ml4425 Microsemi Corporation, ml4425 Datasheet - Page 11

• Manufacturer Part Number: ml4425
• Description: Sensorless Bldc Motor Controller
• Manufacturer: Microsemi Corporation

Run Mode (Back EMF Sensing)
At the end of ramp mode the controller goes into run
mode. In run mode, the back EMF sensing is enabled and
commutation is now under the control of the phase locked
loop. Motor speed is now regulated by the speed control
loop.
PWM SPEED CONTROL
Speed control is accomplished by setting a speed
command at SPEED SET with an input voltage from 0 to
6.9V (V
determined by the external components R
There are a number of methods that can be used to control
the speed command of the ML4425. One is to use a 10k
potentiometer from V
connected to SPEED SET. If SPEED SET is controlled from
a microcontroller, one of its DACs can be used with V
as its input reference.
The speed command is compared with the sensed speed
from SPEED FB through a transconductance error
amplifier. The output of the speed error amplifier is SPEED
COMP. SPEED COMP is clamped between one diode drop
above 3.9V (approximately 4.6V) and one diode drop
below 1.7V (approximately 1V) to prevent speed loop
“wind-up”. Speed loop compensation components are
connected to this pin as shown in Figure 9. The speed loop
compensation components are calculated as follows:
Where f
R
C
SC
SC
REF
SB
=
=
FB A
FB B
FB C
2
). The accuracy of the speed command is
f
is the speed loop bandwidth in Hz.
SB
p
™
™
26 9
f
K
10
SB
.
e
SAMPLER
™
BACK
™
EMF
25 98 696
750nA
C
N V
.
REF
SC
Figure 8. ML4425 Start-up Circuitry for Controlling the Align and Ramp Times
™
+
V DD
to ground with the wiper
MOTOR
.
1.5V
C AT
™
™
t
C
m
OF COMMUTATION
–
+
VCO
2
TO RESET INPUT
STATE MACHINE
™
f
SB
750nA
C AT
VCO
2
V DD
and C
1.5V
VCO
C RT
(9a)
(9b)
REF
.
–
+
C RT
The voltage on SPEED COMP is compared with a ramp
oscillator to create a PWM duty cycle. The PWM ramp
oscillator creates a sawtooth function from 1.7V to 3.9V
as shown in Figure 9. A negative clamp at one diode drop
below 1.7V (approximately 1V) starts the oscillator on
power up. The frequency of the ramp oscillator is set by a
capacitor to ground C
following equation:
Where f
cycle from the speed control loop is gated the current
limit one shot that controls the LA, LB, and LC output
drivers.
10k
Figure 9. Speed Control Loop Component Connections
500nA
V REF
C SC
C
R SC
T
C RR
=
PWM
C RR
V DD
f
C T
PWM
1
is the PWM frequency in Hz. The PWM duty
2 4
.
™
SPEED
V
50
FB
SPEED FB
SPEED SET
SPEED COMP
C T
m
FILTER
A
TO
SPEED FB
1.7V
IOS
C VCO
FROM
and is selected using the
CONTROLLED
OSCILLATOR
VOLTAGE
+
–
20kHz
1.7V
3.9V
R VCO
VCO/TACH
+
–
PWM ON/OFF
FROM I LIMIT
ML4425
ONE-SHOT
LOGIC &
OUTPUT
DRIVERS
GATING
(10)
TO
11