LM2907,LM2917 AN-162 LM2907 Tachometer/Speed Switch Building Block Applications Literature Number: SNAA088
INTRODUCTION Frequency to voltage converters are available in a number of forms from a number of sources, but invariably require significant additional components before they can be put to use in a given situation. The LM2907, LM2917 series of devices was developed to overcome these objections. Both input and output interface circuitry is included on chip so that a minimum number of additional components is required to complete the function.
required to return the capacitor on pin 2 to the high level voltage is duplicated and used to charge the capacitor connected to pin 3. Thus in one cycle of input the capacitor on pin 3 gets charged twice with a charge of CV. Input Hysteresis Amplifier The equivalent schematic diagram is shown in Figure 2. Q1 through Q11 comprise the input hysteresis amplifier.
3 *Note: This connection made on LM2907-8 and LM2917-8 only. Note: Pin numbers refer to 14-pin package. **Note: This connection made on LM2917 and LM2917-8 only. FIGURE 2.
tor, R3 (Figure 2) which is located in a floating isolation pocket, and secondly by clamp diode D1. Since the voltage swing on the base of Q1 is thus restricted, the only restriction on the allowable voltage on pin 1 is the breakdown voltage of the 10 kX resistor. This allows input swings to g 28V. In 14-pin versions the link to D1 is opened in order to allow the base of Q1 to be biased at some higher voltage. Q5 clamps the negative swing on the base of Q1 to about 300 mV.
there must be some current flowing in the resistor above the 3 mA circuit current to operate the regulator. As an example, if the raw supply varies from 9V to 16V, a resistance of 470X will minimize these zener voltage variations to 160 mV. If the resistor goes under 400X or over 600X the zener variation quickly rises above 200 mV for the same input variation. Take care also that the power dissipation of the IC is not exceeded at higher supply voltages. Figure 4 shows suitable dropping resistor values.
placed in series with the output to protect the LED and the output transistor. levels are available but large noise signals on ground are also present. To take full advantage of the common-mode rejection of the input differential stage, a balanced bias configuration must be provided. One such circuit is illustrated in Figure 5(e). With this arrangement, the effective commonmode rejection may be virtually infinite, owing to the input hysteresis. This circuit has no hysteresis in it, i.e.
Figure 7 illustrates methods for protecting against these and other transients. Figure 7(a) shows a typical situation in which the power supply to the LM2907 can be provided through a dropping resistor and regulated by an external zener diode Z1, but the output drive is required to operate from the full available supply voltage. In this case, a separate protection zener Z2 must be provided if the voltage on the power line is expected to exceed the maximum rated voltage of the LM2907.
The concept of building blocks requires that a function be performed in the same way as it can be mathematically defined. In other words, a frequency to voltage converter will provide an output voltage proportional to frequency which is independent of the input voltage or other input parameters, except the frequency. In the same way, the output voltage will be zero when the input frequency is zero. These features are built into the LM2907.
the output current is conducted along the supply line so that a local current sensing device in the supply line can be used to get a direct reading of the frequency at the remote location where the electronics may also be situated. The small zero speed offset due to the device quiescent current may be compensated by offsetting the zero on the display device. This also permits one display device to be shared between several inputs. or visual warning of speeds in excess of that level.
Automotive Tachometer Not all inputs are derived from variable reluctance magnetic pickups; for example, in spark ignition engines the tachometer is generally driven from the spark coil. An interface circuit for this situation is shown in Figure 14. This tachometer can be set up for any number of cylinders by linking the appropriate timing resistor as illustrated. A 500X trim resistor can be used to set up final calibration.
the non-inverting input. As speed increases, the charge pump puts charge into capacitor C2 and causes the output VOUT to fall in proportion to speed. The output current of the op amp transistor is used to provide an analog drive to the motor. Thus as the motor speed approaches the reference level, the current is proportionately reduced to the motor so that the motor gradually comes up to speed and is maintained without operating the motor in a switching mode.
The output of the tachometer is proportional to the product of supply voltage, input frequency, a capacitor and a resistor. Any one of these may be used as the input variable or they may be used in combination to produce multiplication. An example of a capacitive transducer is illustrated in Figure 20, where a fixed input frequency is employed either from the 60 Hz line as a convenient source or from a stable oscillator.
TL/H/7451 – 38 FIGURE 20. Capacitive Transducer The linearity of voltage controlled oscillators can be improved by employing the LM2907 as a feedback control element converting the frequency back to voltage and comparing with the input voltage. This can often be a lower cost solution to linearizing the VCO than by working directly on the VCO itself in the open loop mode. The arrangement is illustrated in Figure 22.
The staircase generator shown in Figure 19 can be used as an A-D converter. A suitable configuration is shown in Figure 25. To start a convert cycle the processor generates a reset pulse to discharge the integrating capacitor C2. Each complete clock cycle generates a charge and discharge cycle on C1. This results in two steps per cycle being added to C2. As the voltage on C2 increases, clock pulses are re- turned to the processor.
amp/comparator is connected with negative feedback with a diode in the loop so that the amplifier can only pull down on the load and not pull up. In this way, the outputs from the two devices can be joined together and the output will be the lower of the two input speeds. In Figure 27 the output emitter of the onboard op amp provides the pullup required to provide a select-high situation where the output is equal to the higher of two speeds.
TL/H/7451 – 49 VOUT is proportional to the higher of the two input wheel speeds TL/H/7451 – 50 FIGURE 27. ‘‘Select-High’’ Circuit TL/H/7451 – 51 FIGURE 28.
Transmission and Clutch Control Functions CONCLUSION Electric clutches can be added to automotive transmissions to eliminate the 6% slip which typically occurs during cruise and which results in a 6% loss in fuel economy. These devices could be operated by a pair of LM2907’s as illustrated in Figure 29. Magnetic pickups are connected to input and output shafts of the transmission respectively and provide frequency inputs f1 and f 2 to the circuit.
LM2907 Tachometer/Speed Switch Building Block Applications LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: AN-162 1.
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