LM231, LM331
SNOSBI2B –JUNE 1999–REVISED MARCH 2013
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However, if the input comparator still detects pin 7 higher than pin 6 when pin 5 crosses ⅔ VCC, the flip-flop will
not be reset, and the current at pin 1 will continue to flow, trying to make the voltage at pin 6 higher than pin 7.
This condition will usually apply under start-up conditions or in the case of an overload voltage at signal input.
During this sort of overload the output frequency will be 0. As soon as the signal is restored to the working range,
the output frequency will be resumed.
The output driver transistor acts to saturate pin 3 with an ON resistance of about 50Ω. In case of over voltage,
the output current is actively limited to less than 50 mA.
The voltage at pin 2 is regulated at 1.90 VDC for all values of i between 10 μA to 500 μA. It can be used as a
voltage reference for other components, but care must be taken to ensure that current is not taken from it which
could reduce the accuracy of the converter.
Basic Voltage-to-Frequency Converter (Figure 15)
The simple stand-alone V-to-F converter shown in Figure 15 includes all the basic circuitry of Figure 14 plus a
few components for improved performance.
A resistor, RIN=100 kΩ ±10%, has been added in the path to pin 7, so that the bias current at pin 7 (−80 nA
typical) will cancel the effect of the bias current at pin 6 and help provide minimum frequency offset.
The resistance RS at pin 2 is made up of a 12 kΩ fixed resistor plus a 5 kΩ (cermet, preferably) gain adjust
rheostat. The function of this adjustment is to trim out the gain tolerance of the LM231/331, and the tolerance of
Rt, RL and Ct.
For best results, all the components should be stable low-temperature-coefficient components, such as metal-film
resistors. The capacitor should have low dielectric absorption; depending on the temperature characteristics
desired, NPO ceramic, polystyrene, Teflon or polypropylene are best suited.
A capacitor CIN is added from pin 7 to ground to act as a filter for VIN. A value of 0.01 μF to 0.1 μF will be
adequate in most cases; however, in cases where better filtering is required, a 1 μF capacitor can be used.
When the RC time constants are matched at pin 6 and pin 7, a voltage step at VIN will cause a step change in
fOUT. If CIN is much less than CL, a step at VIN may cause fOUT to stop momentarily.
A 47Ω resistor, in series with the 1 μF CL, provides hysteresis, which helps the input comparator provide the
excellent linearity.
*Use stable components with low temperature coefficients. See APPLICATIONS INFORMATION.
**0.1μF or 1μF, See PRINCIPLES OF OPERATION.
Figure 15. Simple Stand-Alone V-to-F Converter
with ±0.03% Typical Linearity (f = 10 Hz to 11 kHz)
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