A1126_16 Datasheet, PDF (11/14 Pages) Allegro MicroSystems – Chopper-Stabilized Omnipolar Hall-Effect Switch

A1126
Chopper-Stabilized Omnipolar Hall-Effect Switch
Power Derating
The device must be operated below the maximum junction
temperature of the device, T_J(max) . Under certain combina-
tions of peak conditions, reliable operation may require derating
supplied 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_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.
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_θJC, is
relatively small component of R_θJA. 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. 

× ^PD ^{= V}IN ^IIN
(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_IN = 12 V, I_IN = 4 mA, and R_θJA = 140 °C/W, then:
× ^{  P}D ^{= V}IN ^IIN ^{= 12 V} × ^{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-
able power level, without exceeding T_J(max) , at a selected R_θJA
and T_A.
Example: Reliability for V_CC at T_A =_ 150°C, package UA, using a
single-layer PCB.
Observe the worst-case ratings for the device, specifically:
R_θJA =_ 165 °C/W, T_J(max) _  =_ 165°C, V_CC(max) _ =_ 24_ V, and
I_CC(max) _  = 4 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 ÷ 165 °C/W =  91 mW
Finally, invert equation 1 with respect to voltage:
  V_CC(est) = P_D(max) ÷  I_CC(max)  = 91 mW ÷ 4 mA =  23 V
The result indicates that, at T_A, the application and device can
dissipate adequate amounts of heat at voltages ≤V_CC(est) .
Compare V_CC(est) to V_CC(max) . If V_CC(est) ≤ V_CC(max) , then
reliable operation between V_CC(est) and V_CC(max) requires
enhanced R_θJA. If V_CC(est) ≥ V_CC(max) , then operation
between V_CC(est) and V_CC(max) is reliable under these condi-
tions.
Allegro MicroSystems, LLC
11
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Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com