ML4435 Ver la hoja de datos (PDF) - Micro Linear Corporation
Número de pieza
componentes Descripción
Fabricante
ML4435 Datasheet PDF : 14 Pages
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PRELIMINARY
ML4435
FUNCTIONAL DESCRIPTION
MOTOR START-UP
When power is first applied to the ML4435, the motor is at
rest and thus, 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 ML4435
uses a minimum VCO frequency to begin commutating the
motor. This low frequency commutation is set by the 0.2V
clamp on RVCO, this provides a commutation frequency
at 1/30th of the maximum frequency.
RUN MODE
After the Back EMF sensing PLL has locked on to the
motor’s position, the motor is running in closed loop con-
trol. At this point, the speed control loop should force the
motor speed to the speed that corresponds to the SPEED
SET voltage.
PWM SPEED CONTROL
Speed control is accomplished by setting a speed com-
mand at SPEED SET (pin 5) with an input voltage from 0.2
to 6V. The accuracy of the speed command is determined
by the external components RVCO and CVCO. There are
a number of methods to control the speed command on
the ML4435. One method is to use a potentiometer from
RT to ground with the wiper going to SPEED SET. If SPEED
SET is controlled from a microcontroller, a DAC that uses
RT as its input reference can be used. The RT voltage must
be buffered connecting it to external circuits. The speed
command is compared with the sensed speed from SPEED
FB minus 0.7V (pin 18) through a transconductance error
amplifier. The output of the speed error amplifier is SPEED
COMP (pin 3). SPEED COMP is clamped between 8.2V
and 2.2V. A signal of 8.2V corresponds to full PWM duty
cycle and 2.2V corresponds to 0% duty cycle. Speed
loop compensation components are placed on this pin as
shown in Figure 11.
The speed loop compensation components are calculated
as follows:
CSC 2
=
1.44
×
NxKexVMOTOR × RVCO
2 × π × J × RI × freq2
×
C VCO
Rsc
=
2×π
×
10
freq
×
Csc2
The voltage on SPEED COMP (pin 3) is compared with a
triangle wave oscillator to create a PWM duty cycle. The
PWM oscillator creates a triangle wave function from 3V
to 7V as shown in Figure 11. The frequency of the triangle
wave oscillator is set by a resistor to ground on RT (pin 6).
RT can be selected from the graph in Figure 12.
The PWM duty cycle from the speed control loop is gated
the pulse-by-pulse current limit that controls the LA, LB,
and LC output drivers.
SPEED
SPEED
SET
ERROR AMP
–
5
–
+
+
LEVEL SHIFT
0. V
.2 - 0. V
SPEED
COMP
3
CSE
2.2 + 0. V
V
TRIAN LE
WAVE 3V
COMPARATOR
SPEED FB
PWM
COMPARATOR
+
–
Figure 11. PWM Oscillator Circuit
000
00
Csc1 = 10 x Csc2
Where freq is the speed loop bandwidth in Hz.
0
0
00
PWM FRE UENCY kHz
Figure 12. RT vs PWM Frequency
May, 2000 PRELIMINARY DATASHEET
9
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