L6238 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6238' PDF : 35 Pages View PDF
L6238
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
Figure 24: Effect of Slow Slew Rate.
Figure 23: Output Voltage Slew Rate vs Rslew.
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
~800µs resulting in a mask time period of 200µs.
As can be seen, the excessively long slew rate
24/35
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