The LM2907 and LM2917 devices are monolithic frequency-to-voltage converters with a high
gain op amp designed to operate a relay, lamp, or other load when the input frequency reaches or
exceeds a selected rate. The tachometer uses a charge pump technique and offers frequency doubling
for low-ripple, full-input protection in two versions (8-pin LM2907 and LM2917), and its output
swings to ground for a zero frequency input.
The op amp is fully compatible with the tachometer 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 VCC up to a maximum VCE of 28
V.
The two basic configurations offered include an 8-pin device with a
input and an internal connection
between the tachometer output and the op amp noninverting 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 tachometer 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 tachometer 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.
