TLE4926C-HTN E6747

Data Sheet

Page 3 of 27

Figure 2: Block Diagram of TLE4926C

Basics of self-calibration

A magnetic signal generated by a typical toothed wheel looks somewhat like a

sinusoid. Optimum switching points lie near the zero crossings of the curve. Due to

backbiasing conditions and imperfections in the IC the signal is superposed by an

offset. Therefore the main task to accomplish is to remove this offset. This is done by

measuring the minimum and maximum values of the curve and by calculating the

resulting offset. By subtracting this offset from the original signal, the signal is

centered on its zero crossings and these can be detected by a normal comparator.

The detection of the minimum and maximum values as well as the complete signal

correction strategy is implemented in a digital way. Therefore first the signal has to

be digitized.

Digitizing the signal

A tracking A/D converter basically does signal conversion from analog into digital

domain. The converter has a resolution of 6 bits. This (including some averaging

calculation described later on) is sufficient for characterizing the signal if the signal is

not too small. Therefore, a programmable gain amplifier (PGA) enhances the A/D

converter. Its amplification can be modified by powers of two. 7 different positions are

possible; the amplification range lies between ½ and 32 (Full scale A/D converter

range referred to full-scale range of the offset- Dac). The gain of the PGA tracks the

amplitude of the signal so that sufficient resolution of the converter is ensured. The

gain of the PGA is decreased by 1 step (a factor of 2) whenever a signal overrun in

the tracking converter is detected. On the other hand, the gain of the PGA is

increased by 1 step, when during an offset update low signal amplitude is detected.

This means, that the actual minimum value is larger or equal 50H (on an 8 bit base

referred to the averaged Gain- Dac) and the actual maximum value is smaller than

hyst

comp

digital

Hall probes

+

-

Q

n-channel

open

drain

actual switching

level

min

max

algorithm

overtemperature

& short-circuit

protection

power supply regulator

analog

supply

digital

supply

bias for temperature

& technology

compensation

main

comp

GND

clamping

clamping & reverse

voltage protection

interface

Offset

DAC

enable

+

-

+

-

Tracking ADC

amplifier

oscillator

filter

reset