the PCB with least exposed copper.

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

RθJA=126°C/W, TJ(max) =165°C, VCC(max)=28V, and

ICC(max) = 16.8 mA.

Calculate the maximum allowable power level, PD(max). First,

invert equation 3:

ΔTmax = 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÷126 °C/W=119mW

Finally, invert equation 1 with respect to voltage:

VCC(est) = PD(max) ÷ ICC(max)= 119mW÷16.8mA=7.1 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

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

VCC(max) is reliable under these conditions.

This value applies only to the voltage drop across the

ATS651LSH chip. If a protective series diode or resistor is used,

the effective maximum supply voltage is increased.

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

VS(max) = 7.1 V + 0.7 V = 7.8 V

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