LM2917MX-8/NOPB Integrated Circuits (ICs)

Due to market price fluctuations,if you need to purchase or consult the price.You can contact us or emial to us:   sales@allicdata.com

1. Description

The LM2907 and LM2917MX-8/NOPB 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 lowripple, 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 ground-referenced tachometer 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.

2. Features

    1. Ground Referenced Tachometer Input Interfaces Directly With Variable Reluctance Magnetic Pickups

    2. Op Amp Has Floating Transistor Output

    3. 50-mA Sink or Source to Operate Relays, Solenoids, Meters, or LEDs

    4. Frequency Doubling For Low Ripple

    5. Tachometer Has Built-In Hysteresis With Either Differential Input or Ground Referenced Input

    6. ±0.3% Linearity (Typical)

    7. Ground-Referenced Tachometer is Fully Protected From Damage Due to Swings Above VCC and Below Ground

    8. Output Swings to Ground For Zero Frequency Input

    9. Easy to Use; VOUT = fIN × VCC × R1 × C1

  10. Zener Regulator on Chip allows Accurate and Stable Frequency to Voltage or Current Conversion (LM2917)

3. Applications

    1. Over- and Under-Speed Sensing

    2. Frequency-to-Voltage Conversion (Tachometer)

    3. Speedometers

    4. Breaker Point Dwell Meters

    5. Hand-Held Tachometers

    6. Speed Governors

    7. Cruise Control

    8. Automotive Door Lock Control

    9. Clutch Control

  10. Horn Control

  11. Touch or Sound Switches

4. Pin configuration

5. Pin Description

6. Feature Description

This device features a Schmitt-Trigger comparator that is the first stage in converting the input signal. Every time the output of the comparator flips between high and low correlates to a half cycle elapsing on the input signal. On the LM29x7-8 devices, one terminal of this comparator is internally connected to ground. This requires that the input signal cross zero volts in order for device to detect the frequency. On the LM29x7 devices, the input terminals to the Schmitt-Trigger comparator are both available for use. This open terminal allows the potential at which the comparator’s output is flipped to be applied externally. This allows the device to accept signals with DC offset or compare differential inputs.

The output voltage generated by the charge pump is fed in the noninverting terminal of a high gain op amp. This op amp then drives and uncommitted bipolar junction transistor. This allows the LM2907 to be configured a variety of ways to meet system needs. The output voltage can be buffered and used to drive a load or an output threshold can be given to trigger a load switch .

7. Application Information

The LM2907 series of tachometer circuits is designed for minimum external part count applications and maximum versatility. To fully exploit its features and advantages, first examine its theory of operation. The first stage of operation is a differential amplifier driving a positive feedback flip-flop circuit. The input threshold voltage is the amount of differential input voltage at which the output of this stage changes state. Two options (8-pin LM2907 and LM2917) have one input internally grounded so that an input signal must swing above and below ground and exceed the input thresholds to produce an output. This is offered specifically for magnetic variable reluctance pickups which typically provide a single-ended AC output. This single input is also fully protected against voltage swings to ±28 V, which are easily attained with these types of pickups. The differential input options (LM2907, LM2917) give the user the option of setting his own input switching level and still have the hysteresis around that level for excellent noise rejection in any application. Of course to allow the inputs to attain common-mode voltages above ground, input protection is removed and neither input should be taken outside the limits of the supply voltage being used. It is very important that an input not go below ground without some resistance in its lead to limit the current that will then flow in the epi-substrate diode. Following the input stage is the charge pump where the input frequency is converted to a DC voltage. To do this requires one timing capacitor, one output resistor, and an integrating or filter capacitor. When the input stage changes state (due to a suitable zero crossing or differential voltage on the input) the timing capacitor is either charged or discharged linearly between two voltages whose difference is VCC/2. The n in one half cycle of the input frequency or a time equal to 1/2 fIN the change in charge on the timing capacitor is equal to VCC/2 × C1. The average amount of current pumped into or out of the capacitor is shown in Equation 4.