ML4428
C
SNS
A capacitor to ground at this pin sets the ON time of the 6
current sense pulses used for position detection at start-up
and at low speeds. The ON time is set by:
T
ON
= C
SNS
(35.7k)
Referring to Figure 5, each of the 6 current sense pulses is
governed by a rise time with a time constant of L/R where L
is the inductance of the motor network with 2 windings
shorted and R is the total resistance in series with the motor
between the supply rails. R includes the ON-resistance of
the power-FETs and R
SNS
. The R
DSON
of the high side FET
should match that of the low side FET. L is a function of
rotor position. Each pulse will have a peak value
V
SENSEPEAK
of
V
SENSEPEAK
−
T
ON
V
MOTOR
1
−
e
L / R
=
R
SNS
R
What is important for sensing rotor position is the
amplitude difference between each of the three pairs of
current sense pulses. This can be seen by triggering on
I
SNS
on an oscilloscope with the RC
VCO
pin shorted to
ground. One should see the current waveform of Figure 5.
Allowing the peak current sense pulse to reach an
amplitude of 0.5V (by adjusting C
SNS
, and hence T
ON
) or,
allowing the difference between the maximum and
minimum of the 6 pulses to be >50mV, should suffice for
adequate rotor position sensing. A good starting value for
T
ON
is 200µs, requiring C
SNS
= 5.6nF.
R
INIT
The initial time interval between sample pulses during
start-up is set by R
INIT
. This time interval (t
INIT
) occurs
while the RC
VCO
pin is less than 0.25 volts.
R
INIT
=
3.43 t
INIT
C
VCO
where
R
=
0.75
×
R
L
−
L
+
2
×
R
SDON
+
R
SENSE
L
=
0.75
×
L
L
−
L
(
)
DIRECTION
STATE
REVERSE
FORWARD
A
B
C
D
E
F
N3
N1
OFF
OFF
ON
ON
OFF
OFF
N2
N2
OFF
OFF
OFF
OFF
ON
ON
OUTPUTS
N1
N3
ON
ON
OFF
OFF
OFF
OFF
P3
P1
ON
OFF
OFF
OFF
OFF
ON
P2
P2
OFF
ON
ON
OFF
OFF
OFF
P1
P3
OFF
OFF
OFF
ON
ON
OFF
INPUT SAMPLES
FORWARD
PH2
PH1
PH3
PH2
PH1
PH3
REVERSE
PH2
PH3
PH1
PH2
PH3
PH1
Table 1. Commutation States.
3.75V
C
VCO
2.0V
VCO OUT
A
B
C
D
E
F
A
Figure 6. Commutation Timing and Sequencing.
10
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
