1999 June 10

20

Philips Semiconductors

Preliminary specification

5 V spindle & VCM driver combo

TDA5345HT

The error amplifier compares the VcmDacOut(49) input command and the VcmSenseOut(18) sense amplifier output

signal to generate the control voltage of the power drivers.

(6)

Finally, the transconductance gain of the VCM loop is given by the following equation:

(7)

VCM 12-bit resistive ladder DAC:

The VCM loop input voltage is provided by an internal signed 12-bit resistive ladder DAC. The output voltage is a linear

function of the input code written in register #7 (low gain) or register #8 (high gain), bits [11:0] :

Table 10 Dac12 output voltage versus the input code (1 LSB = 125

µV) when writing in register #7 :

Table 11 Dac12 output voltage versus the input code (1 LSB = 500

µV) when writing in register #8 :

Warning : at power up, the 12-bit DAC needs to be programmed so that it is in a defined state.

VCM back-EMF amplifier

To prevent any actuator crash on the disk when a ramp load is used, an internal VCM back-EMF amplifier is build to

monitor the actuator speed. Indeed, the VCM back-EMF is a picture of the actuator speed. The voltage across the VCM

motor is divided in several contributions:

INPUT CODE

DAC12 VOLTAGE

1.732 V -1 LSB

1.482 V

1.482 V -1 LSB

1.232 V

INPUT CODE

DAC12 VOLTAGE

2.482 V - 1LSB

1.482 V

1.482 V -1 LSB

0.482 V

SENSE

ERROR

to the power

+

-

+

-

VcmInput

VcmCompensIn

G=3

Fig.12 VCM transconductance gain schematic.

drivers

AMPLIFIER

AMP

VcmSenseInP

VcmSenseInM

23

22

VcmRef

18

VcmCompensOut

17

20

19

VcmDacOut

VcmRef

–

R

in

-----------------------------------------------------------------

VcmSenseOut

VcmRef

–

R

FB

-------------------------------------------------------------------------

3R

sense

×

I

VCM

×

R

FB

----------------------------------------------

=

=

G

VCM

A

( )

V

( )

⁄

[]

I

VCM

VcmDacOut

VcmRef

–

-----------------------------------------------------------------

R

FB

R

in

----------

1

3R

sense

×

---------------------------

×

0.4

R

sense

-----------------

=

==