LM2907N

LM2907N - Frequency to Voltage Converter with Op Amp/Comparator

Overview

The LM2907N (and its LM2917N counterpart) is a monolithic frequency-to-voltage converter IC featuring a high-gain operational amplifier/comparator. Designed for driving relays, lamps, or other loads when the input frequency meets or exceeds a predefined threshold, this device is widely used in tachometer and speed-sensing applications. Its charge pump architecture ensures low ripple, while built-in protection mechanisms enhance reliability in automotive and industrial environments.

Key Features

• Ground-Referenced Tachometer Input: Directly interfaces with variable reluctance magnetic pickups.
• High-Current Output: Op amp/comparator with floating transistor output capable of sinking or sourcing 50 mA to drive relays, solenoids, LEDs, or meters.
• Frequency Doubling: Reduces output ripple for stable voltage conversion.
• Built-In Hysteresis: Supports differential or ground-referenced inputs for noise immunity.
• Integrated Protection: Fully protected against voltage swings above V_CC and below ground.
• High Linearity: Typical linearity of ±0.3% ensures accurate frequency-to-voltage conversion.
• Zener Diode (LM2917N only): Includes a built-in zener for voltage regulation.

Applications

The LM2907N is versatile and suited for a broad range of applications, including:

• Speed Sensing: Over/under-speed detection in industrial and automotive systems.
• Tachometers: Frequency-to-voltage conversion for RPM measurement.
• Automotive Systems: Cruise control, door lock control, clutch control, and horn control.
• Instrumentation: Speedometers, breaker point dwell meters, and handheld tachometers.
• Switching Circuits: Touch or sound-activated switches.
• Governors: Speed regulation in mechanical systems.

Electrical Characteristics

The following specifications are critical for design and implementation:

• Supply Voltage (V_CC): 12V typical (refer to datasheet for absolute maximum ratings).
• Output Current: 50 mA sink/source capability.
• Frequency Range: Compatible with low-frequency signals (specific range depends on external components).
• Linearity Error: ±0.3% typical.
• Input Protection: Withstands voltage swings beyond supply rails.

Absolute Maximum Ratings (Note 1)

• Supply Voltage (V_CC): 28V (non-repetitive).
• Input Voltage: ±28V (protected).
• Output Voltage: 28V.
• Output Current: 50 mA continuous.
• Operating Temperature Range: -40°C to +85°C (industrial grade).
• Storage Temperature Range: -65°C to +150°C.

Note 1: Exceeding these ratings may cause permanent damage. Functional operation under these conditions is not guaranteed.

Package Information

The LM2907N is available in multiple package options, including:

• Dual-In-Line (DIP):
  • LM2907N-8: 8-pin DIP (Package Number N08E).
  • LM2907N: 14-pin DIP (Package Number N14A).
• Small Outline (SOIC):
  • LM2907M-8: 8-pin SOIC (Package Number M08A).
  • LM2917M: 14-pin SOIC (Package Number M14A).

Pin Configuration (14-Pin DIP Example)

• Pin 1: Frequency Input (C1).
• Pin 2: Ground (GND).
• Pin 3: Op Amp Output (Output 1).
• Pin 4: Op Amp Inverting Input (-IN).
• Pin 5: Op Amp Non-Inverting Input (+IN).
• Pin 6: V_CC (Supply Voltage).
• Pin 7: Charge Pump Capacitor (C2).
• Pin 8: Output Transistor Collector (Open Collector).
• Pins 9-14: Additional connections (varies by model).

Functional Description

The LM2907N operates by converting an input frequency into a proportional DC voltage. The charge pump circuitry doubles the input frequency to minimize ripple, while the built-in op amp/comparator provides flexibility for driving external loads. The device includes hysteresis to prevent output chatter near threshold frequencies, making it ideal for noisy environments.

Key Functional Blocks

• Frequency-to-Voltage Converter: Charge pump with frequency doubling.
• Op Amp/Comparator: Configurable for threshold detection or linear amplification.
• Output Stage: Open-collector transistor for high-current switching.

Design Considerations

When integrating the LM2907N into a circuit, consider the following:

• Input Signal Conditioning: Ensure proper filtering for noisy signals (e.g., magnetic pickup outputs).
• External Components: Select charge pump capacitors (C1, C2) and resistors per datasheet guidelines to achieve desired frequency response.
• Load Compatibility: Verify output current requirements for relays or LEDs.
• Thermal Management: Dissipate heat adequately when driving high-current loads.

Conclusion

The LM2907N is a robust and flexible solution for frequency-to-voltage conversion and speed sensing. Its integrated protection, high output drive, and low ripple make it suitable for demanding automotive and industrial applications. Engineers can leverage its op amp/comparator for custom threshold detection or analog signal processing, while its compact package options enable space-efficient designs.