L6238S datasheet(21/31 Pages) STMICROELECTRONICS

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sistive network as shown in figure 4.12 can be

used to emulate the Bemf of the motor.

The actual Bemf zero-crossing is 30 electrical de-

grees (50% of a commutation interval) away from

the optimal switch point. A digital counter circuit

measures 50% of the previous interval to deter-

mine the next interval’s commutation delay from

the zero crossing. During the low RPM stages of

start up the long commutation intervals may

cause the counter to overflow, in which case 50%

of the max count will be less than 50% of the

ideal commutation interval. Therefore, the torque

will not be optimal until the desired commutation

interval is less than the dynamic range of the

counter.

4.9 Center Tap Protection

Spindle Motors with a high number of windings

exhibit a transformer coupling effect that in some

cases can cause relatively high currents to flow

through the center tap input.

Current flowing out of the center tap pin as high

as 25mA has been observed with certain motors.

Figure 4-12: Bemf Emulator

R1 1K

DS1

TO CENTER

TAP INPUT

D95IN317

Figure 4-13

Figure 4-11: Bemf Amplifier.

BEMF

-35

-25

ViBEMF(mV)

SLOPE=0

SLOPE=1

D95IN316

L6238S

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