LM2907/LM2917 Frequency to Voltage Converter
May 2003
LM2907/LM2917
Frequency to Voltage Converter
General Description
The LM2907, LM2917 series are monolithic frequency to
voltage converters with a high gain op amp/comparator de-
signed to operate a relay, lamp, or other load when the input
frequency reaches or exceeds a selected rate. The tachom-
eter uses a charge pump technique and offers frequency
doubling for low ripple, full input protection in two versions
(LM2907-8, LM2917-8) and its output swings to ground for a
zero frequency input.
The op amp/comparator is fully compatible with the tachom-
eter and has a floating transistor as its output. This feature
allows either a ground or supply referred load of up to 50 mA.
The collector may be taken above V
CC
up to a maximum V
CE
of 28V.
The two basic configurations offered include an 8-pin device
with a
ground referenced tachometer
input and an internal
connection between the tachometer output and the op amp
non-inverting input. This version is well suited for single
speed or frequency switching or fully buffered frequency to
voltage conversion applications.
The more versatile configurations provide differential ta-
chometer input and uncommitted op amp inputs. With this
version the tachometer input may be floated and the op amp
becomes suitable for active filter conditioning of the tachom-
eter output.
Both of these configurations are available with an active
shunt regulator connected across the power leads. The
regulator clamps the supply such that stable frequency to
voltage and frequency to current operations are possible
with any supply voltage and a suitable resistor.
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Only one RC network provides frequency doubling
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Zener regulator on chip allows accurate and stable
frequency to voltage or current conversion (LM2917)
Features
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Ground referenced tachometer input interfaces directly
with variable reluctance magnetic pickups
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Op amp/comparator has floating transistor output
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50 mA sink or source to operate relays, solenoids,
meters, or LEDs
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Frequency doubling for low ripple
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Tachometer has built-in hysteresis with either differential
input or ground referenced input
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Built-in zener on LM2917
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±
0.3% linearity typical
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Ground referenced tachometer is fully protected from
damage due to swings above V
CC
and below ground
Applications
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Over/under speed sensing
Frequency to voltage conversion (tachometer)
Speedometers
Breaker point dwell meters
Hand-held tachometer
Speed governors
Cruise control
Automotive door lock control
Clutch control
Horn control
Touch or sound switches
Advantages
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Output swings to ground for zero frequency input
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Easy to use; V
OUT
= f
IN
x V
CC
x R1 x C1
Block and Connection Diagrams
Dual-In-Line and Small Outline Packages, Top Views
00794201
00794202
Order Number LM2907M-8 or LM2907N-8
See NS Package Number M08A or N08E
Order Number LM2917M-8 or LM2917N-8
See NS Package Number M08A or N08E
© 2003 National Semiconductor Corporation
DS007942
www.national.com
NSC [ NATIONAL SEMICONDUCTOR ]
