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參數(shù)資料
| 型號(hào)： | A1423 |
| 廠商： | Allegro MicroSystems, Inc. |
| 元件分類： | 速度傳感器和接近開關(guān) |
| 英文描述： | High Accuracy Analog Speed Sensor with Integrated Filter Capacitor |
| 中文描述： | 高精度模擬與集成濾波電容器速度傳感器 |
| 文件頁(yè)數(shù)： | 15/16頁(yè) |
| 文件大?。?/td> | 1675K |
| 代理商： | A1423 |
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15
A1421-DS Rev. 0
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
High Accuracy Speed Sensor with Integrated Filter Capacitor
A1421, A1422, A1423
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
θ
JA
, is a
fi
gure 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
θ
JC
, is
relatively small component of R
θ
JA
. Ambient air temperature,
T
A
, and air motion are signi
fi
cant 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
×
I
IN
T = P
D
×
R
θ
JA
T
J
= T
A
+
T
(1)
(2)
(3)
For example, given common conditions such as: T
A
= 25°C,
V
CC
= 12 V, I
CC
= 4.2 mA, and R
θ
JA
= 177 °C/W, then:
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
θ
JA
and T
A
.
P
D
= V
CC
×
I
CC
= 12 V
×
4.2 mA = 50 mW
T = P
D
×
R
θ
JA
= 50 mW
×
177 °C/W = 9°C
T
J
= T
A
+
T = 25°C + 9°C = 34°C
Example
Reliability for V
CC
at T
A
=
150°C, package L-I1, using minimum-
K PCB
Observe the worst-case ratings for the device, speci
fi
cally:
R
θ
JA
=
177°C/W, T
J(max)
=
165°C, V
CC(max)
=
26.5
V, and
I
CC(max)
=
7.0
mA.
Calculate the maximum allowable power level, P
D(max)
. First,
invert equation 3:
T
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)
=
T
max
÷ R
θ
JA
= 15°C ÷ 177 °C/W = 91 mW
Finally, invert equation 1 with respect to voltage:
V
CC(est)
= P
D(max)
÷ I
CC(max)
= 91 mW ÷ 7.0 mA = 13 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 reli-
able operation between V
CC(est)
and V
CC(max)
requires enhanced
R
θ
JA
. If V
CC(est)
≥
V
CC(max)
, then operation between V
CC(est)
and
V
CC(max)
is reliable under these conditions.
Preliminary - Subject to Change
Without Notice June 16, 2004
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