Self-Calibrating, Zero-Speed Differential
Gear Tooth Sensor IC with Continuous Update
ATS643LSH
14
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 L-I1, using 
minimum-K PCB
Observe the worst-case ratings for the device, specifically:  
R
qJA 
=
 
126癈/W, T
J(max)  
=
 
165癈, V
CC(max) 
=
 
24
 
V, and 
I
CC(max)  
=
 
16
 
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 ?126 癈/W = 119 mW
Finally, invert equation 1 with respect to voltage:
     V
CC(est)
 = P
D(max) 
? I
CC(max)
 = 119 mW ?16 mA = 7 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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