L6238S datasheet(19/31 Pages) STMICROELECTRONICS

19 / 31 page

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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