UCC2626, UCC3626

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

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

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 k

Ω

R_TACH = 25 k

Ω

R_TACH = 100 k

Ω

R_TACH = 500 k

Ω

C_TACH – Tachometer Timing Capacitance – F

TACHOMETER ON-TIME

vs

TIMING CAPACITANCE

(3)

(4)

(5)