Datasheet

 
SLUS318B – APRIL 1999 – REVISED JANUARY 2002
10
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APPLICATION INFORMATION
programming the tachometer
The UCC3626 tachometer consists of a precision 5-V monostable, triggered by either a rising or falling edge
on any of the three Hall inputs, HALLA, HALLB, and HALLC. The resulting TACH_OUT waveform is a variable
duty-cycle square wave whose frequency is proportional to motor speed, as given by:
TACH_OUT +
V P
20
Hz
where P is the number of motor pole pairs and V is motor velocity in RPM.
The on time of the monostable is programmed via timing resistor R_TACH and capacitor C_TACH according
to the following equation:
t
ON
+ R_TACH C_TACH sec
Figure 5 provides a graph of on times for various combinations of R_TACH and C_TACH. On time is typically
set to a value less than the minimum TACH_OUT period as given by:
t
PERIOD (min)
+
20
V
MAX
P
sec
where P is the number of motor pole pairs and V is motor velocity in RPM.
SYNCH
CT
WITHOUT SYNCH
WITH SYNCH
Figure 4. Oscillator Waveforms
Figure 5
1.E–06
1.E–05
1.E–04
1.E–03
1.E–02
1.E–01
1.E+00
1.E–10 1.E–09 1.E–08 1.E–07 1.E–06
R_TACH = 250 kΩ
R_TACH = 25 kΩ
R_TACH = 100 kΩ
R_TACH = 500 kΩ
t
ON
– Tachometer On–Time – s
C_TACH – Tachometer Timing Capacitance – F
TACHOMETER ON-TIME
vs
TIMING CAPACITANCE
(3)
(4)
(5)