ML4425 Ver la hoja de datos (PDF) - Micro Linear Corporation
Número de pieza
componentes Descripción
Fabricante
ML4425 Datasheet PDF : 16 Pages
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ML4425
FUNCTIONAL DESCRIPTION (Continued)
BACK EMF SENSING PLL COMMUTATION CONTROL
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:
CSPEEDFB1
=
0.25
KO1
M
ln
NS 2
d
2
100
fVCO2
(6a)
0 5 RSPEEDFB
= 2 M ln
d
100
fVCO
NS KO1 1- M
(6b)
0 5 CSPEEDFB2 = CSPEEDFB1 M - 1
(6c)
START-UP SEQUENCE
When power is first applied to the ML4425 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 ML4425 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.
Align Mode (RESET)
Before the motor can be started, the rotor must be in a
known position. When power is first applied to the
ML4425, 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
set by a capacitor connected to the CAT pin as shown in
Figure 8. CAT is charged by a constant 750µA current from
GND to 1.5 V until the align comparator trips to end the
align mode. A starting point for CAT is calculated as
follows:
CAT
=
tS
7.5 10-7
1.5V
amp
(7)
If the align time is not long enough to allow the rotor to
settle for reliable starting, then increase CAT until the
desired performance is achieved.
10
CSPEEDFB1
RSPEEDFB
CSPEEDFB2
FB A
22
FB B
23
FB C
24
20
SPEED
FB
VDD
BACK 500nA
EMF
SAMPLER
VOLTAGE
CONTROLLED
OSCILLATOR
VCO/TACH
13
R
A
F
B
E
C
D
COMMUTATION
STATE MACHINE
PHASE
LOCKED
LOOP
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
commutation onto the motor's back EMF. The amount of
time the ML4425 stays in ramp mode is determined by a
capacitor connected to the CRT pin as shown in Figure 8.
CRT is charged by a constant 750µA current from GND to
1.5 V until the ramp comparator trips to end the ramp
mode. This gives a fixed ramp time. CRT is calculated as
follows:
CRT
=
2p
J 5 10-7 amp
IMAX Kt 3 N
KV
(8)
The rate at which the ML4425 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
from CRR to SPEED FB can be added to slow down the
ramping rate. The optimal ramp rate is based on the motor
and load parameters and is can be adjusted by varying
the value of CRR.
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