LM2917 National Semiconductor, LM2917 Datasheet - Page 5

• Manufacturer Part Number: LM2917
• Description: LM2907/LM2917 Frequency to Voltage Converter
• Manufacturer: National Semiconductor

Applications Information
The LM2907 series of tachometer circuits is designed for
minimum external part count applications and maximum ver-
satility In order to fully exploit its features and advantages
let’s examine its theory of operation The first stage of oper-
ation is a differential amplifier driving a positive feedback
flip-flop circuit The input threshold voltage is the amount of
differential input voltage at which the output of this stage
changes state Two options (LM2907-8 LM2917-8) have
one input internally grounded so that an input signal must
swing above and below ground and exceed the input
thresholds to produce an output This is offered specifically
for magnetic variable reluctance pickups which typically pro-
vide a single-ended ac output This single input is also fully
protected against voltage swings to
attained with these types of pickups
The differential input options (LM2907 LM2917) give the
user the option of setting his own input switching level and
still have the hysteresis around that level for excellent noise
rejection in any application Of course in order to allow the
inputs to attain common-mode voltages above ground input
protection is removed and neither input should be taken
outside the limits of the supply voltage being used It is very
important that an input not go below ground without some
resistance in its lead to limit the current that will then flow in
the epi-substrate diode
Following the input stage is the charge pump where the
input frequency is converted to a dc voltage To do this
requires one timing capacitor one output resistor and an
integrating or filter capacitor When the input stage changes
state (due to a suitable zero crossing or differential voltage
on the input) the timing capacitor is either charged or dis-
charged linearly between two voltages whose difference is
V
time equal to 1 2 f
capacitor is equal to V
current pumped into or out of the capacitor then is
The output circuit mirrors this current very accurately into
the load resistor R1 connected to ground such that if the
pulses of current are integrated with a filter capacitor then
V
Where K is the gain constant typically 1 0
Typical Applications
CC
O
T
Q
e
2 Then in one half cycle of the input frequency or a
e
i
c c
i
c(AVG)
R1 and the total conversion equation becomes
V
O
e
e
C1
V
IN
CC c
c
the change in charge on the timing
CC
V
2
CC
2
f
IN c
c
c
C1 The average amount of
(2f
C1
IN
g
c
)
28V which are easily
e
R1
V
c
CC c
K
Minimum Component Tachometer
f
IN c
C1
5
The size of C2 is dependent only on the amount of ripple
voltage allowable and the required response time
CHOOSING R1 AND C1
There are some limitations on the choice of R1 and C1
which should be considered for optimum performance The
timing capacitor also provides internal compensation for the
charge pump and should be kept larger than 500 pF for very
accurate operation Smaller values can cause an error cur-
rent on R1 especially at low temperatures Several consid-
erations must be met when choosing R1 The output current
at pin 3 is internally fixed and therefore V
than or equal to this value If R1 is too large it can become
a significant fraction of the output impedance at pin 3 which
degrades linearity Also output ripple voltage must be con-
sidered and the size of C2 is affected by R1 An expression
that describes the ripple content on pin 3 for a single R1C2
combination is
It appears R1 can be chosen independent of ripple howev-
er response time or the time it takes V
new voltage increases as the size of C2 increases so a
compromise between ripple response time and linearity
must be chosen carefully
As a final consideration the maximum attainable input fre-
quency is determined by V
USING ZENER REGULATED OPTIONS (LM2917)
For those applications where an output voltage or current
must be obtained independent of supply voltage variations
the LM2917 is offered The most important consideration in
choosing a dropping resistor from the unregulated supply to
the device is that the tachometer and op amp circuitry alone
require about 3 mA at the voltage level provided by the
zener At low supply voltages there must be some current
flowing in the resistor above the 3 mA circuit current to op-
erate the regulator As an example if the raw supply varies
from 9V to 16V a resistance of 470
ner voltage variation to 160 mV If the resistance goes un-
der 400
above 200 mV for the same input variation
V
RIPPLE
e
or over 600
V
2
CC
c
f
C1
C2
MAX
c
e
the zener variation quickly rises
CC
C1
1
b
C1 and I
c
I
V
2
CC c
V
CC
will minimize the ze-
OUT
2
I
O
f
2
IN c
R1 must be less
to stabilize at a
C1
TL H 7942– 8
pk-pk