A1425 Allegro MicroSystems, A1425 Datasheet - Page 11

• Manufacturer Part Number: A1425
• Description: High Accuracy Analog Speed Sensor with Integrated Filter Capacitor and Dual Zero-Crossing Output Signal
• Manufacturer: Allegro MicroSystems

AC-Coupled Operation
Steady-state magnet and system offsets are eliminated using an
on-chip differential band-pass fi lter. The low and high frequency
poles of this band-pass fi lter are set using internal integrated
capacitors and resistors. The differential structure of this fi lter
improves the ability of the IC to reject single-ended noise on
the ground (GND pin) or supply line (VCC pin) and, as a result,
makes it more resistant to electromagnetic interference typically
seen in hostile remote-sensing environments.
Power Supply Protection
The A1425 contains an on-chip voltage regulator and can oper-
ate over a wide supply voltage range. In applications that operate
the device from an unregulated power supply, transient protec-
tion must be added externally. For applications using a regulated
line, EMI/RFI protection may still be required. The circuit
shown in fi gure 2 is the most basic confi guration required for
proper device operation.
Power Derating
The device must be operated below the maximum junction
temperature of the device, T
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
the Allegro MicroSystems Web site.)
The Package Thermal Resistance, R
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
Figure 2. Basic application circuit
the output driver.
A1425-DS Rev. 0b
Preliminary - Subject to Change
Without Notice November 10, 2004
High Accuracy Analog Speed Sensor with Integrated Filter Capacitor and Dual Zero-Crossing Output Signal
0.1 uF
4
A1425
1
3
J(max)
J
. (Thermal data is also available on
. A pull-up resistor is required with
2
. Under certain combinations of
θJA
, is a fi gure of merit sum-
R
PU
VCC
VOUT
A1425
traces. Radiation from the die through the device case, R
relatively small component of R
T
overmolding.
The effect of varying power levels (Power Dissipation, P
be estimated. The following formulas represent the fundamental
relationships used to estimate T
For example, given common conditions such as: T
V
A worst-case estimate, P
able power level (V
at a selected R
Example
Reliability for V
K PCB
Observe the worst-case ratings for the device, specifi cally:
R
I
Calculate the maximum allowable power level, P
invert equation 3:
This provides the allowable increase to T
power dissipation. Then, invert equation 2:
Finally, invert equation 1 with respect to voltage:
The result indicates that, at T
dissipate adequate amounts of heat at voltages ≤V
Compare V
able operation between V
R
V
CC(max)
A
θJA
θJA
CC
CC(max)
P
V
, and air motion are signifi cant external factors, damped by
∆T
D(max)
CC(est)
∆T = P
= 12 V, I
P
T
. If V
= 177°C/W, T
D
J
max
= T
= V
= 7.0 mA.
is reliable under these conditions.
= T
= ∆T
= P
CC(est)
A
D
CC(est)
CC
+ ∆T = 25°C + 9°C = 34°C
×
J(max)
CC
D(max)
θJA
×
max
R
CC
= 4.2 mA, and R
≥ V
I
θJA
and T
to V
CC
J(max)
∆T = P
÷ R
P
T
– T
at T
CC(max)
÷ I
D
CC(max)
J
= 12 V
= 50 mW
θJA
= T
CC(max)
= V
A
CC(max)
A
A
= 165°C, V
= 165 °C – 150 °C = 15 °C
D(max)
= 150°C, package L-I1, using minimum-
.
A
= 15°C ÷ 177 °C/W = 91 mW
CC(est)
D
IN
, I
+ ∆T
×
, then operation between V
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
×
CC(max)
A
×
. If V
= 91 mW ÷ 7.0 mA = 13 V
, the application and device can
, represents the maximum allow-
R
×
4.2 mA = 50 mW
J
I
and V
θJA
θJA
, at P
IN
θJA
177 °C/W = 9°C
CC(est)
. Ambient air temperature,
CC(max)
), without exceeding T
= 177 °C/W, then:
D
CC(max)
.
≤ V
J
= 26.5 V, and
resulting from internal
CC(max)
(1)
(2)
(3)
requires enhanced
D(max)
CC(est)
A
= 25°C,
, then reli-
CC(est)
. First,
.
θJC
D
), can
J(max)
, is
and
11
,