EVAL6470H Datasheet, PDF (34/70 Pages) STMicroelectronics – dSPIN fully integrated microstepping motor driver with motion engine and SPI

Phase current control
7
Phase current control
L6470
The L6470 controls the phase current applying a sinusoidal voltage to motor windings.
Phase current amplitude is not directly controlled but depends on phase voltage amplitude,
load torque, motor electrical characteristics and rotation speed. Sinewave amplitude is
proportional to the motor supply voltage multiplied by a coefficient (K_VAL). K_VAL ranges from
0 to 100% and the sinewave amplitude can be obtained through the following formula:
Equation 1
V_OUT = V_S ⋅ K_VAL
Different K_VAL values can be programmed for acceleration, deceleration and constant speed
phases and when the motor is stopped (HOLD phase) through the KVAL_ACC, KVAL_DEC,
KVAL_RUN and KVAL_HOLD registers (see Section 9.1.10). KVAL value is calculated
according to the following formula:
Equation 2
K_VAL = [(K_{VAL_X} + BEMF_COMP) × VSCOMP × K_THERM] × microstep
where K_{VAL_X} is the starting K_VAL value programmed for present motion phase (KVAL_ACC,
KVAL_DEC, KVAL_RUN or KVAL_HOLD), BEMF_COMP is the BEMF compensation curve
value, VSCOMP and K_THERM are the motor supply voltage and winding resistance
compensation factors and microstep is the current microstep value (fraction of target peak
current).
The L6470 offers various methods to guarantee a stable current value, allowing the
compensation of:
● low speed optimization (Section 7.3 7.3)
● back electromotive force value (Section 7.4 7.4)
● motor supply voltage variation (Section 7.5 7.5)
● windings resistance variation (Section 7.67.6).
7.1
PWM sinewave generators
The two voltage sinewaves applied to the stepper motor phases are generated by two PWM
modulators.
The PWM frequency (f_PWM) is proportional to the oscillator frequency (f_OSC) and can be
obtained through the following formula:
Equation 3
f_PWM
=
-----^f--^O----^S---^C-------
512 ⋅ N
⋅
m
'N' is the integer division factor and 'm' is the multiplication factor. 'N' and 'm' values can be
programmed by the F_PWM_INT and F_PWM_DEC parameters in the CONFIG register
(see Table 28 and Table 29, Section 9.1.21).
Available PWM frequencies are listed in Section 9.1.21 from Table 30 to Table 33.
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