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LM331N Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM331N
Description  Precision Voltage-to-Frequency Converters
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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 6 page
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Typical Performance Characteristics (Continued)
Typical Applications
PRINCIPLES OF OPERATION OF A SIMPLIFIED
VOLTAGE-TO-FREQUENCY CONVERTER
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 3 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 3, which consists of the simplified block diagram of
the LM231/331 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=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=i x t, 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 an-
other cycle.
The current flowing into C
L is exactly IAVE = i x (1.1xRtCt)x
f, 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 2)
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 tem-
perature coefficient, and typically changes less than 12%
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 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
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 23 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 23 V
CC, the flip-flop will not be
reset, and the current at pin 1 will continue to flow, in its at-
tempt to make the voltage at pin 6 higher than pin 7. This
Power Drain vs V
SUPPLY
DS005680-34
Output Saturation Voltage vs
I
OUT (Pin 3)
DS005680-35
Nonlinearity Error, Precision
F-to-V Converter (
Figure 7)
DS005680-36
DS005680-4
FIGURE 3. Simplified Block Diagram of Stand-Alone
Voltage-to-Frequency Converter and
External Components
www.national.com
6




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