ML4425 PRODUCT SPECIFICATION
6 REV. 1.0.2 7/2/01
Functional Description
General
The ML4425 provides all the circuitry for sensorless speed
control of 3-phase Brushless DC (BLDC) motors. Controller
functions include start-up circuitry, back-EMF commutation
control, Pulse Width Modulation (PWM) speed control,
fixed OFF-time current limiting, braking, and undervoltage
protection.
The start-up circuitry aligns the motor to a known position,
then ramps up the motor speed to generate a back-EMF
signal. A back-EMF sampling circuit controls commutation
timing by forming a Phase Locked Loop (PLL). The commu-
tation control circuitry also outputs a speed feedback
(SPEED FB) signal used in the speed control loop. The
speed control loop consists of an error amplifier and PWM
comparator that produce a PWM duty cycle for speed regula-
tion. Motor current is limited by a fixed OFF-time PWM
shutdown comparator that is controlled by an external sense
resistor. Commutation control, PWM speed control, and
current limiting are combined to produce the output driver
signals. Six output drivers are used to provide gating signals
to an external 3 phase bridge power stage sized for the
BLDC motor voltage and current requirements. Additional
functions include a braking function and undervoltage
protection circuit to shut down the output drivers in the event
of a low voltage condition on V
DD
of the ML4425.
Component Selection
Selecting external components for the ML4425 requires
calculations based on the motor’s electrical and mechanical
parameters. The following is a list of the motor parameters
needed for these calculations :
• DC motor supply voltage – V
MOTOR
(V)
• Maximum operating current – I
MAX
(A)
• Number of magnetic poles – N
• Back EMF constant – K
e
(V-s/Rad)
• Motor torque constant – K
t
(Nm/A) (K
t
= K
e
in SI units)
• Maximum speed of operation RPM
MAX
(RPM)
• Moment of inertia of the motor and load – J (Kg-m
2
)
• Viscous damping factor of the motor and load – ζ
If one or more of the above values is not known, it is still
possible to pick components for the ML4425, but some
experimentation may be necessary to determine the optimal
values. All quantities are in SI units unless otherwise speci-
fied. The following formulas should be considered as a start-
ing point for optimization. All calculations for capacitors and
resistors should be used as the first approximation for select-
ing the closest standard value.
Power Supply and Reference
The supply voltage (V
DD
) is nominally 12V ±10%. A 100nF
bypass capacitor to ground should be placed as close as pos-
sible to V
DD
. A 6.9V voltage reference output (V
REF
) is pro-
vided to set the speed command and current limit of the
ML4425. A 137kΩ from R
REF
to GND is required to set up a
reference current for internal functions.
Output Drivers
The output drivers LA, LB, LC, HA, HB, and HC provide
totem pole output drive signals for a 3 phase bridge power
stage. All control functions in the ML4425 translate to out-
puts at these pins. LA, LB, and LC provide the low-side
drive signals for phases A, B, and C of the 3 phase power
stage and are 12V active high signals. HA, HB, and HC
provide the high-side signals and are 12V active low signals.
Figure 1. Using R
SENSE
in a 3-Phase 12V Power Stage
MOTOR
PHASE C
MOTOR
PHASE B
MOTOR
PHASE A
V
MOTOR
12V
R
SENSE
DC SUPPLY
CAPACITOR
HA
LA
HB
LB
HC
LC
