BEMF Zero crossing detection internal peripheral

AN4370

6/23

DocID025336 Rev 1

Figure 2. BEMF detection stepping of BEMFCNT

To avoid detecting a false Z crossing event due to the PWM switching noise (ringing: see

Figure 3), we recommend reading the floating phase as far as possible from the switching

edges, this is why the output of the selected comparator can be sampled by the appropriate

edge of the appropriate PWM signal (Driving PWM signal or Active Freewheeling PWM

signal). Therefore, for LOW DUTY CYCLES we recommend using the GND comparator for

PWM on the HS MOSFETs and the VSMS comparator for PWM on the LS MOSFETs to

detect free of noise Z crossing event, in this case, in fact, the floating phase will be sampled

usually at the end of the active freewheeling PWM TON (this means close to the end of the

comparator to detect free of noise Z crossing events. In this case, in fact, the sampling will

be performed always at the end of the driving PWM TON. While in the latter case the output

driving PWM signal. In the previous case the output of the GND/BUS comparator can be

either sampled at the turning OFF command edge of the active freewheeling PWM signal for

BEMFPOL = 1 or at the turning ON command edge of the driving PWM signal (this option is

mandatory when there is no active freewheeling). The BEMFBY bit allows switching from

the PWM sampling source to the internal 10MHz sampling source, mandatory selection for

a PWM with a 100% Duty Cycle. Table 1 provides the PWM sampling edge according to the

bits configuration.

GAPGCFT00039

Table 1. PWM edge used for filtering the selected BEMF Z crossing comparator output

BEMFMOD

BEMFSW

BEMF comparator

selected

BEMFPOL

BEMF

sampling

Comment

0

0 (PWM on

HS)

GND comparator

0

HS turn-on

To be used for passive

freewheeling. We recommend

selecting the GND comparator

output for low PWM duty cycles.

0

0 (PWM on

HS)

GND comparator

1

LS turn-off

To be used for active

freewheeling. We recommend

selecting the GND comparator

output for low PWM duty cycles.