L6238 datasheet(23/35 Pages) STMICROELECTRONICS

23 / 35 page

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.

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-

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.

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.

Figure 20: Motor Pole.

Figure 21: Effect of Cm.

Figure 22: Correct Compensation.

L6238

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