Applications Information

PRINCIPLES OF OPERATION

The LM231/331 are monolithic circuits designed for accu-

racy 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 2 and consists of a switched current

source, input comparator, and 1-shot timer.

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 2, 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, V

x, 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 R

tCt. During

this period, the current i will flow out of the switched current

source and provide a fixed amount of charge,Q=ixt, into

the capacitor, C

L. 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

C

L 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 C

L is exactly IAVE = i x (1.1xRtCt)xf,

and the current flowing out of C

L is exactly Vx/RL . VIN/RL.

If V

IN is doubled, the frequency will double to maintain this

balance. Even a simple V-to-F converter can provide a fre-

quency precisely proportional to its input voltage over a wide

range of frequencies.

Detail of Operation, Functional Block

Diagram (Figure 1)

The block diagram shows a band gap reference which pro-

vides a stable 1.9 V

DC output. This 1.9 VDC is well regulated

over a V

S range of 3.9V to 40V. It also has a flat, low

temperature coefficient, and typically changes less than 1⁄2%

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/R

S 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 R

S flip-flop.

The timing function consists of an R

S flip-flop and a timer

comparator connected to the external R

tCt network. When

the input comparator detects a voltage at pin 7 higher than

pin 6, it sets the R

S flip-flop which turns ON the current

switch and the output driver transistor. When the voltage at

pin 5 rises to 2⁄3 V

CC, the timer comparator causes the RS

flip-flop to reset. The reset transistor is then turned ON and

the current switch is turned OFF.

However, if the input comparator still detects pin 7 higher

than pin 6 when pin 5 crosses 2⁄3 V

CC, 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 over-

load 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 50

Ω. 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 V

DC 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 3)

The simple stand-alone V-to-F converter shown in Figure 3

includes all the basic circuitry of Figure 2 plus a few compo-

nents for improved performance.

A resistor, R

IN=100 k

Ω ±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 R

S at pin 2 is made up of a 12 k

Ω fixed

resistor plusa5k

Ω (cermet, preferably) gain adjust rheostat.

The function of this adjustment is to trim out the gain toler-

ance of the LM231/331, and the tolerance of R

t,RL and Ct.

For best results, all the components should be stable low-

temperature-coefficient components, such as metal-film re-

sistors. The capacitor should have low dielectric absorption;

depending on the temperature characteristics desired, NPO

ceramic, polystyrene, Teflon or polypropylene are best

suited.

A capacitor C

IN is added from pin 7 to ground to act as a filter

for V

IN. 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 V

IN will cause

a step change in f

OUT.IfCIN is much less than CL, a step at

V

IN may cause fOUT to stop momentarily.

00568004

FIGURE 2. Simplified Block Diagram of Stand-Alone

Voltage-to-Frequency Converter and

External Components