8

ML4411/ML4411A

The motor will start more consistently and tolerate a wider

variation in open loop step rate if there is some damping

on the motor (such as head drag) during the open loop

modes.

The tolerance of the open loop step VCO acceleration

dF

dt

VCO







variables can be digitally “calibrated” out by the

microprocessor using the following procedure:

1. Reset the IC by holding pin 16 low for at least 5µs.

2. Go into open loop step mode with no current on

the motor and measure the difference between the

first two complete VCO periods with the PWM

signal at 50% duty cycle:

ENABLE E/A = (see below)

PWM OUT = 50%

I(RAMP)

VCO/TACH OUT

PWM OUT

IN

MicroP

ML4411

Figure 7. Auto-Calibration of Open-Loop Step

Rate.

by changing the PWM duty cycle from the

Micro (D.C.)

DC NEW

F

DESIRED

F

MEASURED

VCO

VCO

..(

)

%

()

()

=×

50

∆

∆

4. Use new computed duty cycle for open loop

stepping mode and proceed with a normal start-up

sequence.

If this auto calibration is used ENABLE E/A can be tied

permanently high, eliminating a line from the Micro.

Since there is offset associated with the Phase Detector

Error Amp (E/A), more current than is being injected by

(into pin 20) if the error amp were enabled during the

rise and the motor would not step properly. The effect of

E/A offset can also be canceled out by the auto calibration

algorithm described above allowing the E/A to be

permanently enabled.

A

sC

V

OTA

=

.

PWM AND LINEAR CURRENT CONTROL

To facilitate speed control, the ML4411 includes two

current control loops — linear and PWM (figure 9). The

amplifier’s (A2) output modulates the gates of the 3 N-

channel MOSFET’s when OTA OUT is tied to OTA IN, or

can modulate a single MOSFET gate tied to OTA OUT.

When operated in this mode, OTA IN is tied to 12V, and

N1-N3 are saturated switches. This method produces the

lowest current ripple at the expense of an extra MOSFET.

The linear current control modulates the gates of the

external MOSFET drivers. Amplifier A2 is a

transconductance amplifier which amplifies the difference

A2 is:

−

.

pole given by

OTA

C

=

.

This time constant should be fast enough so that the

mismatch of the N-Channel MOSFETs.

goes above 1V, a bias current of about –300µA will flow

out of pin 12 and the N outputs will be inhibited. Bringing

pin (1Ký on Figure 10) should be less than 1.5Ký.

The noise filter time constant should be great enough to

filter the leading edge current spike when the N-FETs turn

on but small enough to avoid excessive phase shift in the

OUTPUT DRIVERS

The motor’s source drivers (P1 thru P3) are open-collector

NPN’s with internal 16Ký pull-up resistors. N3is inhibited

Drivers N1 through N3 are totem-pole outputs capable of

sourcing and sinking 10mA. Switching noise in the

external MOSFETs can be reduced by adding resistance in

series with the gates.