LM231, LM331

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SNOSBI2C – JUNE 1999 – REVISED SEPTEMBER 2015

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

9.1 Application Information

9.1.1 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 13, which consists of the simplified block diagram of the LMx31 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.

9.1.2 Principles of Operation

The LMx31 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

LMx31 is shown in Figure 13 and consists of a switched current source, input comparator, and 1-shot timer.

Figure 13. Simplified Block Diagram of Stand-Alone

Voltage-to-Frequency Converter and

External Components

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