544

Motor driver ICs

FDD spindle motor driver

BA6486FS

The BA6486FS is a one-chip IC designed for driving 12V FDD spindle motors. This high-performance IC employs a
3-phase, full-wave soft switching drive system, and contains a digital servo, an index amplifier, and a power save circuit.
The compactly packaged IC reduces the number of external components required.

Applications

Floppy disk drivers

Features

1) 12V supply voltage.
2) 3-phase, full-wave soft switching drive system.
3) Digital servo circuit.
4) Power save circuit.

5) Hall power supply switch.
6) Motor speed changeable.
7) Index amplifier.

Absolute maximum ratings (Ta = 25

Recommended operating conditions (Ta = 25

547

Motor driver ICs

BA6486FS

Circuit operation

(1)

Motor drive circuit

The motor driver is based on a 3-phase, full-wave soft
switching, current drive system in which the position of
rotor is sensed by Hall elements. The total drive current
of motor is sensed by a small resistor (R

) and regulated

through a voltage comparison. The IC consists of Hall
amplifiers, an amplitude control circuit, a driver, an error
amplifier, and current feedback ampifier (Fig. 1).
The waveforms of different steps along the signal path
from the Hall elements to the motor driver output are
shown in Fig. 2. The Hall amplifiers receive the Hall ele-
ments voltage signals as differential signals. Next, by de-
ducting the voltage signal of Hall elements 2 from the
voltage signal of Hall elements 1, current signal H1,
which has a phase 30 degrees ahead of Hall element 1,
is created. Current signals H2 and H3 are created like-
wise. The amplitude control circuit then amplifies the H1,
H2, and H3 signal according to the current feedback am-
plifier signal. Then, drive current signals are produced at
A1, A2, and A3 by applying a constant magnification fac-
tor. Because a soft switching system is employed, the
drive current has low noise and a low total current ripple.
The total drive current is controlled by the error amplifier
input voltage. The error amplifier has a voltage gain of
about �11dB (a factor of 0.28). The current feedback am

plifier regulates the total drive current, so that the error
amplifier output voltage (V1) is equal to the VR

voltage,

which has been voltage-converted from the total drive
current through the R

pin. If V1 exceeds the current lim-

iter voltage (V

), the constant voltage V

takes prece-

dence, and a current limit is applied at the level of V

/ R

The current feedback amplifier tends to oscillate be-
cause it receives all the feedback with a gain of 0dB. To
prevent this oscillation, connect an external capacitor to
the C

pin for phase compensation and for reducing the

high frequency gain.
(2)

Speed control circuit

The speed control circuit is a non-adjustable digital servo
system that uses a frequency locked loop (FLL). The cir-
cuit consists of an 1 / 2 frequency divider, an FG amplifier,
and a speed discriminator (Fig. 14).
An internal reference is generated from an external clock
signal input. The 1 / 2 frequency divider reduces the fre-
quency of the OSC signal. The FG amplifier amplifies the
minute voltage generated by the motor FG pattern and
produces a rectangular-shaped speed signal. The FG
amplifier gain (G

= 42dB, typical) is determined by the

internal resistance ratio.

548

Motor driver ICs

BA6486FS

For noise filtering, a high-pass filter is given by C3 and a
resistor of 1.6k

(typical), and a low-pass filter is given

by C4 and a resistor of 200k

(typical). The cutoff fre-

quencies of high-pass and low-pass filters (f

and f

, re-

spectively) are given by:

The C3 and C4 capacitances should be set so as to satis-
fy the following relationship:
f

where f

is the FG frequency. Note that the FG amplifier

inputs have a hysteresis.
The speed discriminator divides the reference clock and
compares with the reference frequency, and then outputs
an error pulse according to the frequency difference. The
motor rotational speed N is given in the following formula.

fosc is the reference clock frequency,
n is (speed discriminator count)

z is the FG tooth number.

The discriminator count depends on the speed control
pin voltage.
The integrator flattens out the error pulse of the speed
discriminator and creates a control signal for the motor
drive circuit (Fig. 15).
(3)

Index amplifier

The index amplifier is a hysteresis amplifier with a typical
hysteresis width of +18mV and �22mV. The input pin is
not biased internally.
(4)

Other circuits

Start / stop circuit

The start / stop circuit puts the IC to the operating state
when the control pin is LOW, and to the standby state (cir-
cuit current is nearly zero) when the control pin is HIGH.
The Hall device bias switch, which is linked to the
start / stop circuit, is turned off during the standby state,
so that the Hall device current is shut down.

Thermal shutdown circuit

This circuit shuts down the IC currents when the chip
junction temperature is increased to about 175

C (typi-

cal). The thermal shutdown circuit is deactivated when
the temperature drops to about 155

C (typical).

1.6k

200k

(1)

N=60

fosc

Электронный компонент: BA6486FS