Datasheet

Circuit description L6229Q
18/28 Doc ID 15209 Rev 3
5.6 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 17. 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 16). The off time t
PULSE
can be
set using the external RC network (R
PUL
, C
PUL
) connected to the pin RCPULSE. Figure 18
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 1 nF … 100 nF and R
PUL
in the range
20 kΩ … 100 kΩ.
By connecting the tachometer pin to an external pull-up resistor, the output signal average
value V
M
is proportional to the frequency of the hall effect signal and, therefore, to the motor
speed. This realizes a simple frequency-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 16. Tacho operation waveforms
V
M
t
PULSE
T
------------------
V
DD
⋅=
T
t
PULSE
H
1
V
TACHO
H
2
H
3
V
M
V
DD