AN1625 STMicroelectronics, AN1625 Datasheet - Page 26

• Manufacturer Part Number: AN1625
• Description: L6235 THREE PHASE BRUSHLESS DC MOTOR DRIVER
• Manufacturer: STMicroelectronics

AN1625 APPLICATION NOTE
Also the duty cycle of this signal, so its average value is proportional to the motor rotation speed. Simply inte-
grating the square-wave a voltage proportional to the motor speed will be available to realize a speed loop, as
in Figure 28: R
to a voltage proportional to the desired speed (V
speed error signal. Providing this signal to the Vref input of the L6235, which sets the current in the motor wind-
ings, the speed error will act on the motor modifying its torque, in order to maintain the speed at a constant value.
R1 and R2 set the maximum current in the motor by limiting the voltage at the V
Figure 28. Tacho output allows easy implementation of a speed loop.
Defining
K
D
J
T
n
f
G
G
G
and neglecting the current ripple due to PWM control, the expression of the control loop transfer function (see
Figure 29) is:
26/39
tacho
M
I
R
T
m
op-amp
w-v
1
= C
=R
= J/D
=V
= n
fb
2
/ R
· R
pullup
= R
SENSE ·
m
fb2
fb2
/ 2
· n · t
PULSE
/R
fb1
(R
pulse
1
and C
+R
/ 2
t
PULSE
2
)
C
Motor torque constant
Dynamic friction torque
Motor + Load inertia moment
Load resistant torque
Mechanical time constant
Number of polar couples
Motor rotation speed
Tacho output frequency
Integrator time constant
PULSE
PULSE
define the fixed on-time (t
RC
H
H
H
1
2
3
PULSE
R
V
PULSE
TACHO
TAC HO
L6235
OUT
V
speed
BLDC Motor
1
SENSE
OUT
V
2
speed
OUT
pullup
A
R
R
SENSE
SENSE
3
pullup
V
R
) by the op-amp; the output of the op-amp represents the
ref
fb1
B
PULSE
[Nm/A]
[Nms/rad]
[kgm2]
[Nm]
[s]
[rad/s]
[Hz]
[s]
[Vs/rad]
[1/ ]
R
C
fb
fb2
) of the tacho output, integrated and compared
V
OP-AM P
V
REF
R
1
R
2
ref
pin.