PRELIMINARY
ML4435
FUNCTIONAL DESCRIPTION
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.
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.
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.
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.
SPEED FB
SPEED
ERROR AMP
SPEED
–
SET
+
5
3
–
+
PWM SPEED CONTROL
LEVEL SHIFT
0. V
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.
.2 - 0. V
SPEED
COMP
PWM
CSE
COMPARATOR
2.2 + 0. V
+
V
–
TRIAN LE
WAVE
COMPARATOR
3V
Figure 11. PWM Oscillator Circuit
000
The speed loop compensation components are calculated
as follows:
1.44 ×NxKexVMOTOR ×RVCO × CVCO
CSC 2 =
2× π × J×RI× freq2
00
10
Rsc =
2× π × freq × Csc2
Csc1 = 10 x Csc2
0
0
00
PWM FRE UENCY kHz
Where freq is the speed loop bandwidth in Hz.
Figure 12. RT vs PWM Frequency
May, 2000
PRELIMINARY DATASHEET
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
