Two-Wire, Differential, Vibration-Resistant Sensor
with Speed and Direction Output
ATS699LSN
POWER DERATING
The device must be operated below the maximum junction tem-
perature of the device (TJ(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 TJ. (Thermal data is also available on the
Allegro website.)
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 SN, using a
single-layer PCB.
Observe the worst-case ratings for the device, specifically:
RθJA =150°C/W, TJ(max) =165°C, and ICC(mean) = 13 mA.
(Note: For variant –xxWPx, at maximum target frequency,
ICC(LOW) = 8 mA, ICC(HIGH) = 16 mA, and maximum pulse
widths, the result is a duty cycle of 84% and thus a worst-case
mean ICC of 14.8 mA).
The Package Thermal Resistance (RθJA) 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 (RθJC) is relatively Calculate the maximum allowable power level, PD(max). First,
small component of RθJA. Ambient air temperature (TA) and air
motion are significant external factors, damped by overmolding.
invert equation 3:
ΔTmax = TJ(max) – TA = 165°C–150°C = 15°C
The effect of varying power levels (Power Dissipation, PD), can
be estimated. The following formulas represent the fundamental
relationships used to estimate TJ, at PD.
This provides the allowable increase to TJ resulting from internal
power dissipation. Then, invert equation 2:
ꢀꢀꢀꢀPD(max) = ΔTmax ÷RθJA =15°C÷150°C/W=100 mW
PD = VIN
I
(1)
(2)
(3)
×
IN
Finally, invert equation 1 with respect to voltage:
ꢀ
ꢀ
ΔT = PD
R
θJA
×
ꢀ VCC(est) = PD(max) ÷ ICC(max)= 100 mW÷14.8 mA= 6.8 V
TJ = TA + ΔT
The result indicates that, at TA, the application and device can
dissipate adequate amounts of heat at voltages ≤VCC(est).
For example, given common conditions such as: TA= 25°C,
VCC = 12 V, ICC = 6 mA, and RθJA = 150°C/W, then:
Compare VCC(est) to VCC(max). If VCC(est) ≤ VCC(max), then
reliable operation between VCC(est) and VCC(max) requires
enhanced RθJA. If VCC(est) ≥ VCC(max), then operation between
VCC(est) and VCC(max) is reliable under these conditions.
PD = VCC
I
= 12 V 6 mA = 72 mW
CC
×
×
ꢀ
ΔT = PD
R
= 72 mW 150°C/W = 10.8°C
θJA
×
×
TJ = TA + ΔT = 25°C + 10.8°C = 35.8°C
13
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
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