LM2907, LM2917
FREQUENCY-TO-VOLTAGE CONVERTERS
SLFS011A – MARCH 1986 – REVISED JULY 1993
APPLICATION INFORMATION
The LM2907 and LM2917 frequency-to-voltage converter circuits provide maximum versatility with a minimum
of external parts. The first stage of each device is a differential comparator. The single-input 8-pin versions have
one input grounded so that an input signal must swing above and below GND and exceed the input thresholds
to produce an output. This version is specifically for magnetic variable-reluctance pickups, which typically
provide a single-ended ac output. These single-ended inputs are fully protected against voltage swings to
±28 V, which are easily attained by this type of pickup.
The differential-input 14-pin versions provide the option of setting the input reference level, maintaining
hysteresis around that level to provide excellent noise rejection in any application. The input protection is
removedinthe14-pinversions. Therefore, neitherofthedifferentialinputsshouldexceedthelimitsofthesupply
voltage. An input must not go below GND without a resistance in the lead to limit the current that flows in the
episubstrate diode. The charge-pump circuit that follows the input state produces a dc output voltage
proportional to the input frequency. The charge-pump circuit (see Figures 1 and 2) consists of a timing capacitor
(C1), an output resistor (R1), and an integrating or filter capacitor (C2). When the input changes state (due to
a suitable zero crossing or differential voltage on the input), the timing capacitor is either charged or discharged
linearlywithaconstantcurrentof200µAthroughCAP1betweentwovoltageswhosedifferenceisV /2.Within
CC
one-half cycle of the input frequency or a time equal to 1/2f, the change in charge on C1 is equal to (V /2)C1.
CC
The average amount of current pumped into or out of the capacitor is:
V
Q
T
CC
CAP1 current (average)
C1 •
• 2f
V
• f • C1
CC
2
The output of the charge pump accurately mirrors the CAP1 current into the load resistor (R1) connected to
CPO. If the pulses of current are integrated with a filter capacitor, the output voltage is the average CAP1 current
times R1 and the total equation becomes:
V
V
• f • C1 • R1 • K
CC
O
where K is the gain factor, which is typically one.
The size of C2 is dependent only on the amount of ripple allowable and the required response time.
selection of R1, C1, and C2
To achieve optimum performance, there are some limitations to be considered in the selection of R1 and C1.
The timing capacitor controls the RC time and provides internal compensation for the charge-pump circuit. For
very accurate operation, it should be 100 pF or greater. Smaller values, especially at lower temperatures, can
cause an error current through R1. V /R1 must be less than or equal to the output current at CPO, which is fixed
O
typically at 200 µA. If R1 is too large, it becomes a significant fraction of the output impedance at CPO, which
degrades the linearity. In addition, ripple voltage must be considered when selecting R1. The size of C2 is
directly affected by the size of R1. An expression that describes the ripple content at CPO is:
V
CC
2
C1
C2
C1
200
•
•
•
•
volts peak-to-peak
)
V
(1
V
f
ripple
CC
where:
C1 and C2 are in farads,
is in volts, and
V
CC
f is in hertz.
5
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