U211B Datasheet, PDF (6/30 Pages) ATMEL Corporation – Phase Control IC with Overload Limitation for Tacho Applications

7. Frequency-to-voltage Converter
The internal frequency-to-voltage converter (f/V converter) generates a DC signal on pin 10
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
pin 8 compares the tacho voltage to a switch-on threshold of typically -100 mV. The switch-off
threshold is -50 mV. The hysteresis guarantees very reliable operation even when relatively sim-
ple tacho generators are used.
The tacho frequency is given by:
f
=
--ⁿ----
60
×
p (Hz)
where:
n
= Revolutions 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 10. The conversion constant is determined by C₅, its charge
transfer voltage of V_ch, R₆ (pin 10) and the internally adjusted charge transfer gain.
G_i
I_--1---0-
I₉
= 8.3
k = G_i × C₅ × R₆ × V_ch
The analog output voltage is given by
V_O = k × f
The values of C₅ and C₆ must be such that for the highest possible input frequency, the maxi-
mum output voltage V_O does not exceed 6 V. While C₅ is charging up, the R_i on pin 9 is
approximately 6.7 kΩ. 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 10 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 increasing 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.
6 U211B
4752B–INDCO–09/05