DYNAMIC, PEAK-DETECTING,

DIFFERENTIAL HALL-EFFECT

GEAR-TOOTH SENSORS

13

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APPLICATIONS INFORMATION — Continued

Capacitor Requirements. The choice of the capacitor at

pin 3 (C3) defines the minimum operating speed of the

target. This capacitor (0.1

µF minimum) is required to

stabilize the internal amplifiers as well as to eliminate the

signal offsets. Typically, a 0.33

µF low-leakage ceramic

capacitor is recommended. Values greater than 0.47

µF

should not be used as this may cause high-speed perfor-

mance degradation.

Capacitor leakage current at pin 3 will cause degrada-

tion in the low-speed performance of the device. Excess

capacitor leakage can result in the sensor changing output

state without movement of the gear tooth being sensed.

In addition to the capacitor leakage, it is extremely impor-

tant to minimize the leakage at the PC board and between

the pins of the sensor. Up to 50 nA of external leakage

can be tolerated at the capacitor pin node to ground.

Choice of low-leakage-current potting compounds and the

use of clean PC board techniques are extremely impor-

tant.

Operation From an Unregulated Power Supply. In

automotive applications, where the device receives its

power from an unregulated supply such as the battery, full

protection is generally required so that the device can

withstand the many supply-side transients. Specifications

for such transients vary between car manufacturers, and

protection-circuit design should be optimized for each

application. In the circuit below, a simple

Zener-controlled regulator is constructed using discrete

components. The RC low-pass filter on the supply line

(R1C1) and a low-value supply bypass capacitor (CS) can

be included, if necessary, so as to minimize susceptibility

to EMI/RFI. The npn transistor should be chosen with

sufficiently high forward breakdown voltage so as to

withstand supply-side transients. The series diode should

be chosen with sufficiently high reverse breakdown

capabilities so as to withstand the most negative tran-

sient. The current-limiting resistor (RZ) and the Zener

diode should be sized for power dissipation requirements.

3

X

X

+

-

12

4

Vcc

Dwg. EH-008A

20

Ω

RL

0.033

µF

0.22

µF

100 pF

OUTPUT

SUPPLY

0.033

µF

2.5 k

Ω

6.8 V

C3

C 2

C1

R 1

R Z

C S

Additional applications Information on gear-tooth and

other Hall-effect sensors is provided in the Allegro Elec-

tronic Data Book AMS-702 or Application Note 27701.