Chopper-Stabilized, Two-Wire
Hall-Effect Latch
A1244
9
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
The device must be operated below the maximum junction tem-
perature 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 Web site.)
ThePackageThermalResistance,R
θJA
, 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.
ItsprimarycomponentistheEffectiveThermalConductivity,K,
of the printed circuit board, including adjacent devices and traces.
Radiationfromthediethroughthedevicecase,R
θJC
, is relatively
smallcomponentofR
θJA
. Ambient air temperature, T
A
, and air
motion are significant external factors, damped by overmolding.
Theeffectofvaryingpowerlevels(PowerDissipation,P
D
), can
be estimated. The following formulas represent the fundamental
relationships used to estimate T
J
, at P
D
.
P
D
=V
IN
×
I
IN
(1)
ΔT = P
D
×
R
θJA
(2)
T
J
= T
A
+ΔT (3)
For example, given common conditions such as: T
A
= 25°C,
V
CC
= 12V, I
CC
= 4 mA, and R
θJA
= 140 °C/W, then:
P
D
=V
CC
×
I
CC
=12V
×
4 mA = 48 mW
ΔT = P
D
×
R
θJA
= 48 mW
×
140°C/W=7°C
T
J
= T
A
+ ΔT=25°C+7°C=32°C
A worst-case estimate, P
D
(max), represents the maximum allow-
ablepowerlevel(V
CC
(max), I
CC
(max)), without exceeding
T
J
(max),ataselectedR
θJA
and T
A
.
Example:ReliabilityforV
CC
at T
A
=
150°C, package LH, using a
low-KPCB.
Observe the worst-case ratings for the device, specifically:
R
θJA
=
110 °C/W, T
J
(max)
=
165°C,V
CC
(max)
=
24V,and
I
CC
(max)
=
17
mA.
Calculate the maximum allowable power level, P
D
(max). First,
invert equation 3:
ΔT
max
= T
J
(max) – T
A
=165
°C
–
150
°C = 15
°C
This provides the allowable increase to T
J
resulting from internal
power dissipation. Then, invert equation 2:
P
D
(max)
= ΔT
max
÷ R
θJA
=15°C÷110°C/W=136mW
Finally, invert equation 1 with respect to voltage:
V
CC(est)
= P
D
(max)
÷ I
CC
(max) =136mW÷17mA = 8V
The result indicates that, at T
A
, the application and device can
dissipateadequateamountsofheatatvoltages≤V
CC(est)
.
CompareV
CC(est)
toV
CC
(max).IfV
CC(est)
≤V
CC
(max), then reli-
ableoperationbetweenV
CC(est)
andV
CC
(max) requires enhanced
R
θJA
.IfV
CC(est)
≥V
CC
(max),thenoperationbetweenV
CC(est)
andV
CC
(max) is reliable under these conditions.
Power Derating