A1120LUA-T【PDF 14/18 页】IC SW HALL EFFECT CHOPPER 3-SIP

Chopper Stabilized Precision Hall Effect Switches

A1120, A1121, A1122,

A1123, and A1125

14

Allegro MicroSystems, LLC

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 supplied

power or improving the heat dissipation properties of the appli-

cation. This section presents a procedure for correlating factors

affecting operating T

J

. (Thermal data is also available on the

Allegro MicroSystems website.)

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

= 1.6 mA, and R

qJA

= 165 癈/W, then:

P

D

= V

CC

?/DIV>

I

CC

= 12 V

?/DIV>

1.6 mA = 19 mW

DT = P

D

?/DIV>

R

qJA

= 19 mW

?/DIV>

165 癈/W = 3癈

T

J

= T

A

+ DT = 25癈 + 3癈 = 28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 LH, using a

minimum-K PCB.

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

R

qJA

=

228癈/W, T

J

(max)

=

165癈, V

CC

(max)

=

24 V, and

I

CC

(max)

=

4

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 ?228 癈/W = 66 mW

Finally, invert equation 1 with respect to voltage:

V

CC(est)

= P

D

(max)

? I

CC

(max) = 66 mW ?4 mA = 16.5 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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