ASIL-Compliant Hall-Effect Wheel Speed Sensor IC

with Speed and Direction Output

A19300

15

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 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 or PD), can

be estimated. The following formulas represent the fundamental

relationships used to estimate TJ, at PD.

PD = VIN

× IIN

(1)

Δ

T = PD

× RθJA

(2)

TJ = TA + ΔT

(3)

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

VCC = 12 V, ICC = 7.15 mA, and RθJA = 213°C/W, then:

PD = VCC

× ICC = 12 V × 7.15 mA = 85.8 mW

Δ

T = PD

× RθJA = 85.8 mW × 213°C/W = 18.3°C

TJ = TA + ΔT = 25°C + 18.3°C = 43.3°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 UB, using

minimum-K PCB.

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

RθJA=213°C/W, TJ(max) =165°C, VCC(max)= 24 V, and

ICC(AVG) = 14.8 mA. ICC(AVG) is computed using ICC(HIGH)(max)

and ICC(LOW)(max), with a duty cycle of 84% computed from

tw(REV)(max) on-time and tw(PRE)(min) off-time (pulse width proto-

col).

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

invert equation 3:

Δ

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÷213°C/W=70.4mW

Finally, invert equation 1 with respect to voltage:

VCC(est) = PD(max) ÷ ICC(AVG)= 70.4mW÷14.8mA=4.8 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.

POWER DERATING