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TDA5155X

Pre-amplifier for Hard Disk Drive (HDD) with MR-read/inductive write heads

Philips Electronics 

Philips Semiconductors

Pre-amplifier for Hard Disk Drive (HDD)

with MR-read/inductive write heads

Preliminary specification

TDA5155

8 FUNCTIONAL DESCRIPTION

8.1 Read mode

The read mode disables the write circuitry to save power

while reading. The read circuitry is disactivated for write,

sleep and standby modes. The read circuitry may also be

biased during write mode to shorten transients.

The selected head is connected to a multiplexed low-noise

read amplifier. The read amplifier has low-impedance

inputs nRx and nRy (n is the number of the head) and

low-impedance outputs RDx and RDy. The signal polarity

is non-inverting from x and y inputs to x and y outputs.

Ambient magnetic fields at the MR elements result in a

relative change in MR resistance:

d--R--R---M-M--R--R--

This change produces a current variation:

dIMR = IMR × d--R--R---M-M--R--R-- ,

where IMR is the bias current in the MR element.

The current variation is amplified to form the read data

output signal voltage, which is available at RDx and RDy.

AC coupling between MR elements and amplifier stages

prevents the amplifier input stages from overloading by DC

voltages across the MR elements. A fast settling

procedure shortens DC settling transients.

An on-chip generated stable temperature reference

voltage (1.32 V), available at the Rext pin, is dropped

across an external resistor (10 kΩ) to form a global

reference current for the write and the MR bias currents.

The MR bias current DACs are programmed through the

serial interface according to the following formula:

IMR = 0.5 × -1-R--0---e-k--x-Ω-t-- (10 + 16d4 + 8d3 + 4d2 + 2d1 + d0)

(in mA), where d4-d0 are bits (either logic 0 or logic 1).

At power-up all bits are set to logic 0, which results in a

default MR current of 5 mA. The adjustable range of the

MR currents is 5 mA to 20.5 mA. The MR bias currents are

equal for the two stripes of each head. The gain amplifier

is 1-bit programmable. The amplifier gain can be set to its

nominal value or to the nominal value +3 dB.

8.2 Write mode

To minimize power dissipation, the read circuitry may be

disabled in write mode. The write circuitry is disabled in

read, sleep and standby modes. In write mode, a

programmable current is forced through the selected

two-terminal inductive write head. The push-pull output

drivers yield near rail-to-rail voltage swings for fast current

polarity switching.

The write data input can be either voltage or current input

(see Chapter 12). In voltage mode, the differential write

data inputs WDIx(v) and WDIy(v) are PECL (Positive

Emitter Coupled Logic) compatible. The write data flip-flop

can either be used or passed-by. In the case that the write

data flip-flop is used, current polarity is toggled at the

falling edges of

Vdata = V-----W----D----I-x----(--v--)----2–----V----W-----D----I-y---(--v---)-

Switching to write mode initializes the data flip-flop so that

the write current flows in the write head from x to y. In the

case that the write data flip-flop is not used, the signal

polarity is non-inverting from x and y inputs to

x and y outputs.

The write current magnitude is controlled through on-chip

DACs. The write current is defined as follows:

IWR = 1--R--0---e-k--x-Ω-t-- (20 + 16d4 + 8d3 + 4d2 + 2d1 + d0)

(in mA), where d4-d0 are bits (either logic 0 or logic 1).

The adjustable range of the write current is 20 mA to

51 mA. At power-up, the default values

d4 = d3 = d2 = d1 = d0 = logic 0 are initialized,

corresponding to IWR = 20 mA. IWR is the current provided

by the write drivers: the current in the write coil and in the

damping resistor together. The static current in the write

coil is

1----I-+-W----R-RR---------hd-- ,

where Rh is the resistance of the coil including leads and

Rd is the damping resistor.

1997 Apr 08

8

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