U209B_07 Datasheet, PDF (7/19 Pages) ATMEL Corporation – Phase Control IC for Tacho Applications

U209B
3.5 Frequency-to-voltage Converter
The internal frequency-to-voltage converter (f/V converter) generates a DC signal on pin 9 which
is proportional to the rotational speed, using an AC signal from a tacho generator or a light beam
whose frequency is in turn dependent on the rotational speed. The high impedance input with a
switch-on threshold of typically -100 mV gives very reliable operation even when relatively sim-
ple tacho generators are employed. The tacho frequency is given by:
f
=
--ⁿ----
60
p(Hz)
n = revolution per minute
p = number of pulses per revolution
The converter is based on the charge pumping principle. With each negative half wave of the
input signal, a quantity of charge determined by C₅ is internally amplified and then integrated by
C₆ at the converter output on pin 9. The conversion constant is determined by C₅, its charging
voltage of V_ch, R₆ (pin 9) and the internally adjusted charge amplification G_i.
k = G_i × C₅ × R₆ × V_ch
The analog output voltage is given by
where:
V_o
=k× f
V_ch = 6.7 V
G_i
= 8.3
The values of C₅ and C₆ must be such that for the highest possible input frequency, the maxi-
mum output voltage V₀ does not exceed 6 V. The R_i on pin 8 is approximately 6 kΩ while C₅ is
charging up. To obtain good linearity of the f/V converter the time constant resulting from R_i and
C₅ should be considerably less (1/5) than the time span of the negative half cycle for the highest
possible input frequency. The amount of remaining ripple on the output voltage on pin 9 is
dependent on C₅, C₆ and the internal charge amplification.
∆V_O
=
G_-----i---×_------V_{----c--h-----}×_-----C_-----5-
C₆
The ripple ∆V_o can be reduced by using larger values of C₆, however, the maximum conversion
speed will then also be reduced.
The value of this capacitor should be chosen to fit the particular control loop where it is going to
be used.
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4765C–INDCO–02/07