SLFS040

– DECEMBER 1993

POST OFFICE BOX 655303

DALLAS, TEXAS 75265

10

PRINCIPLES OF OPERATION

voice-coil driver system

The voice-coil full-bridge power amplifier is capable of

0.4-A output current and has a total drain-to-source

on-state resistance (2

amplifier that detects load current with a single resistor in series with the voice-coil motor (VCM). The VCM

current is commanded via serial port A. Full-scale current is controlled by the value of the VCM current sense

resistor (Rsvcm). The retract control circuitry is fully integrated and does not require external isolation of the

device power supply terminals such as an external isolation transistor/diodes. Voice-coil driver auxiliary digital

control functions include an output-disable and low-power sleep mode following a retract of the VCM to the

landing zone. During a fault condition, the VCM drivers are disabled.

VCM current control

The voice-coil motor current is controlled by an internal 8-bit digital-to-analog converter (DAC), four adjustable

gain ranges, and the external current-sense resistor (Rsvcm). The four gain ranges (see functional block

diagram) are system dependent and provide the current needed to ensure that the head assembly swings

across the whole platter. Selecting one of the four gain ranges is accomplished via port A, bits 8 and 9 (see

Table 1 for gain settings and Table 4 for bit definitions). During the start mode, the 8-bit DAC is used to start the

spindle (see Figure 5). In the run mode, the 8-bit DAC is used to program the VCM current. Port A (bits 0

–7)

controls the 8-bit DAC (see Table 4). The VCM current-control equation is given as follows:

where:

G) /(16

Rsvcm)

Rsvcm (the external VCM current sense resistor) typically 0.47

1%

G (the VCM current gain scale)

B0

–B7 (port-A control bits)

Maximum VCM current is load dependent. The following equation should be used as a guideline to determine

maximum VCM current:

where:

Rtotal = Rsvcm + 2

Table 1. VCM Gain-Range Scale Truth Table

GAIN SWITCH SETTING

(see functional block diagram)

SERIAL PORT A GAIN SETTINGS

(see Table 4)

GS1

GS2

GS3

GR0

GR1

GAIN (G)

Open

Open

Open

0

0

0.250

Closed

Open

Open

0

1

0.333

Open

Closed

Open

1

0

0.500

Open

Open

Closed

1

1

1.000