Principle and performance parameters of LM2917 voltage converter

1. Overview

LM2917 is a monolithic integrated frequency-voltage converter. The chip contains a high-gain operational amplifier/comparator. When the input frequency reaches or exceeds a given value, the output can be used to drive switches, indicator lights or other loads. The included tachometer uses charge pump technology and has a frequency multiplication function for low ripple. In addition, LM2917 also has a complete input protection circuit. At zero frequency input, the output logic swing of LM2917 is zero.

1.1 Main Features

The LM2917 has the following features:

Only one RC network is required for frequency multiplication;
the chip has a Zener diode adjustment circuit that can perform accurate frequency-voltage (current) conversion;
the ground-referenced tachometer input can be directly interfaced with a variable reluctance pickup;
the operational amplifier/comparator uses a floating transistor output;
the 50mA output sink current or drive capability can drive switches, solenoids, meters, light-emitting diodes, etc.;
it has a frequency multiplication function for low ripple;
the tachometer has a hysteresis, differential input or a single-ended input referenced to ground;
the typical linearity is ±0.3%;

the ground-referenced tachometer has a complete protection circuit and will not be damaged by inputs above the VCC value or below the ground reference.

1.2 Application Areas

The LM2917 can be applied to the following areas:

• Overspeed/underspeed detection;
• Frequency to voltage conversion (tachometer);
• Speedometer;
• Handheld tachometer;
• Speed ​​monitor;
• Tour control;
• Door lock control;
• Clutch control;
• Speaker control;
• Touch or sound switch.

1.3 Electrical Performance Parameters

The main electrical performance parameters of LM2917 are listed in Table 1.

2. Working Principle

Figure 1 shows the functional block diagram of LM2917. The functions of each pin are as follows:

• Pins 1 and 11 are the inputs of the operational amplifier/comparator;
• Pin 2 is connected to the timing capacitor of the charge pump;
• Pin 3 connects the output resistor and the integral capacitor of the charge pump;
• Pins 4 and 10 are the input terminals of the operational amplifier;
• Pin 5 is the output, taken from the emitter of the output transistor;
• Pins 6, 7, 13, and 14 are not used; Pin 8 is the collector of the output transistor, usually connected to the power supply;
• Pin 9 is the positive power supply terminal;
• Pin 12 is the negative power supply terminal and is usually grounded.

The operational amplifier/comparator is fully compatible with the tachometer. It uses a floating transistor as the output terminal and has a strong output drive capability. It can drive a load referenced to ground or power supply with a current of 50mA. The collector potential of the output transistor can be higher than VCC, and the maximum allowed voltage VCE is 28V.

Differential input terminals are used in the circuit, and the user can set the input conversion level by himself, and the hysteresis is also around the set level, so good noise suppression can be obtained. Of course, in order to make the input have a common-mode voltage when it is higher than the ground voltage, no input protection circuit is used, but the input voltage level cannot exceed the power supply voltage range. It is particularly noteworthy that the input level cannot be lower than the ground level when the input is not connected to the series protection resistor.

When the charge pump converts the frequency from the input stage to a DC voltage, this conversion requires an external timing capacitor C1, an output resistor R1, and an integrating capacitor or a filter capacitor C2. When the output of the input stage changes state (this may occur due to a suitable zero-crossing voltage or differential input voltage at the input end), the timing capacitor is linearly charged or discharged between two voltage values ​​with a voltage difference of VCC/2. In the half cycle of the input frequency signal, the charge change on the timing capacitor is C1VCC/2, and the average current pumped into the capacitor or the average current flowing out of the capacitor is:

△Q/T=iC(AVG)=fINC1VCC

The output circuit accurately sends this current to the load resistor (output resistor) R1, and the other end of the R1 resistor is grounded. In this way, the pulsed current is integrated by the filter capacitor to obtain the output voltage:

VO=VCCfINC1R1K

, where K is the gain constant.

The value of capacitor C2 depends on the size of the ripple voltage and the response time required in the actual application.

3. Typical application circuit

A noteworthy issue in the application is how to choose resistor R1 and capacitor C1.

To achieve the best performance, the appropriate resistor R1 and capacitor C1 must be carefully selected. The timing capacitor also provides internal compensation for the charge pump. In order for the device to achieve accurate conversion results, its value should be greater than 500pF. Too small a capacitor value will produce error current on R1, especially in low-temperature applications. The output current of the device's pin 3 is internally fixed, so the VO/R1 value must be less than or equal to this fixed value. If R1 is too large, it will affect the output impedance of pin 3, and the linearity of the device's frequency-to-voltage conversion will deteriorate. In addition, the output ripple voltage and the effect of R1 on the value of C2 must be considered. The ripple VRIPPLE at pin 3 can be calculated as follows:

VRIPPLE=VCCC1[1-(VCCfINC1/I2)]/2C2

The choice of R1 has nothing to do with ripple. However, the response time, that is, the time required for the output VOUT to settle to a new value, will increase with the increase of C2 value, so a careful compromise must be made between ripple, response time and linearity. The maximum frequency of the input signal allowed by the device is determined by VCC, C1 and I2.

LM2917 is very suitable for applications that require output voltage or current to be independent of power supply voltage changes, because it uses a Zener diode to adjust the output end. However, a resistor must be connected in series with the power supply end, and the resistance value must be reasonably selected. The tachometer current and the operating current of the operational amplifier circuit in the chip need to be at least 3mA. In low-voltage applications, it is necessary to ensure that the current in the resistor is greater than 3mA so that the Zener diode voltage adjustment can be normal. For example, when the power supply voltage changes from 9V to 16V, a 470Ω resistor in series can reduce the Zener diode voltage change to 160mV.