LM231, LM331
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SNOSBI2B –JUNE 1999–REVISED MARCH 2013
APPLICATIONS INFORMATION
PRINCIPLES OF OPERATION
The LM231/331 are monolithic circuits designed for accuracy and versatile operation when applied as voltage-to-
frequency (V-to-F) converters or as frequency-to-voltage (F-to-V) converters. A simplified block diagram of the
LM231/331 is shown in Figure 14 and consists of a switched current source, input comparator, and 1-shot timer.
Figure 14. Simplified Block Diagram of Stand-Alone
Voltage-to-Frequency Converter and
External Components
Simplified Voltage-to-Frequency Converter
The operation of these blocks is best understood by going through the operating cycle of the basic V-to-F
converter, Figure 14, which consists of the simplified block diagram of the LM231/331 and the various resistors
and capacitors connected to it.
The voltage comparator compares a positive input voltage, V1, at pin 7 to the voltage, Vx, at pin 6. If V1 is
greater, the comparator will trigger the 1-shot timer. The output of the timer will turn ON both the frequency
output transistor and the switched current source for a period t=1.1 RtCt. During this period, the current i will flow
out of the switched current source and provide a fixed amount of charge, Q = i × t, into the capacitor, CL. This will
normally charge Vx up to a higher level than V1. At the end of the timing period, the current i will turn OFF, and
the timer will reset itself.
Now there is no current flowing from pin 1, and the capacitor CL will be gradually discharged by RL until Vx falls
to the level of V1. Then the comparator will trigger the timer and start another cycle.
The current flowing into CL is exactly IAVE = i × (1.1×RtCt) × f, and the current flowing out of CL is exactly Vx/RL ≃
VIN/RL. If VIN is doubled, the frequency will double to maintain this balance. Even a simple V-to-F converter can
provide a frequency precisely proportional to its input voltage over a wide range of frequencies.
Detail of Operation, Functional Block Diagram
The block diagram (FUNCTIONAL BLOCK DIAGRAM) shows a band gap reference which provides a stable 1.9
VDC output. This 1.9 VDC is well regulated over a VS range of 3.9V to 40V. It also has a flat, low temperature
coefficient, and typically changes less than ½% over a 100°C temperature change.
The current pump circuit forces the voltage at pin 2 to be at 1.9V, and causes a current i=1.90V/RS to flow. For
Rs=14k, i=135 μA. The precision current reflector provides a current equal to i to the current switch. The current
switch switches the current to pin 1 or to ground, depending upon the state of the RS flip-flop.
The timing function consists of an RS flip-flop and a timer comparator connected to the external RtCt network.
When the input comparator detects a voltage at pin 7 higher than pin 6, it sets the RS flip-flop which turns ON the
current switch and the output driver transistor. When the voltage at pin 5 rises to ⅔ VCC, the timer comparator
causes the RS flip-flop to reset. The reset transistor is then turned ON and the current switch is turned OFF.
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