Typical Applications
(Continued)
PRINCIPLES OF OPERATION OF A SIMPLIFIED
VOLTAGE-TO-FREQUENCY CONVERTER
The LM131 is a monolithic circuit designed for accuracy and
versatile operation when applied as a voltage-to-frequency
(V-to-F) converter or as a frequency-to-voltage (F-to-V) con-
verter A simplified block diagram of the LM131 is shown in
Figure 2
and consists of a switched current source input
comparator and 1-shot timer
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 LM131 and the various resistors and capacitors con-
nected 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
e
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
e
i
c
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 capaci-
tor 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
e
i
c
(1 1
c
R
t
C
t
)
c
f and the current flowing out of C
L
is exactly V
x
R
L
j
V
IN
R
L
If V
IN
is doubled the frequency will double to main-
tain this balance Even a simple V-to-F converter can pro-
vide a frequency precisely proportional to its input voltage
over a wide range of frequencies
DETAIL OF OPERATION FUNCTIONAL BLOCK
DIAGRAM (FIGURE
1a
)
The block diagram shows a band gap reference which pro-
vides 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 tem-
perature coefficient and typically changes less than %
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
e
1 90V R
S
to flow For
R
s
e
14k i
e
135
mA
The precision current reflector pro-
vides a current equal to i to the current switch The current
switch switches the current to pin 1 or to ground depending
on 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
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
However if the input comparator still detects pin 7 higher
than pin 6 when pin 5 crosses
V
CC
the flip-flop will not
be reset and the current at pin 1 will continue to flow in its
attempt 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 It should be
noted that 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 50X In case of overvoltage 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
mA
to 500
mA
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
PRINCIPLES OF OPERATION OF BASIC VOLTAGE-
TO-FREQUENCY CONVERTER (FIGURE
1
)
The simple stand-alone V-to-F converter shown in
Figure 1
includes all the basic circuitry of
Figure 2
plus a few compo-
nents for improved performance
A resistor R
IN
e
100 kX
g
10% has been added in the path
to pin 7 so that the bias current at pin 7 (
b
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 kX fixed
resistor plus a 5 kX (cermet preferably) gain adjust rheo-
stat The function of this adjustment is to trim out the gain
tolerance of the LM131 and the tolerance of R
t
R
L
and C
t
TL H 5680–4
FIGURE 2 Simplified Block Diagram of Stand-Alone
Voltage-to-Frequency Converter Showing LM131 and
External Components
6
NSC [ NATIONAL SEMICONDUCTOR ]
