ML4426
FUNCTIONAL DESCRIPTION (Continued)
The maximum frequency on the VCO pin is found by:
C
R
VCO
VCO
(3)
fMAX = 0.05 NRPMMAX
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.
SPEED
FB
C
R
VCO
VCO
BACK EMF SAMPLER
FROM
BACK EMF
SAMPLER
& RAMP
VCO/TACH
VOLTAGE
CONTROLLED
OSCILLATOR
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
RESET
GENERATOR
(FROM C
)
AT
from the motor phase leads that is below the V
of the
DD
ML4426. The phase sense input impedance is 8kW. This
requires a series resistor RES1 from the motor phase lead
as shown in Figure 6 based on the following equation:
4.3V
C
VCO
RES1= 670 / V V
- 10V
(4)
1
6
MOTOR
2.3V
The back EMF sampler takes the motor phase voltages
5V
0V
divided down to signals that are less than V (12V
DD
VCO/TACH
nominal) and calculates the neutral point of the motor by
the following equation:
Figure 5. External VCO Component Connections
PH1+PH2+ PH3
Neutral =
(5)
3
This allows the ML4426 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.
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.
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
FB A
RES1
MOTOR ΦA
NEUTRAL
SIMULATOR
FB B
FB C
RES2
RES3
MOTOR ΦB
MOTOR ΦC
1
8kΩ
g
=
m
ΦA + ΦB + ΦC
6
+
–
TO
SPEED FB
SIGN
CHANGER
4kΩ
4kΩ
4kΩ
4kΩ
4kΩ
MULTIPLEXER
4kΩ
COMMUTATION
STATE MACHINE
Figure 6. Back EMF Sampler Detailed Block Diagram
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
