LM231AN National Semiconductor, LM231AN Datasheet - Page 7

• Manufacturer Part Number: LM231AN
• Description: Voltage / Frequency (V/F & F/V) Converter IC
• Manufacturer: National Semiconductor

Specifications of LM231AN

• Frequency: 100kHz
• No. Of Pins: 8
• Mounting Type: Through Hole
• Converter Type: Voltage/Frequency
• Operating Temperature Max: 85°C
• Peak Reflow Compatible (260 C): No
• Converter Function: VFC
• Full Scale Frequency: 100
• Power Supply Requirement: Single
• Single Supply Voltage (typ): 5/9/12/15/18/24/28V
• Single Supply Voltage (max): 40V
• Single Supply Voltage (min): 4V
• Dual Supply Voltage (typ): Not RequiredV
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temperature (min): -25C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Commercial
• Package Type: MDIP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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
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
this period, the current i will flow out of the switched current
source and provide a fixed amount of charge, Q = i x t, into
the capacitor, C
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
of V1. Then the comparator will trigger the timer and start
another cycle.
The current flowing into C
and the current flowing out of C
If V
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.
L
FIGURE 2. Simplified Block Diagram of Stand-Alone
will be gradually discharged by R
IN
is doubled, the frequency will double to maintain this
Voltage-to-Frequency Converter and
L
. This will normally charge V
External Components
L
is exactly I
x
, at pin 6. If V1 is greater, the
L
is exactly V
L
until V
AVE
= i x (1.1xR
x
x
falls to the level
x
up to a higher
/R
L
t
C
. V
t
. During
t
C
00568004
IN
t
) x f,
/R
L
.
7
Detail of Operation, Functional Block
Diagram (Figure 1)
The block diagram shows a band gap reference which pro-
vides a stable 1.9 V
over a V
temperature 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=1.90V/R
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
The timing function consists of an R
comparator connected to the external R
the input comparator detects a voltage at pin 7 higher than
pin 6, it sets the R
switch and the output driver transistor. When the voltage at
pin 5 rises to
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
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 50Ω. 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
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
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
resistor plus a 5 kΩ (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
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
for V
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
a step change in f
V
IN
may cause f
IN
S
. A value of 0.01 µF to 0.1 µF will be adequate in most
flip-flop.
S
range of 3.9V to 40V. It also has a flat, low
IN
IN
=100 kΩ
2
⁄
3
is added from pin 7 to ground to act as a filter
OUT
V
S
OUT
CC
DC
at pin 2 is made up of a 12 kΩ fixed
S
to stop momentarily.
, the timer comparator causes the R
. If C
flip-flop which turns ON the current
output. This 1.9 V
±
10%, has been added in the path to
IN
is much less than C
2
⁄
3
V
CC
, the flip-flop will not be
S
S
flip-flop and a timer
to flow. For R
DC
t
C
DC
t
is well regulated
network. When
for all values of
t
IN
, R
www.national.com
L
, a step at
will cause
L
and C
s
=14k,
1
⁄
2
%
S
t
.