ML4411A Просмотр технического описания (PDF) - Micro Linear Corporation

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ML4411A

Sensorless Spindle Motor Controller

Micro Linear Corporation 

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





dFVCO

dt





depends on the tolerances of KVCO, IRAMP, C1,

C2, and CVCO. For more optimum spin up times, these

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)

ICMD = 0V

PWM OUT = 50%

MicroP

PWM OUT

ML4411

I(RAMP)

IN

VCO/TACH OUT

Figure 7. Auto-Calibration of Open-Loop Step

Rate.

3. Compute a correction factor to adjust IRAMP current

by changing the PWM duty cycle from the

Micro (D.C.)

D.C.(NEW) = 50% × ∆FVCO(DESIRED)

∆FVCO(MEASURED)

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

IRAMP may be taken out of pin 20 if the offset is positive

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

open loop stepping mode. In that case, VVCO would not

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.

AV

=

1.875 × 10−4

sCOTA

PWM AND LINEAR CURRENT CONTROL

To facilitate speed control, the ML4411 includes two

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

linear control loop senses the motor current on the ISENSE

terminal through RSENSE. An internal current sense

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

between ICMD and ISENSE. The transconductance gain of

A2 is:

gm = 1.875 × 10−4

The current loop is compensated by COTA which forms a

pole given by

ωP

=

9.375 × 10−4

COTA

This time constant should be fast enough so that the

current loop settles in less than 10% of TVCO at the

highest motor speed to avoid torque ripple to VTH

mismatch of the N-Channel MOSFETs.

The ISENSE input pin should be kept below 1V. If ISENSE

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

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

ISENSE below 0.7V removes the bias current to its normal

level. For this reason, the noise filter resistor on the ISENSE

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

ISENSE signal.

OUTPUT DRIVERS

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

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

until P3 is within 1.4V (typ) of VCC2 on the ML4411A.

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.

8

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