A1202_16 Datasheet, PDF (10/13 Pages) Allegro MicroSystems – Continuous-Time Bipolar Switch Family

A1202
and A1203
Continuous-Time Bipolar Switch Family
POWER DERATING
Power Derating
× ^{  P}D ^{= V}CC ^ICC ^{= 12 V} × ^{4 mA = 48 mW}
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 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. 
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 (V_CC(max), I_CC(max)), 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
minimum-K 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) } = 7.5 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:
× ^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_CC = 12 V, I_CC = 4 mA, and R_θJA = 140 °C/W, then:
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 ÷ 7.5 mA = 12.1 V
The result indicates that, at T_A, the application and device can dis-
sipate 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 reli-
able 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 conditions.
Allegro MicroSystems, LLC
10
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com