TMC603 DATA SHEET (V. 1.06 / 26. Mar. 2009)
22
Calculating the commutation frequency fCOM of the motor:
SRPM is the rotation velocity in RPM
nPOLE is the pole count of the actual motor, or the double of the number of pole pairs
The spike suppression time can be chosen as high, as the commutation frequency required for
maximum motor velocity allows. As a thumb rule, we take half of this time to have enough spare.
Example:
Given a 4 pole motor operating at 4000 RPM:
CSUP = 6.25nF. The nearest value is 6.8nF.
5.4.2 Adjusting the hallFX filter frequency
The filter block needs to separate the motors’ back EMF from the chopper pulses. Thus, the target is,
to filter away as much commutation noise as possible, while maintaining as much of the back EMF
signal as possible. Therefore, we need to find a cut-off frequency in between the chopper frequency
and the electrical frequency of the motor. Since we do not want to change the frequency within the
application, we use the nominal or maximum motor velocity to calculate its electrical frequency. The
chopper frequency is given by the system, typically about 20 kHz.
The electrical frequency of the motor is:
Since the filter has a logarithmic behavior, as a thumb rule we can make a logarithmic mean-value as
follows:
With the cut-off frequency being about 1/390 of the switched capacitor clock frequency fSCCLK the
following results as a thumb rule:
The result shall be checked against minimum limit of 250 kHz and maximum limit of 4 MHz, however,
the actual frequency is quite uncritical and can be varied in a wide range.
Example:
Given a 4 pole motor operating at 4000 RPM with a 20 kHz chopper frequency:
fEL = 133 Hz
fCUTOFF = 1.6 kHz
fSCCLK = 0.64 MHz
The result is well within the limits, however, the frequency in a practical application can be
chosen between 300 kHz and 1.5 MHz.
5.4.3 Block commutation chopper scheme for hallFX
hallFX works perfectly with nearly every motor. You can use a standard block commutation scheme,
but the chopper must fulfill the following: The coils must be open for some percentage of the chopper
period, in order to allow the back-EMF of the motor to influence the coil voltages. The motor direction
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