Datasheet

L6229
14/25
Figure 16. 60° Hall Sensor Sequence.
3.6 TACHO
A tachometer function consists of a monostable, with constant off time (t
PULSE
), whose input is one Hall Effect
signal (H
1
). It allows developing an easy speed control loop by using an external op amp, as shown in Figure
18. For component values refer to Application Information section.
The monostable output drives an open drain output pin (TACHO). At each rising edge of the Hall Effect Sensors
H
1
, the monostable is triggered and the MOSFET connected to pin TACHO is turned off for a constant time
t
PULSE
(see Figure 17). The off time t
PULSE
can be set using the external RC network (R
PUL
, C
PUL
) connected
to the pin RCPULSE. Figure 19 gives the relation between t
PULSE
and C
PUL
, R
PUL
. We have approximately:
t
PULSE
= 0.6 · R
PUL
· C
PUL
where C
PUL
should be chosen in the range 1nF … 100nF and R
PUL
in the range 20K Ω … 100K Ω .
By connecting the tachometer pin to an external pull-up resistor, the output signal average value V
M
is propor-
tional to the frequency of the Hall Effect signal and, therefore, to the motor speed. This realizes a simple Fre-
quency-to-Voltage Converter. An op amp, configured as an integrator, filters the signal and compares it with a
reference voltage V
REF
, which sets the speed of the motor.
Figure 17. Tacho Operation Waveforms.
H
1
H
1
H
2
H
2
H
2
H
2
H
2
H
3
H
3
H
3
H
3
H
3
H
1
H
1
H
1
H
1
H
3
H
2
1 2 3b 4 5 6b
= H
= L
V
M
t
PULSE
T
--------- ---------
V
DD
⋅=
T
t
PULSE
H
1
V
TACHO
H
2
H
3
V
M
V
DD