ML4426
FUNCTIONAL DESCRIPTION (Continued)
BACK EMF SENSING PLL COMMUTATION CONTROL
C
R
SPEEDFB1
SPEEDFB
Three blocks form a phase locked loop that locks the
commutation clock onto the back EMF signal: the
commutation state machine, the voltage controlled
oscillator, and the back EMF sampler. The complete phase
locked loop is illustrated in Figure 7. The phased locked
loop requires a lead lag filter that is set by external
components on SPEED FB. The components are selected
as follows:
C
SPEEDFB2
20
SPEED
FB
FB A
FB B
FB C
V
22
23
24
DD
VCO/TACH
500nA
BACK
EMF
SAMPLER
VOLTAGE
CONTROLLED
OSCILLATOR
13
ꢀ
ꢂ
ꢂ
ꢂ
ꢃ
ꢅ
ꢅ
ꢅ
2
KO1
M
NS
CSPEEDFB1 = 0.25
2
ꢀ
ꢃ
(6a)
d
lnꢀ ꢃ
ꢂ ꢅ
ꢁ100ꢄ
2
ꢂ
ꢂ
ꢁ
ꢅ
ꢅ
ꢄ
fVCO
ꢂ
ꢅ
ꢁ
ꢄ
R
F/R
12
d
fVCO
NS KO1
= 2 M lnꢀ ꢃ
A
PHASE
LOCKED
LOOP
RSPEEDFB
ꢂ ꢅ
(6b)
(6c)
F
B
C
ꢁ100ꢄ
0
1- M
5
CSPEEDFB2 = CSPEEDFB1 M - 1
0 5
E
D
COMMUTATION
STATE MACHINE
START-UP SEQUENCE
When power is first applied to the ML4426 and the motor
is at rest, the back EMF is equal to zero. The motor needs
to be rotating for the back EMF sampler to lock onto the
rotor position and commutate the motor. The ML4426 uses
an open loop start-up technique to bring the rotor from rest
up to a speed fast enough to allow back EMF sensing.
Start-up is comprised of three modes: align mode, ramp
mode, and run mode.
Figure 7. Back EMF Commutation Phase Locked Loop
Ramp Mode
At the end of align mode the controller goes into ramp
mode. Ramp mode starts commutating through the states
A through F as shown in Table 1. This ramps up the
commutation frequency, and therefore the motor speed,
for a fixed length of time. This allows the motor to reach a
sufficient speed for the back EMF sampler to lock
Align Mode (RESET)
Before the motor can be started, the rotor must be in a
known position. When power is first applied to the
ML4426, the controller is reset into the align mode. Align
mode turns on the output drivers LB, HA, and HC which
aligns the motor into a position 30 electrical degrees
before the center of the first commutation state. This is
shown as state R in the commutation states of Table 1.
Align mode must last long enough to allow the motor and
its load to settle into this position. The align mode time is
commutation onto the motor's back EMF. The amount of
time the ML4426 stays in ramp mode is determined by a
capacitor connected to the C pin as shown in Figure 8.
RT
C
is charged by a constant 750µA current from GND to
RT
1.5 V until the ramp comparator trips to end the ramp
mode. This gives a fixed ramp time. C is calculated as
RT
follows:
7
2 J 510 amp KV
set by a capacitor connected to the C pin as shown in
AT
CRT
=
(8)
Figure 8. C is charged by a constant 750µA current from
AT
IMAX Kt 3N
GND to 1.5 V until the align comparator trips to end the
align mode. A starting point for C is calculated as
The rate at which the ML4426 ramps up the motor speed
is determined by a fixed 500µA current source on the
SPEED FB pin. The current sources charges up the PLL
filter components causing the VCO frequency to ramp up.
During ramp mode, the back EMF sampler is disabled to
allow control of the ramping to be set only by the 500µA
current source. The ramp based on the SPEED FB filter is
generally too fast for the motor to keep up, so a capacitor
AT
follows:
7
tS 7.510 amp
CAT
=
(7)
1.5V
If the align time is not long enough to allow the rotor to
settle for reliable starting, then increase C until the
AT
desired performance is achieved.
from C to SPEED FB can be added to slow down the
RR
ramping rate. The optimal ramp rate is based on the motor
and load parameters and is can be adjusted by varying
the value of C
.
RR
10
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
