L6238S datasheet(16/31 Pages) STMICROELECTRONICS

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put switched to phase 2 with output A high and C

low with the Bemf amplifier monitoring output B.

In order to prevent commutation current noise be-

ing detectedm as a false zero crossing, a mask-

ing circuit automatically blanks out all incoming

signals as soon as a zero crossing is detected.

When the next commutation occurs an internal

counter starts counting down to set the time that

the masking pulse remains.

The counter is initially loaded with a number that

is equal to time that is always 25% of the previous

phase period or 15 electrical degrees. The time-

out of the masking pulse shown for reference at

the bottom of figure 4-1. Thus the actual masking

period is the total of the time from the detected

zero crossing to the phase commutation, plus

25% of the previous period. The mask pulse op-

eration is further discussed in section 4.6, Slew

Rate Control and PWM operation.

After the masking period, the Bemf voltage at out-

put B is monitored for a zero crossing. Upon de-

tection of the crossing, the output is commutated

after the selected phase delay insuring maximum

torque. The spin sense waveform at the bottom of

the figure indicates that this output signal toggles

with each zero crossing.

4.2 Brake Delay

When Run/Brake is brought low, a brake is initi-

ated. Referring to figure 4-2, SW1 is opened and

the brake delay capacitor, Cbrake, is allowed to

discharge towards groun via Rbrake.

At the same time, switches SW2 through SW7

bring the gates of the output FETs to ground halt-

ing conduction, causing the motor to coast. While

the motor is coasting, the Bemf is used to park

the heads. When Cbrake reaches a voltage that

is below the turn ON threshold of Q1, Switches

SW8, 9, and 10 bring the gates of the lower driv-

ers to Vbrake potential. This enables the lower

FETs causing a braking action.

The analog and logic supplies are not monitored

in the L6238S, since the L6244 already monitors

this voltage and initiates a Park function when

either supply drops to a predeterminated level.

Figure 4-2

L6238S

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