Specifications

tor, R3 (Figure 2) which is located in a floating isolation
pocket, and secondly by clamp diode D1. Since the voltage
swing on the base of Q1 is thus restricted, the only restric-
tion on the allowable voltage on pin 1 is the breakdown
voltage of the 10 kX resistor. This allows input swings to
g
28V. In 14-pin versions the link to D1 is opened in order to
allow the base of Q1 to be biased at some higher voltage.
Q5 clamps the negative swing on the base of Q1 to about
300 mV. This prevents substrate injection in the region of
Q1 which might otherwise cause false switching or errone-
ous discharge of one of the timing capacitors.
The differential input options (LM2907-14, LM2917-14), give
the user the option of setting his own input switching level
and still having the hysteresis around that level for excellent
noise rejection in any application.
HOW TO USE IT
Basic f to V Converter
The operation of the LM2907, LM2917 series is best under-
stood by observing the basic converter shown in
Figure 3. In
this configuration, a frequency signal is applied to the input
of the charge pump at pin 1. The voltage appearing at pin 2
will swing between two values which are approximately 1/4
(V
CC
)
b
V
BE
and 3/4 (V
CC
)
b
V
BE
. The voltage at pin 3 will
have a value equal to V
CC
#
f
IN
#
C1
#
R1
#
K, where K is
the gain constant (normally 1.0).
The emitter output (pin 4) is connected to the inverting input
of the op amp so that pin 4 will follow pin 3 and provide a
low impedance output voltage proportional to input frequen-
cy. The linearity of this voltage is typically better than 0.3%
of full scale.
Choosing R1, C1 and C2
There are some limitations on the choice of R1, C1 and C2
(Figure 3) which should be considered for optimum perform-
ance. C1 also provides internal compensation for the
charge pump and should be kept larger than 100 pF. Small-
er values can cause an error current on R1, especially at
low temperatures. Three considerations must be met when
choosing R1.
First, the output current at pin 3 is internally fixed and there-
fore V3 max, divided by R1, must be less than or equal to
this value.
.
.
. R1
t
V3 max
I
3MIN
where V3 max is the full scale output voltage required
1
3MIN
is determined from the data sheet (150 mA)
Second, if R1 is too large, it can become a significant frac-
tion of the output impedance at pin 3 which degrades linear-
ity. Finally, ripple voltage must be considered, and the size
of C2 is affected by R1. An expression that describes the
ripple content on pin 3 for a single R1, C2 combination is:
V
RIPPLE
e
V
CC
2
#
C1
C2
#
1
b
V
CC
#
f
IN
#
C1
I
2
J
p-p
It appears R1 can be chosen independent of ripple, howev-
er response time, or the time it takes V
OUT
to stabilize at a
new frequency increases as the size of C2 increases, so a
compromise between ripple, response time, and linearity
must be cosen carefully. R1 should be selected according
to the following relationship:
C is selected according to:
C1
e
V3 Full Scale
R1
#
V
CC
#
f
FULL SCALE
Next decide on the maximum ripple which can be accepted
and plug into the following equation to determine C2:
C2
e
V
CC
2
#
C1
V
RIPPLE
#
1
b
V
3
R
1
I
2
J
The kind of capacitor used for timing capacitor C1 will deter-
mine the accuracy of the unit over the temperature range.
Figure 15 illustrates the tachometer output as a function of
temperature for the two devices. Note that the LM2907 op-
erating from a fixed external supply has a negative tempera-
ture coefficient which enables the device to be used with
capacitors which have a positive temperature coefficient
and thus obtain overall stabililty. In the case of the LM2917
the internal zener supply voltage has a positive coefficient
which causes the overall tachometer output to have a very
low temperature coefficient and requires that the capacitor
temperature coefficient be balanced by the temperature co-
efficient of R1.
Using Zener Regulated Options (LM2917)
For those applications where an output voltage or current
must be obtained independently of the supply voltage varia-
tions, the LM2917 is offered. The reference typically has an
11X source resistance. In choosing a dropping resistor from
the unregulated supply to the device note that the tachome-
ter and op amp circuitry alone require about 3 mA at the
voltage level provided by the zener. At low supply voltages,
TL/H/7451–6
FIGURE 3. Basic f to V Converter
4