L9177 datasheet(41/49 Pages) STMICROELECTRONICS

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L9177

Functional description

4.6.3

Auto-adaptative time filter

This characteristic is useful to set the best internal filter time in function of the input signal

frequency.

Tfilter time is function of the duration of the previous period Tn according to following

formula:

Tfilter(n+1) = 1/32*Tn if Tn > Tfilter(n)

The filtering time purpose is filtering very short spikes.

The digital filtering time is applied to internal squared signal (int_vrs), obtained by voltage

comparators.

The output of time filtering block is out_vrs signal.

The filtering time Tfilter is applied to int_vrs signal in two different way:



Rising edge: if int_vrs high level lasts less than Tfilter out_vrs is not set to high level.

In absence of any spikes during input signal rising edge out_vrs signal is expected with

a delay of Tfilter time.



Falling edge: the falling edge of int_vrs is not delayed through time filtering block: after

falling edge for a time Tfilter any other transition on int_vrs signal is ignored

The initial value (Default) and maximum for Tfilter must be considered at RPM_min = 20 e.g.

Tmax filter = 180 μs. The minimum available value is Tmin filter = 2.8 μs.

Figure 33. Auto-adaptative time filter behaviour 1

L9177 Datasheet(PDF) 41 Page - STMicroelectronics