ML4425CH Micro Linear, ML4425CH Datasheet - Page 9

• Manufacturer Part Number: ML4425CH
• Description: Sensorless BLDC Motor Controller
• Manufacturer: Micro Linear

FUNCTIONAL DESCRIPTION
The maximum frequency on the VCO pin is found by:
The voltage at the VCO/TACH pin is equal to the rotor
speed. The voltage at SPEED FB is controlled by the back
EMF sampler.
BACK EMF SAMPLER
The input to the voltage controlled oscillator is the back
EMF sampler. The back EMF sense pins FB A, FB B, and
FB C inputs to the back EMF sampler require a signal
from the motor phase leads that is below the V
ML4425. The phase sense input impedance is 8k . This
requires a series resistor RES1 from the motor phase lead
as shown in Figure 6 based on the following equation:
The back EMF sampler takes the motor phase voltages
divided down to signals that are less than V
nominal) and calculates the neutral point of the motor by
the following equation:
This allows the ML4425 to compare the back EMF signal
to the motor's neutral point without the need for bringing
out an extra wire on a WYE wound motor. For DELTA
wound motors there is no physical neutral to bring out, so
this reference point must be calculated in any case.
The back EMF sampler measures the motor phase that is
not driven (i.e. if LA and HB are on, then phase A is
driven low, phase B is driven high, and phase C is
RES
Neutral
f
MAX
1 670
=
=
MOTOR A
MOTOR C
MOTOR B
0 05
=
.
PH
W
™
F/R
1
/
N RPM
+
V
™
PH
™
3
RES1
RES2
RES3
2
V
+
MOTOR
MAX
PH
3
FB A
FB B
FB C
F/R
-
10
4k
4k
Figure 6. Back EMF Sampler Detailed Block Diagram
V
4k
4k
DD
(Continued)
4k
4k
DD
(12V
of the
(3)
(4)
(5)
MULTIPLEXER
SIMULATOR
A + B + C
NEUTRAL
6
sampled). The sampled phase provides a back EMF signal
that is compared against the neutral of the motor. The
sampler is controlled by the commutation state machine.
The sampled back EMF is compared to the neutral through
an error amplifier. The output of the error amplifier outputs
a charging or discharging current to SPEED FB, which
provides the control voltage to the VCO.
GENERATOR
VCO/TACH
BACK EMF
SAMPLER
& RAMP
FROM
Figure 5. External VCO Component Connections
C VCO
4.3V
2.3V
5V
0V
(FROM C AT )
COMMUTATION
STATE MACHINE
RESET
CHANGER
SIGN
SPEED
C VCO
FB
C VCO
g m =
+
–
CONTROLLED
OSCILLATOR
8k
VOLTAGE
1
TO
SPEED FB
R VCO
ML4425
R VCO
VCO/TACH
9