The LM2907 and LM2917 are monolithic frequency-to-voltage converters. Each device has an output circuit that activates loads such as relays and lamps when the input frequency reaches or exceeds a selected rate. The converter (tachometer) section consists of a comparator driving a charge pump and offers frequency doubling for low ripple, full input protection in 8-pin versions, and an output swing to ground for a zero-frequency input. The output section consists of an operational amplifier, normally operating as a comparator, that drives an output transistor with both the collector and emitter floating. The circuit can either sink or source 40 mA of load current.
Two basic configurations are offered: 8-pin devices and 14-pin devices. Each 8-pin version has a groundreferenced tachometer input and an internal connection between the tachometer output and the operational amplifier input. The 8-pin versions are suited to single-speed or single-frequency switching or fully buffered frequency-to-voltage conversion applications. The more versatile 14-pin versions provide differential tachometer inputs and uncommitted operational amplifier inputs. The tachometer input can be floated, and the operational amplifier becomes suitable for active filter conditioning of the tachometer output. The LM2917 has an active shunt regulator connected across the power leads. The regulator clamps the supply voltage so that stable frequency-to-voltage and frequency-to-current conversions are possible with any supply voltage and a suitable resistor.
The LM2907 and LM2917 are designed for operation from –40°C to 85°C.
Output Swings to Ground for Zero-Frequency Input
Only One RC Network Provides Frequency Doubling for Low Ripple
8-Pin Versions Interface Directly to Variable-Reluctance Magnetic Pickups
Uncommitted Collector and Emitter Outputs Provide 40-mA Sink or Source Current to Operate Relays, Solenoids, Meters, or LEDs
Built-In Hysteresis for Noise Immunity
Linearity Typically ±0.3%
8-Pin Versions Are Fully Protected From Damage Due to TACH Input Swing Above VCC and Below Ground