Si9993CS

Vishay Siliconix

S-60752–Rev. A, 05-Apr-99

FaxBack 408-970-5600, request 70653

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www.siliconix.com

REG5 (Spindle Command) to “00” state. Individual motor

shutdown operation is also possible through the serial port.

Refer to Table 2 for a complete description.

Immediately after detection of the undervoltage condition, all

spindle drivers are turned off while the VCM’s low-side drivers

are turned on to stop the head from any previous movement.

Additionally, all command resistors, except REG1, are reset.

After a time delay, programmed by D5/D4 of REG1, the VCM’s

retract circuitry is turned on. The spindle drivers remained off.

The head is retracted toward the inside diameter of the disk,

or (VCM+ - VCM-) is positive. The voltage clamp across the

VCM, or the maximum head velocity, is limited by the stored

value in D7/D6 of REG1. No programmable current clamp is

available on Si9993CS. The minimum short-circuited VCM

retract current is 120 mA. Refer to Figure 3 for a simplified

schematic.

The head retract time interval, or spindle brake delay time, is

programmed via bit D3/D2 of REG1. After the time-out, the

VCM drivers are turned off (tri-state) and all bits of REG1

except D1/D0 are reset. The state of the spindle motor

low-side drivers are determined by D1/D0) of REG1.

Depending on the motor and the number of disk platters, a

spindle brake at high speed (BEMF voltage of 6 V or higher)

may result in a high amount of energy dissipation in the

LITTLE FOOT drivers. One solution to this problem is to use a

the dissipation limit of the Si9942 be exceeded. In which case,

D1/D0 or REG1 should be programmed for immediate brake.

A lower cost solution, when feasible, is to allow the motor to

spin down to a lower rpm before the low-side drivers are

turned on to brake the motor. In such case, the motor speed

threshold, which is sensed by measuring the motor BEMF via

(Figure 4 shows a 3-V threshold being selected.) Also, as

shown in Figure 4, once the spindle brake is on, an external

switched on to maintain the gate drive to the LITTLE FOOT

FIGURE 3. Simplified Retract Circuit

FIGURE 4. Simplified Spindle Brake Circuit