LM2917N-8 Frequency to Voltage Converter

Overview

The LM2917N-8 is a monolithic frequency-to-voltage converter from the LM2907/LM2917 series, designed to drive relays, lamps, or other loads when the input frequency reaches a specified threshold. This device integrates a high-gain operational amplifier/comparator and utilizes a charge pump technique for frequency doubling, which minimizes output ripple. It is particularly suited for automotive and industrial applications requiring precise frequency monitoring and conversion.

The LM2917N-8 variant includes a built-in zener diode for voltage regulation and provides full input protection against voltage swings above V_CC and below ground. Its ground-referenced tachometer input allows direct interfacing with variable reluctance magnetic pickups, making it ideal for speed sensing applications.

Features

• Ground-referenced tachometer input for direct interfacing with variable reluctance magnetic pickups
• Operational amplifier/comparator with floating transistor output
• 50 mA sink or source capability for driving relays, solenoids, meters, or LEDs
• Frequency doubling for reduced output ripple
• Built-in hysteresis for differential or ground-referenced input
• Integrated zener diode (LM2917-specific feature)
• High linearity (typically ±0.3%)
• Full input protection against voltage swings beyond supply rails

Applications

• Over/under speed sensing
• Frequency-to-voltage conversion (tachometer applications)
• Automotive speedometers and cruise control systems
• Breaker point dwell meters
• Hand-held tachometers
• Speed governors
• Automotive door lock and clutch control
• Horn control circuits
• Touch or sound-activated switches

Electrical Characteristics

Absolute Maximum Ratings

Note 1: Exceeding these ratings may cause permanent device damage.

• Supply Voltage (V_CC): 28V
• Input Voltage Range: -30V to +30V
• Output Sink Current: 50mA
• Output Source Current: 50mA
• Operating Temperature Range: -40°C to +85°C
• Storage Temperature Range: -65°C to +150°C
• Junction Temperature: 150°C

Key Performance Parameters

• Frequency Range: 0Hz to 10kHz
• Linearity Error: ±0.3% typical
• Output Voltage Swing: 0V to V_CC-2V
• Quiescent Current: 6mA typical
• Input Threshold Voltage: 7.5mV typical
• Hysteresis: 30mV typical

Package Information

Available Packages

• LM2917N-8: 8-pin PDIP (N08E package)
• LM2917M-8: 8-pin SOIC (M08A package)

Pin Configuration (8-pin DIP/SOIC)

• Pin 1: Frequency Input
• Pin 2: R₁/C₁ Connection
Pin 3: C₁ Connection
• Pin 4: Ground
• Pin 5: Op-Amp Output
• Pin 6: V_CC (Supply Voltage)
• Pin 7: Output Transistor Collector
• Pin 8: Output Transistor Emitter

Thermal Characteristics

• Thermal Resistance (θ_JA):
  • PDIP Package: 80°C/W
  • SOIC Package: 160°C/W

Functional Description

The LM2917N-8 converts an input frequency into a proportional output voltage using a charge pump technique. The internal frequency doubler improves output ripple characteristics, while the built-in comparator with hysteresis provides clean switching at the threshold frequency. The device can directly interface with variable reluctance sensors common in automotive applications, eliminating the need for additional signal conditioning circuitry.

The operational amplifier/comparator section features an open-collector output capable of sinking up to 50mA, allowing direct drive of relays, lamps, or other loads. The floating transistor output provides flexibility in connecting various load types.

Design Considerations

Input Circuitry

The ground-referenced input stage includes protection against voltage transients and can accept signals from variable reluctance magnetic pickups without additional conditioning. For optimal performance with low-amplitude signals, an external pull-up resistor may be required.

Frequency-to-Voltage Conversion

The conversion gain is determined by external components R₁ and C₁ according to the formula: V_OUT = f_IN × V_CC × R₁ × C₁ × K, where K is a constant determined by the internal charge pump characteristics.

Output Stage

The output transistor can be configured as either a current sink or source, with the emitter terminal (Pin 8) providing flexibility in grounding arrangements. When driving inductive loads, appropriate flyback protection diodes should be included.

Typical Application Circuit

A basic tachometer application would include:

• Variable reluctance sensor connected to Pin 1
• R₁ and C₁ connected between Pins 2 and 3
• Meter or relay connected between V_CC and Pin 7
• Pin 8 grounded for current sink operation