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参数资料
| 型号： | L6238SQT |
| 厂商： | STMICROELECTRONICS |
| 元件分类： | 运动控制电子 |
| 英文描述： | BRUSHLESS DC MOTOR CONTROLLER, 5 A, PQFP64 |
| 封装： | TQFP-64 |
| 文件页数： | 11/31页 |
| 文件大小： | 3465K |
| 代理商： | L6238SQT |
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2)The motor capacitance, CM, itself can inter-
fere with the loop, creating double poles. If
the gain at higher frequencies is sufficiently
high, the double pole slope of 40dB/decade
can cause the phase shift to reach 180°,
re sulting in oscillation.
Figure 4-8 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 unchanged and increasing the
zero, the response indicated by the dashed lines
can be achieved.
4.6 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 OFF 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.
The rate at which the upper and lower drivers turn
OFF is programmable via an external resistor,
Sslew connected to the SLEW RATE pin. This re-
sistor defines an internal current source that is
utilized to limit the voltage slew rate at the outputs
during transition, thus minimizing the load change
that the power supply sees.
To insure proper operation the range of resistor
values indicated should not be exceeded and in
some applications values near the end points
should be avoided as discussed below.
Low Values of Rslew - If a relatively low value of
Rslew is selected, the resultant fast slew rate will
result in increased commutation cross-over cur-
rent, higher EMI, and large amount of commuta-
tion current.
This last case can cause voltage spikes at the
output that can go as much as lV below ground
level. This situation must be avoided in this inte-
grated circuit (as in most) since it causes unpre-
dictable operation.
High Values of Rslew - Higher values of Rslew
result of course in slow slew rates at the outputs
which is, under most conditions, the desired case
since the problems associated with fast rates are
reduced. The additional advantage is lower
acoustical noise.
Problems can occur though if the slew rate for a
Figure 4-8
Figure 4-9: Effect of Slow Slew Rate.
L6238S
19/31
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