L6238 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6238' PDF : 35 Pages View PDF
the pole necessary for loop stability. Thus the
loop bandwidth is actually limited by the motor it-
self.
Figure 19 shows the complete transconductance
loop including compensation, plus the response.
The Bode plot depicts the normal way to achieve
stability in the loop. The pole and zero are used to
set a gain of 20dB at a higher frequency and the
pole of the motor cuts the gain to achieve stabil-
ity.
Loop instability may be caused by two factors:
1)The motor pole is too close to the zero. Refer-
ring to figure 20, the zero is not able to decre-
ment the shift of phase, and when the effect
of the pole is present, the phase shift may
reach 180° and the loop will oscillate. To rec-
tify this situation, the pole/zero must be
shifted at lower frequencies by increasing the
compensation capacitor.
Figure 20: Motor Pole.
L6238
though the gain decrease at a rate of
40dB/decade, the phase does not reach 180°
of shift.
If the gain at higher frequencies is sufficiently
high, the double pole slope of 40dB/decade can
cause the phase shift to reach 180°, resulting in
oscillation. Figure 22 is a Bode plot showing how
to correct this situation. The bold line indicates
the response with relatively high gain at the
higher frequencies. By leaving the pole un-
changed and increasing the zero, the response
indicated by the dashed lines can be achieved.
Figure 22: Correct Compensation.
2)The motor capacitance, CM, itself can inter-
fere with the loop, creating double poles. If
the gain at higher frequencies is low, this dou-
ble pole will not be able to reach a critical
value due to it’s 40dB/decade slope. Figure
21 illustrates performance with low gain. Al-
Figure 21: Effect of Cm.
5.6 Slew Rate Control
A 3-phase motor appears as an inductive load to
the power supply. The power supply sees a dis-
turbance when one motor phase turns OFE and
another turns ON because the FET turn-OFF time
is much shorter than the L/R rise time. Abrupt
FET turn-OFF without a proper snubbing circuit
can even cause current recirculation back into the
supply.
However, the need for a snubber circuit can be
eliminated by controlling the turn-OFF time of the
FETs.
23/35
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