A1148LUA-T【PDF 10/13 页】IC SWITCH HALL EFFECT UNI 3-SIP

Low Current Ultrasensitive Two-Wire

Chopper-Stabilized Unipolar Hall Effect Switches

A1147 and

A1148

9

Allegro MicroSystems, Inc.

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

temperature of the device, T

J(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 T

J

. (Thermal data is also available on

the Allegro MicroSystems Web site.)

The Package Thermal Resistance, R

qJA

, 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, R

qJC

, is

relatively small component of R

qJA

. Ambient air temperature,

T

A

, and air motion are significant external factors, damped by

overmolding.

The effect of varying power levels (Power Dissipation, P

D

), can

be estimated. The following formulas represent the fundamental

relationships used to estimate T

J

, at P

D

.

P

D

= V

IN

?/DIV>

I

IN

(1)

DT = P

D

?/DIV>

R

qJA

(2)

T

J

= T

A

+ 擳 (3)

For example, given common conditions such as: T

A

= 25癈,

V

CC

= 12 V, I

CC

= 4 mA, and R

qJA

= 140 癈/W, then:

P

D

= V

CC

?/DIV>

I

CC

= 12 V

?/DIV>

4 mA = 48 mW

DT = P

D

?/DIV>

R

qJA

= 48 mW

?/DIV>

140 癈/W = 7癈

T

J

= T

A

+ DT = 25癈 + 7癈 = 32癈

A worst-case estimate, P

D(max)

, represents the maximum allow-

able power level (V

CC(max)

, I

CC(max)

), without exceeding T

J(max)

,

at a selected R

qJA

and T

A

.

Example: Reliability for V

CC

at T

A

=

150癈, package UA, using

minimum-K PCB.

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

R

qJA

=

165癈/W, T

J(max)

=

165癈, V

CC(max)

=

24 V, and

I

CC(max)

=

17

mA.

Calculate the maximum allowable power level, P

D(max)

. First,

invert equation 3:

DT

max

= T

J(max)

T

A

= 165

癈

150

癈 = 15

癈

This provides the allowable increase to T

J

resulting from internal

power dissipation. Then, invert equation 2:

P

D(max)

= DT

max

?R

qJA

= 15癈 ?165 癈/W = 91 mW

Finally, invert equation 1 with respect to voltage:

V

CC(est)

= P

D(max)

? I

CC(max)

= 91 mW ?17 mA = 5 V

The result indicates that, at T

A

, the application and device can

dissipate adequate amounts of heat at voltages dV

CC(est)

.

Compare V

CC(est)

to V

CC(max)

. If V

CC(est)

d V

CC(max)

, then reli-

able operation between V

CC(est)

and V

CC(max)

requires enhanced

R

qJA

. If V

CC(est)

e V

CC(max)

, then operation between V

CC(est)

and

V

CC(max)

is reliable under these conditions.

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