LM2917N/NOPB IC

Technical Specifications of LM2917N/NOPB

Datasheet	LM2917N/NOPB datasheet
Category	Integrated Circuits (ICs)
Family	PMIC – V/F and F/V Converters
Manufacturer	Texas Instruments
Series	–
Packaging	Tube
Part Status	Active
Type	Frequency to Voltage
Frequency – Max	10kHz
Full Scale	–
Linearity	±0.3%
Mounting Type	Through Hole
Package / Case	14-DIP (0.300″, 7.62mm)
Supplier Device Package	14-DIP

LM2917N-8/NOPB Description

The LM2907 and LM2917 are monolithic frequency-to-voltage converters that use a high-gain op-amp to power a relay, light, or other loads when the input frequency hits or exceeds a user-defined threshold. The tachometer’s output goes to the ground when the input frequency is zero, and it features frequency doubling for low ripple, full-input protection, and two different variants (8-pin LM2907 and LM2917).

Tachometer compatibility is ensured by the op amp’s floating transistor output. This function supports a 50-mA ground or supply referred load, depending on which is selected. A maximum of 28 VCE can be applied to the collector in addition to VCC. An 8-pin device is connected internally to the tachometer’s output and the op amp’s noninverting input, one of the two default configurations. This revision works splendidly in single-speed or single-frequency switching setups and completely buffered frequency-to-voltage conversions.

LM2917N-8/NOPB Features

• Input Interfaces for Tachometers that Use a Ground Reference.
• Using VRM Pickups (Variable Reluctance Magnetic Pickups), directly.
• Transistor-based output is a feature of operational amplifiers.
• Use as a sink or source of 50 mA current for driving relays, solenoids, meters, or light-emitting diodes.
• The frequency was doubled to reduce the ripple effect.
• Hysteresis can be used with the tachometer’s differential or ground-referenced input.
• Linearity of 0.3% (Typical).
• The tachometer’s ground reference shields it from above- and below-ground voltage fluctuations.
• In the event that the input frequency is zero, the output will go low or ground.
• Quick and Simple; VOUT = fIN VCC R1 C1.
• A built-in Zener regulator on the chip makes a precise and stable frequency-to-voltage or current conversion possible (LM2917).

Detailed Description

Two independent inputs allow for signal monitoring with the LM29x7 frequency-to-voltage converter. To account for the fact that one input in 8-pin devices is internally grounded, the device checks for zero crossings on the other input. These inputs are floating in 14-pin devices, sensing instead when the differential voltage flips sign. This results in a square wave with the same frequency as the input-output by the comparator. A charge pump device converts this square wave’s frequency to a voltage. A 180-A constant current is applied to C1 at the beginning of each positive half-cycle of the input signal, raising its voltage by VCC/2.

Once the input signal starts a negative half cycle, the capacitor is kept at that voltage. Next, 180 A of steady current drains capacitor C1 until its voltage is VCC/2 less. When the voltage reaches this point, it remains until the next positive half-cycle. This causes current pulses to enter and leave capacitor C1 at the input signal’s frequency. At the beginning of each half-cycle, the charge pump injects a positive current pulse into the parallel combination of resistor R1 and capacitor C2. Therefore, the total charge entering and leaving C1 equals the amount leaving pin 3 at the end of each cycle.

Feature Description

● Differential Input

A Schmitt-Trigger comparator is the basis of the input signal conversion in this apparatus. The passage of one-half cycle of the input signal is reflected in the comparator’s high-to-low output transitions. One terminal of this comparator is linked to the ground on the LM29x7-8 devices. The input signal must be more than zero volts for the instrument to pick up the frequency. Each of the LM29x7’s terminals can be used as an input to the comparator’s Schmitt-Trigger. The comparator’s output can be flipped by applying an external voltage to this terminal. This feature allows the device to compare differential inputs and accept signals with a DC offset.

● Output Stage

The charge pump’s output voltage is routed into the high gain op amp’s noninverting terminal. This op-amp drives an uncommitted bipolar junction transistor. The LM2907 can be set up in various ways to suit the system’s requirements. Buffering the output voltage makes it suitable for driving a load, or an output threshold can be specified to activate a load switch.

Device Functional Modes

Devices with a Grounded Input (8-Pin LM2907 and LM2917) Internally, these devices have one of their two Schmitt-Trigger comparator inputs connected to the ground; hence they require an additional external connection to the system ground. With this setup, the other terminal is watched for zero crossings.

● Differential Input Devices (LM2907 and LM2917)

Both Schmitt-Trigger comparator inputs are available on these devices, with pins 1 and 11 broken out to accommodate them. This setup can either take in a differential pair and switch based on voltage difference or give a new switching threshold in the event of signals with a DC offset.

Application Information

The tachometer circuits in the LM2907 family are optimized for use with few external components while still providing a high level of flexibility. Examining its mode of functioning is the first step toward fully capitalizing on its features and advantages. A differential amplifier drives a positive-feedback flip-flop in the first stage of operation. At a certain differential input voltage, the output of this stage will transition from one state to the next. For example, one input is grounded internally with the 8-pin LM2907 and LM2917. Therefore, the input signal must go above and below ground and pass through the thresholds to generate an output. Since magnetic variable reluctance pickups normally only provide a single-ended AC signal, this option is provided for them.

Because voltage swings to 28 V are so simple to achieve with these pickups, this single input is likewise fully secured against them. The hysteresis around the user-set switching level and the strong noise rejection it enables are also available with the differential input options. Turning off input protection allows common-mode voltages to be generated above ground, albeit neither input should be driven higher than the supply voltage it is connected to.

It is critical that an input not descend below ground without some resistance in its lead to limit the current that will flow in the epi-substrate diode. The charge pump is the stage that follows the input stage and is responsible for transforming the input frequency into a DC voltage. It takes an integrating or filter capacitor, an output resistor, and a timing capacitor to accomplish this. A change in state at the input stage results in a linear charge or discharge of the timing capacitor between two voltages with a difference equal to half of the supply voltage, or VCC/2. Hence, the timing capacitor will experience a charge change of VCC/2 C1 in a time equal to 1/2 fIN, where fIN is the input frequency.

Conclusion

To summarize, the LM2907 and LM2917 are excellent frequency-to-voltage converters with a host of features and high levels of flexibility. These devices are ideal for tachometer circuits and can drive components such as relays, solenoids, meters, and light-emitting diodes. With full-input protection, low ripple, and high linearity, these converters offer a quick and simple solution for frequency-to-voltage conversion. When you partner with ICRFQ, you can expect top-quality products and exceptional customer service. We recommend investing in the LM2917N/NOPB to advance your project.

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