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A1425 Schematic ( PDF Datasheet ) - Allegro MicroSystems

Teilenummer A1425
Beschreibung High Accuracy Analog Speed Sensor IC
Hersteller Allegro MicroSystems
Logo Allegro MicroSystems Logo 




Gesamt 13 Seiten
A1425 Datasheet, Funktion
A1425
High Accuracy Analog Speed Sensor IC with Integrated Filter
Capacitor and Dual Zero-Crossing Output Signal
Features and Benefits
Used in sensing motion of ring magnet or ferrous targets
Integrated filter capacitor
Wide operating temperature range
Operation with magnetic input signal frequency from
20 Hz to 20 kHz
Resistant to EMI
Large effective air gaps
4.0 to 26.5 V supply operating range
Output compatible with both TTL and CMOS logic
families
Reverse battery protection
Resistant to mechanical and thermal stress
Accurate true zero-crossing switchpoint
Package: 4 pin SIP (suffix K)
Description
The A1425 AC-coupled Hall-effect sensor IC is a monolithic
integrated circuit that switches in response to changing
differential magnetic fields created by rotating ring magnets
and, when coupled with a magnet, by ferrous targets.The device
is a true zero-crossing detector: the output switches precisely
when the difference in magnetic field strength between the
two Hall elements is zero. A unique dual-comparator scheme
provides for accurate switching at the zero crossing on both the
positive and negative-going regions of the differential signal,
while utilizing hysteresis to prevent false switching. The zero-
crossing nature of this device provides excellent repeatability
and accuracy for crankshaft applications.
Changes in field strength at the device face, which are
induced by a moving target, are sensed by the two integrated
Hall transducers. The transducers generate signals that are
differentially amplified by on-chip electronics.This differential
design provides immunity to radial vibration within the
operating air gap range of the A1425, by rejection of the
common mode signal. Steady-state magnet and system offsets
are eliminated using an on-chip differential band-pass filter.
This filter also provides relative immunity to interference from
electromagnetic sources.
Continued on the next page…
VS+
VCC
(Pin 1)
Functional Block Diagram
Dual Hall
Transducers
Regulator
0.1 uF
Hall
Amp
Gain
Stage
Bandpass Filter Integrated
Tracking Capacitor
VREF
Diagnostic
Circuitry
Comparator
TEST
(Pin 3)
VOUT
(Pin 2)
1425-DSa, Rev.3
GND
(Pin 4)
(Required)






A1425 Datasheet, Funktion
A1425
High Accuracy Analog Speed Sensor IC with Integrated
Filter Capacitor and Dual Zero-Crossing Output Signal
Empirical Results, continued
116
Air Gap (mm)
116
Air Gap (mm)
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6

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A1425 pdf, datenblatt
A1425
High Accuracy Analog Speed Sensor IC with Integrated
Filter Capacitor and Dual Zero-Crossing Output Signal
Power Derating
The device must be operated below the maximum junction
temperature of the device, TJ(max). Under certain combina-
tions of peak conditions, reliable operation may require derating
supplied power or improving the heat dissipation properties of
the application. This section presents a procedure for correlating
factors affecting operating TJ. (Thermal data is also available on
the Allegro MicroSystems Web site.)
The Package Thermal Resistance, RJA, is a 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, RJC, is
relatively small component of RJA. Ambient air temperature,
TA, and air motion are signicant external factors, damped by
overmolding.
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.
×PD = VIN IIN
(1)
 T = PD × RJA
(2)
TJ = TA + ΔT
(3)
For example, given common conditions such as: TA= 25°C,
VCC = 5.0 V, ICC = 4.2 mA, and RJA = 177 °C/W, then:
PD = VCC × ICC = 5.0 V × 4.2 mA = 21.0 mW
 T = PD × RJA = 21.0 mW × 177 °C/W = 3.7°C
TJ = TA + T = 25°C + 3.7°C = 28.7°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 RJA and TA.
Example
Reliability for VCC at TA=150°C, using minimum-K PCB
Observe the worst-case ratings for the device, specically:
RJA=177°C/W, TJ(max) =165°C, VCC(max)=26.5V, and
ICC(max) = 7.0 mA.
Calculate the maximum allowable power level, PD(max). First,
invert equation 3:
Tmax = TJ(max) – TA = 165°C–150°C = 15°C
This provides the allowable increase to TJ resulting from internal
power dissipation. Then, invert equation 2:
PD(max) = Tmax ÷ RJA = 15°C ÷ 177 °C/W = 84 mW
Finally, invert equation 1 with respect to voltage:
VCC(est) = PD(max) ÷ ICC(max) = 84 mW ÷ 7.0 mA = 12 V
The result indicates that, at TA, the application and device can
dissipate adequate amounts of heat at voltages VCC(est).
Compare VCC(est) to VCC(max). If VCC(est) VCC(max), then reli-
able operation between VCC(est) and VCC(max) requires enhanced
RJA. If VCC(est) VCC(max), then operation between VCC(est)
and VCC(max) is reliable under these conditions.
For example, when a standard diode with a 0.7 V drop is used:
VS(max) = 12 V + 0.7 V = 12.7 V
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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