Chopper-Stabilized Precision Hall-Effect Switches
A1120, A1121, A1122,
A1123, and A1125
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
temperature of the device, TJ(max). Under certain combinations of
power or improving the heat dissipation properties of the appli-
cation. This section presents a procedure for correlating factors
affecting operating TJ. (Thermal data is also available on the
Allegro MicroSystems website.)
The Package Thermal Resistance, RθJA, is a figure of merit sum-
marizing the ability of the application and the device to dissipate
Its primary component is the Effective Thermal Conductivity, K,
Radiation from the die through the device case, RθJC, is relatively
small component of RθJA. Ambient air temperature, TA, and air
motion are significant external factors, damped by overmolding.
be estimated. The following formulas represent the fundamental
relationships used to estimate TJ, at PD.
PD= VIN × IIN (1)
ΔT = PD × RθJA (2)
TJ = TA + ΔT (3)
For example, given common conditions such as: TA= 25°C,
VCC = 12 V, ICC = 1.6 mA, and RθJA = 165°C/W, then:
PD= VCC × ICC = 12 V × 1.6 mA = 19 mW
ΔT = PD × RθJA = 19 mW × 165°C/W = 3°C
TJ = TA + ΔT=25°C+3°C=28°C
A worst-case estimate, PD(max), represents the maximum allow-
able power level (VCC(max), ICC(max)), without exceeding TJ(max),
at a selected RθJA and TA.
Example: Reliability for VCC at TA
150°C, package LH, using a
Observe the worst-case ratings for the device, specifically:
= 24 V, and
ICC(max) = 4 mA.
Calculate the maximum allowable power level, PD(max). First,
ΔTmax = TJ(max) – TA = 165
°C = 15
This provides the allowable increase to TJ resulting from internal
PD(max) = ΔTmax ÷ RθJA=15°C÷228°C/W=66mW
VCC(est) = PD(max) ÷ ICC(max) = 66 mW ÷ 4 mA = 16.5 V
The result indicates that, at TA, the application and device can
Compare VCC(est) to VCC(max). If VCC(est)≤VCC(max), then reli-
able operation between VCC(est) and VCC(max)requiresenhanced
RθJA. If VCC(est)≥VCC(max), then operation between VCC(est)
and VCC(max) is reliable under these conditions.