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LM2907/LM2917 Frequency to Voltage Converter

May 2003

LM2907/LM2917 Frequency to Voltage Converter
General Description
The LM2907, LM2917 series are monolithic frequency to voltage converters with a high gain op amp/comparator 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 low ripple, full input protection in two versions (LM2907-8, LM2917-8) and its output swings to ground for a zero frequency input. The op amp/comparator 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 28V. 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 non-inverting input. This version is well suited for single speed or frequency switching or fully buffered frequency to voltage conversion applications. The more versatile configurations provide differential tachometer input and uncommitted op amp inputs. With this version the tachometer input may be floated and the op amp becomes suitable for active filter conditioning of the tachometer output. Both of these configurations are available with an active shunt regulator connected across the power leads. The regulator clamps the supply such that stable frequency to voltage and frequency to current operations are possible with any supply voltage and a suitable resistor. n Only one RC network provides frequency doubling n Zener regulator on chip allows accurate and stable frequency to voltage or current conversion (LM2917)

Features
n Ground referenced tachometer input interfaces directly with variable reluctance magnetic pickups n Op amp/comparator has floating transistor output n 50 mA sink or source to operate relays, solenoids, meters, or LEDs n Frequency doubling for low ripple n Tachometer has built-in hysteresis with either differential input or ground referenced input n Built-in zener on LM2917 n 0.3% linearity typical n Ground referenced tachometer is fully protected from damage due to swings above VCC and below ground

Applications
n n n n n n n n n n n Over/under speed sensing Frequency to voltage conversion (tachometer) Speedometers Breaker point dwell meters Hand-held tachometer Speed governors Cruise control Automotive door lock control Clutch control Horn control Touch or sound switches

Advantages
n Output swings to ground for zero frequency input n Easy to use; VOUT = fIN x VCC x R1 x C1

Block and Connection Diagrams
Dual-In-Line and Small Outline Packages, Top Views

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Order Number LM2907M-8 or LM2907N-8 See NS Package Number M08A or N08E

Order Number LM2917M-8 or LM2917N-8 See NS Package Number M08A or N08E

2003 National Semiconductor Corporation

DS007942

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LM2907/LM2917

Block and Connection Diagrams Dual-In-Line and Small Outline Packages, Top Views

(Continued)

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00794204

Order Number LM2907M or LM2907N See NS Package Number M14A or N14A

Order Number LM2917M or LM2917N See NS Package Number M14A or N14A

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LM2907/LM2917

Absolute Maximum Ratings

(Note 1)

Power Dissipation LM2907-8, LM2917-8 LM2907-14, LM2917-14 See (Note 1) Operating Temperature Range Storage Temperature Range Soldering Information Dual-In-Line Package Soldering (10 seconds) Small Outline Package Vapor Phase (60 seconds) Infrared (15 seconds) 215C 220C 260C -40C to +85C -65C to +150C 1200 mW 1580 mW

If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage Supply Current (Zener Options) Collector Voltage Differential Input Voltage Tachometer Op Amp/Comparator Input Voltage Range Tachometer LM2907-8, LM2917-8 LM2907, LM2917 Op Amp/Comparator 28V 28V 28V 25 mA 28V

28V
0.0V to +28V 0.0V to +28V

See AN-450 "Surface Mounting Methods and Their Effect on Product Reliability" for other methods of soldering surface mount devices.

Electrical Characteristics
VCC = 12 VDC, TA = 25C, see test circuit Symbol TACHOMETER Input Thresholds Hysteresis Offset Voltage LM2907/LM2917 LM2907-8/LM2917-8 Input Bias Current VOH VOL I 2, I3 I3 K Pin 2 Pin 2 Output Current Leakage Current Gain Constant Linearity OP/AMP COMPARATOR VOS IBIAS Input Common-Mode Voltage Voltage Gain Output Sink Current Output Source Current Saturation Voltage VC = 1.0 VE = VCC -2.0 ISINK = 5 mA ISINK = 20 mA ISINK = 50 mA ZENER REGULATOR Regulator Voltage Series Resistance Temperature Stability TOTAL SUPPLY CURRENT RDROP = 470 7.56 10.5 +1 3.8 6 15 V mV/C mA 1.0 40 VIN = 6.0V VIN = 6.0V 0 200 50 10 0.1 0.5 1.0 1.5 3 50 10 500 VCC-1.5V mV nA V V/mV mA mA V V V VIN = 50 mVDC VIN = +125 mVDC (Note 3) VIN = -125 mVDC (Note 3) V2 = V3 = 6.0V (Note 4) I2 = 0, V3 = 0 (Note 3) fIN = 1 kHz, 5 kHz, 10 kHz (Note 5) 0.9 -1.0 1.0 0.3 140 VIN = 250 mVp-p @ 1 kHz (Note 2) VIN = 250 mVp-p @ 1 kHz (Note 2) VIN = 250 mVp-p @ 1 kHz (Note 2) 3.5 5 0.1 8.3 2.3 180 240 0.1 1.1 +1.0 % 10 15 1 mV mV A V V A A Parameter Conditions Min Typ Max Units mV mV

10

25
30

40

Note 1: For operation in ambient temperatures above 25C, the device must be derated based on a 150C maximum junction temperature and a thermal resistance of 101C/W junction to ambient for LM2907-8 and LM2917-8, and 79C/W junction to ambient for LM2907-14 and LM2917-14. Note 2: Hysteresis is the sum +VTH - (-VTH), offset voltage is their difference. See test circuit. Note 3: VOH is equal to 3/4 x VCC - 1 VBE, VOL is equal to 1/4 x VCC - 1 VBE therefore VOH - VOL = VCC/2. The difference, VOH - VOL, and the mirror gain, I2/I3, are the two factors that cause the tachometer gain constant to vary from 1.0. Note 4: Be sure when choosing the time constant R1 x C1 that R1 is such that the maximum anticipated output voltage at pin 3 can be reached with I3 x R1. The maximum value for R1 is limited by the output resistance of pin 3 which is greater than 10 M typically.

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LM2907/LM2917

Electrical Characteristics

(Continued)

Note 5: Nonlinearity is defined as the deviation of VOUT (@ pin 3) for fIN = 5 kHz from a straight line defined by the VOUT @ 1 kHz and VOUT @ 10 kHz. C1 = 1000 pF, R1 = 68k and C2 = 0.22 mFd.

Test Circuit and Waveform

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Tachometer Input Threshold Measurement

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LM2907/LM2917

Typical Performance Characteristics
Total Supply Current Zener Voltage vs Temperature

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Normalized Tachometer Output vs Temperature

Normalized Tachometer Output vs Temperature

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Tachometer Currents I2 and I3 vs Supply Voltage

Tachometer Currents I2 and I3 vs Temperature

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LM2907/LM2917

Typical Performance Characteristics
Tachometer Linearity vs Temperature

(Continued) Tachometer Linearity vs Temperature

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Tachometer Linearity vs R1

Tachometer Input Hysteresis vs Temperature

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Op Amp Output Transistor Characteristics

Op Amp Output Transistor Characteristics

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LM2907/LM2917

Applications Information
The LM2907 series of tachometer circuits is designed for minimum external part count applications and maximum versatility. In order to fully exploit its features and advantages let's 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 (LM2907-8, LM2917-8) 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 28V, 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 in order 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. Then 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 x C1. The average amount of current pumped into or out of the capacitor then is:

CHOOSING R1 AND C1 There are some limitations on the choice of R1 and C1 which should be considered for optimum performance. The timing capacitor also provides internal compensation for the charge pump and should be kept larger than 500 pF for very accurate operation. Smaller values can cause an error current on R1, especially at low temperatures. Several considerations must be met when choosing R1. The output current at pin 3 is internally fixed and therefore VO/R1 must be less than or equal to this value. If R1 is too large, it can become a significant fraction of the output impedance at pin 3 which degrades linearity. Also output ripple voltage must be considered and the size of C2 is affected by R1. An expression that describes the ripple content on pin 3 for a single R1C2 combination is:

It appears R1 can be chosen independent of ripple, however response time, or the time it takes VOUT to stabilize at a new voltage increases as the size of C2 increases, so a compromise between ripple, response time, and linearity must be chosen carefully. As a final consideration, the maximum attainable input frequency is determined by VCC, C1 and I2:

The output circuit mirrors this current very accurately into the load resistor R1, connected to ground, such that if the pulses of current are integrated with a filter capacitor, then VO = ic x R1, and the total conversion equation becomes: VO = VCC x fIN x C1 x R1 x K Where K is the gain constant -- typically 1.0. The size of C2 is dependent only on the amount of ripple voltage allowable and the required response time.

USING ZENER REGULATED OPTIONS (LM2917) For those applications where an output voltage or current must be obtained independent of supply voltage variations, the LM2917 is offered. The most important consideration in choosing a dropping resistor from the unregulated supply to the device is that the tachometer and op amp circuitry alone require about 3 mA at the voltage level provided by the zener. At low supply voltages 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 470 will minimize the zener voltage variation to 160 mV. If the resistance goes under 400 or over 600 the zener variation quickly rises above 200 mV for the same input variation.

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LM2907/LM2917

Typical Applications
Minimum Component Tachometer

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LM2907/LM2917

Typical Applications

(Continued) Zener Regulated Frequency to Voltage Converter

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Breaker Point Dwell Meter

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LM2907/LM2917

Typical Applications

(Continued) Voltage Driven Meter Indicating Engine RPM VO = 6V @ 400 Hz or 6000 ERPM (8 Cylinder Engine)

00794212

Current Driven Meter Indicating Engine RPM IO = 10 mA @ 300 Hz or 6000 ERPM (6 Cylinder Engine)

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LM2907/LM2917

Typical Applications

(Continued) Capacitance Meter VOUT = 1V10V for CX = 0.01 to 0.1 mFd (R = 111k)

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Two-Wire Remote Speed Switch

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LM2907/LM2917

Typical Applications

(Continued) 100 Cycle Delay Switch

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Variable Reluctance Magnetic Pickup Buffer Circuits

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Precision two-shot output frequency equals twice input frequency.

00794217

Pulse height = VZENER

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LM2907/LM2917

Typical Applications
Finger Touch or Contact Switch

(Continued)

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Flashing LED Indicates Overspeed

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Flashing begins when fIN 100 Hz. Flash rate increases with input frequency increase beyond trip point.

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LM2907/LM2917

Typical Applications

(Continued)

Frequency to Voltage Converter with 2 Pole Butterworth Filter to Reduce Ripple

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Overspeed Latch

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LM2907/LM2917

Typical Applications

(Continued)

Some Frequency Switch Applications May Require Hysteresis in the Comparator Function Which can be Implemented in Several Ways:

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00794225 00794226

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LM2907/LM2917

Typical Applications

(Continued)

Changing the Output Voltage for an Input Frequency of Zero

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Changing Tachometer Gain Curve or Clamping the Minimum Output Voltage

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LM2907/LM2917

Anti-Skid Circuit Functions
"Select-Low" Circuit

00794234

VOUT is proportional to the lower of the two input wheel speeds.

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"Select-High" Circuit

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VOUT is proportional to the higher of the two input wheel speeds.

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"Select-Average" Circuit

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LM2907/LM2917

Equivalent Schematic Diagram

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*This connection made on LM2907-8 and LM2917-8 only. **This connection made on LM2917 and LM2917-8 only.

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LM2907/LM2917

Physical Dimensions
unless otherwise noted

inches (millimeters)

8-Lead (0.150" Wide) Molded Small Outline Package, JEDEC Order Number LM2907M-8 or LM2917M-8 NS Package Number M08A

Molded SO Package (M) Order Number LM2907M or LM2917M NS Package Number M14A

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LM2907/LM2917

Physical Dimensions

inches (millimeters) unless otherwise noted (Continued)

Molded Dual-In-Line Package (N) Order Number LM2907N-8 or LM2917N-8 NS Package Number N08E

Molded Dual-In-Line Package (N) Order Number LM2907N or LM2917N NS Package Number N14A

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LM2907/LM2917 Frequency to Voltage Converter

Notes

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