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ML4411 Datasheet(PDF) 8 Page - Micro Linear Corporation

No. de pieza ML4411
Descripción Electrónicos  Sensorless Spindle Motor Controller
Download  14 Pages
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Fabricante Electrónico  MICRO-LINEAR [Micro Linear Corporation]
Página de inicio  http://www.microlinear.com
Logo MICRO-LINEAR - Micro Linear Corporation

ML4411 Datasheet(HTML) 8 Page - Micro Linear Corporation

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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

 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%
I(RAMP)
VCO/TACH OUT
PWM OUT
IN
MicroP
ML4411
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.)
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
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.
A
sC
V
OTA
=
×
1 875 10 4
.
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
OTA
C
=
×
9 375 10 4
.
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.


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