L6238 datasheet(24/35 Pages) STMICROELECTRONICS

24 / 35 page

Referring back to figure 18, the rate at which the

upper and lower drivers turn OFF is programma-

ble via an external resistor, Rslew connected to the

SLEW RATE pin. This resistor defines a current

which is utilized internally to limit the voltage slew

rate at the outputs during transition, thus minimiz-

ing the load change that the power supply sees.

Figure 23 is a plot of the slew rate that will be ob-

tained as a function of the resistor connected to

the SLEW RATE pin. The voltage at the this pin

is typically 2.4V.

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

given application is too slow. Figure 5-10 is an os-

cillograph taken on a device that had a fairly large

value for Rslew and failed to spin up and phase

lock a motor.

The problem manifests itself as the motor begins

to spin up. At lower RPMs, the Bemf of the motor

is relatively small resulting in higher amounts of

commutation current. In figure 24, the upper

waveform is the voltage appearing at OUTPUT

relative to the CENTER TAP input. The lower

waveform is the actual output of the Bemf ampli-

fier available on special engineering prototypes.

The oscillograph was taken just as the problem

occured. The period between zero crossings was

~800

µs resulting in a mask time period of 200µs.

As can be seen, the excessively long slew rate

Figure 23: Output Voltage Slew Rate vs Rslew.

Figure 24: Effect of Slow Slew Rate.

L6238

24/35

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