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
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,
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
t
C
t
. 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, C
L
. This will
normally charge V
x
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 R
L
until V
x
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 I
AVE
= i × (1.1×R
t
C
t
) × f, and the current flowing out of C
L
is exactly V
x
/R
L
≃
V
IN
/R
L
. If V
IN
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
shows a band gap reference which provides a stable 1.9
V
DC
output. This 1.9 V
DC
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 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/R
S
to flow. For
R
s
=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
t
C
t
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
⅔
V
CC
, the timer comparator
causes the R
S
flip-flop to reset. The reset transistor is then turned ON and the current switch is turned OFF.
Copyright © 1999–2013, Texas Instruments Incorporated
Product Folder Links:
7
TI [ TEXAS INSTRUMENTS ]
