ML4426
FUNCTIONAL DESCRIPTION
GENERAL
The ML4426 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 commutation 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 regulation. 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 ML4426.
If one or more of the above values is not known, it is still
possible to pick components for the ML4426, but some
experimentation may be necessary to determine the
optimal values. All quantities are in SI units unless
otherwise specified. The following formulas should be
considered as a starting point for optimization. All
calculations for capacitors and resistors should be used as
the first approximation for selecting 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 possible to V
DD
. A 6.9V voltage reference output
(V
REF
) is provided to set the speed command and current
limit of the ML4426. A 137kW 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 ML4426 translate to
outputs 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.
COMPONENT SELECTION
Selecting external components for the ML4426 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 –
z
VMOTOR
12V
DC SUPPLY
CAPACITOR
HA
HB
MOTOR
PHASE A
HC
MOTOR
PHASE B
LC
MOTOR
PHASE C
LA
LB
RSENSE
Figure 1. Using R
SENSE
in a 3-Phase 12V Power Stage
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
