LM1819

LM1819 Air-Core Meter Driver February 1995 LM1819 Air-Core Meter Driver General Description The LM1819 is a function generator driver for air-core (moving-magnet) meter movements A Norton amplifier and an NPN transistor are included on chip for signal conditioning as required Driver outputs are self-centering and develop g 4 5V swing at 20 mA Better than 2% linearity is guaranteed over a full 305-degree operating range Features Y Y Y Y Self-centering 20 mA outputs 12V operation Norton amplifier Function generator Applications Y Y Y Air-core meter driver Tachometers Ruggedized instruments Typical Application TL H 5263 – 1 FIGURE 1 Automotive Tachometer Application Circuit shown operates with 4 cylinder engine and deflects meter pointer (270 ) at 6000 RPM Order Number LM1819M or LM1819N See NS Package Number M14A or N14A TRW Type X463UW Polycarbonate Capacitor RN60D Low TC Resistor ( g 100 ppm) Components Required for Automotive Load Dump Protection Available from FARIA Co P O Box 983 Uncasville CT 06382 Tel 203-848-9271 C1995 National Semiconductor Corporation TL H 5263 RRD-B30M115 Printed in U S A Absolute Maximum Ratings If Military Aerospace specified devices are required please contact the National Semiconductor Sales Office Distributors for availability and specifications Supply Voltage V a (pin 13) Power Dissipation (note 1) 20V 1300 mW Operating Temperature Storage Temperature Lead Temp (Soldering 10 seconds) BVCEO b 40 C to a 85 C b 65 C to b 150 C 260 C 20VMIN Electrical Characteristics VS e 13 1V TA e 25 C unless otherwise specified Symbol IS VREG Parameter Supply Current Regulator Voltage Regulator Output Resistance VREF Reference Voltage Reference Output Resistance Norton Amplifier Mirror Gain hFE NPN Transistor DC Gain Function Generator Feedback Bias Current Drive Voltage Extremes Sine and Cosine Sine Output Voltage with Zero Input Function Generator Linearity k Function Generator Gain Pin(s) 13 11 11 4 4 56 9 10 1 2 12 2 V1 e 5 1V ILOAD e 20 mA V8 e VREF FSD e 305 Meter Deflection DV8 50 75 53 75 Conditions Zero Input Frequency (See Figure 1 ) IREG e 0 mA IREG e 0 mA to 3 mA IREF e 0 mA IREF e 0 mA to 50 mA IBIAS j 20 mA 09 19 81 85 13 5 21 53 10 125 10 g4 g4 5 Min Typ Max 65 89 Units mA V X 23 V kX 11 mA V a 350 g1 7 b 350 0 mV %FSD V 56 75 Note 1 For operation above 25 C the LM1819 must be derated based upon a 125 C maximum junction temperature and a thermal resistance of 76 C W which applies for the device soldered in a printed circuit board and operating in a still-air ambient Application Hints AIR-CORE METER MOVEMENTS Air-core meters are often favored over other movements as a result of their mechanical ruggedness and their independence of calibration with age A simplified diagram of an aircore meter is shown in Figure 2 There are three basic pieces a magnet and pointer attached to a freely rotating axle and two coils each oriented at a right angle with respect to the other The only moving part in this meter is the axle assembly The magnet will tend to align itself with the vector sum of H fields of each coil where H is the magnetic field strength vector If for instance a current passes through the cosine coil (the reason for this nomenclature will become apparent later) as shown in Figure 3(a) the magnet will align its magnetic axis with the coil’s H field Similarly a current in the sine coil (Figure 3(b) ) causes the magnet to align itself with the sine H field If currents are applied simultaneously to both sine and cosine coils the magnet will turn to the direction of the vector sum of the two H fields (Figure 3(c)) H is proportional to the voltage applied to a coil Therefore by varying both the polarity and magnitude of the coil voltages the axle assembly can be made to rotate a full 360 The LM1819 is designed to drive the meter through a minimum of 305 TL H 5263 – 2 FIGURE 2 Simplified Diagram of an Air Core Meter 2 Application Hints (Continued) TL H 5263 – 3 (a) (b) (c) FIGURE 3 Magnet and pointer position are controlled by the H field generated by the two drive coils In an air-core meter the axle assembly is supported by two nylon bushings The torque exerted on the pointer is much greater than that found in a typical d’Arsonval movement In contrast to a d’Arsonval movement where calibration is a function of spring and magnet characteristics air-core meter calibration is only affected by the mechanical alignment of the drive coils Mechanical calibration once set at manufacture can not change Making pointer position a linear function of some input is a matter of properly ratioing the drive to each coil The H field contributed by each coil is a function of the applied current and the current is a function of the coil voltage Our desired result is to have i (pointer deflection measured in degrees) proportional to an input voltage i e kVIN 1 where k is a constant of proportionality with units of degrees volt The vector sum of each coils’ H field must follow the deflection angle i We know that the axle assembly always points in the direction of the vector sum of HSINE and HCOSINE This direction (see Figure 4 ) is found from the formula (i) e arctan l HSINE l l HCOSINE l 2 Recalling some basic trigonometry (i) e arctan(sin (i) cos(i )) 3 Comparing 3 to 2 we see that if HSINE varies as the sine of i and HCOSINE varies as the cosine of i we will generate a net H field whose direction is the same as i And since the axle assembly aligns itself with the net H field the pointer will always point in the direction of i THE LM1819 Included in the LM1819 is a function generator whose two outputs are designed to vary approximately as the sine and cosine of an input A minimum drive of g 20 mA at g 4V is available at pins 2 (sine) and 12 (cosine) The common side of each coil is returned to a 5 1V zener diode reference and fed back to pin 1 For the function generator k j 54 V (in equation 1) The input (pin 8) is internally connected to the Norton amplifier’s output VIN as considered in equation 1 is actually the difference of the voltages at pins 8 (Norton output function generator input) and 4 Typically the reference voltage at pin 4 is 2 1V Therefore i e k(V8bVREF) e 54 (V8b2 1) 4 As V8 varies from 2 1V to 7 75V the function generator will drive the meter through the chip’s rated 305 range Air-core meters are mechanically zeroed during manufacture such that when only the cosine coil is driven the pointer indicates zero degrees deflection However in some applications a slight trim or offset may be required This is accomplished by sourcing or sinking a DC current of a few microamperes at pin 4 A Norton amplifier is available for conditioning various input signals and driving the function generator A Norton amplifier was chosen since it makes a simple frequency to voltage converter While the non-inverting input (pin 6) bias is at one diode drop above ground the inverting input (5) is at 2 1V equal to the pin 4 reference Mirror gain remains essentially flat to IMIRROR e 5 mA The Norton amplifier’s output (8) is designed to source current into its load To bypass the Norton amplifier simply ground the non-inverting input tie the inverting input to the reference and drive pin 8 (Norton output function generator input) directly An NPN transistor is included on chip for buffering and squaring input signals Its usefulness is exemplified in Figures 1 6 where an ignition pulse is converted to a rectangular waveform by an RC network and the transistor The emitter is internally connected to ground It is important not to allow the base to drop below b5Vdc as damage may occur The 2 1V reference previously described is derived from an 8 5V regulator at pin 11 Pin 11 is used as a stable supply for collector loads and currents of up to 5 mA are easily accommodated 3 TL H 5263 – 4 FIGURE 4 The vector sum of HCOSINE and HSINE points in a direction i measured in a clockwise direction from HCOSINE Application Hints (Continued) TACHOMETER APPLICATION A measure of the operating level of any motor or engine is the rotational velocity of its output shaft In the case of an automotive engine the crankshaft speed is measured using the units ‘‘revolutions per minute’’ (RPM) It is possible to indirectly measure the speed of the crankshaft by using the signal present on the engine’s ignition coil The fundamental frequency of this signal is a function of engine speed and the number of cylinders and is calculated (for a four-stroke engine) from the formula f e n0 120 (Hz) (5) where n e number of cylinders and 0 e rotational velocity of the crankshaft in RPM From this formula the maximum frequency normally expected (for an 8 cylinder engine turning 4500RPM) is 300 Hz In certain specialized ignition systems (motorcycles and some automobiles) where the coil waveform is operated at twice this frequency ( f e 0 60) These systems are identified by the fact that multiple coils are used in lieu of a single coil and distributor Also the coils have two outputs instead of one A typical automotive tachometer application is shown in Figure 1 The coil waveform is filtered squared and limited by the RC network and NPN transistor The frequency of the pulse train at pin 9 is converted to a proportional voltage by the Norton amplifier’s charge pump configuration The ignition circuit shown in Figure 5 is typical of automotive systems The switching element ‘‘S’’ is opened and closed in synchronism with engine rotation When ‘‘S’’ is closed energy is stored in Lp When opened the current in Lp diverts from ‘‘S’’ into C The high voltage produced in Ls when ‘‘S’’ is opened is responsible for the arcing at the spark plug The coil voltage (see Figure 6 ) can be used as an input to the LM1819 tachometer circuit This waveform is essentially constant duty cycle D4 rectifies this waveform thereby preventing negative voltages from reaching the chip C4 and R5 form a low pass filter which attenuates the high frequency ringing and R7 limits the input current to about 2 5mA R6 acts as a base bleed to shut the transistor OFF when ‘‘S’’ is closed The collector is pulled up to the internal regulator by RREG The output at pin 9 is a clean rectangular pulse Many ignition systems use magnetic hall effect or optical sensors to trigger a solid state switching element at ‘‘S ’’ These systems (see the LM1815) typically generate pulses of constant width and amplitude suitable for driving the charge pump directly The charge pump circuit in Figure 7 can be operated in two modes constant input pulse width (C1 acts as a coupling capacitor) and constant input duty cycle (C1 acts as a differentiating capacitor) The transfer functions for these two modes are quite diverse However deflection is always directly proportional to R2 and ripple is proportional to C2 The following variables are used in the calculation of meter deflection symbol description n number of cylinders 0 0IDLE engine speed at redline and idle RPM pointer deflection at redline degrees charge pump input pulse width seconds peak to peak input voltages volts maximum desired ripple degrees function generator gain degrees volt f f IDLE input frequency at redline and idle Hz Where the NPN transistor and regulator are used to create a pulse VIN e 8 5V Acceptable ripple ranges from 3 to 10 degrees (a typical pointer is about 3 degrees wide) depending on meter damping and the input frequency The constant pulse width circuit is designed using the following equations VIN k 3 mA (1) 100 mAk R1 10e (2) C1 t R1 R1i 120R1i e e (3) R2 VINek f VINn0ek 1 1 e (4) C2 e R2Di f IDLE R2Din0IDLE The constant duty cycle equations are as follows RREG t 3 kX R1 s VINx104 bRREG C1 s e 10(RREG a R1) RZ e i 3 54n0C1 e i 425 f C1 C2 e 425C1 Di The values in Figure 1 were calculated with n e 4 0 e 6000RPM i e 270 degrees e e 1 ms VIN is VREGb0 7V and Di e 3 degrees in the constant duty cycle mode For distributorless ignitions these same equations will apply if 0 60 is substituted for f i e VIN Di k 4 5 TL H 5263 – 12 Equivalent Schematic Typical Applications TL H 5263 – 9 FIGURE 5 Typical Pulse-Squaring Circuit for Automotive Tachometers TL H 5263 – 11 TL H 5263–10 FIGURE 6 Waveforms Encountered in Automotive Tachometer Circuit FIGURE 7 Tachometer Charge Pump Voltage Driven Meter with Norton Amplifier Buffer TL H 5263 – 5 Deflection e 54 (VIN b 7)R2 R1 (degrees) 0 to 305 deflection is obtained with 7 to 5V input Full scale deflection is adjusted by trimming R2 6 Typical Applications (Continued) Unbuffered Voltage Driven Meter TL H 5263 – 6 Deflection e 54(VIN b 2 1) (degrees) 0 to 305 deflection is obtained for inputs of 2 1 to 7 75V Full scale deflection is adjusted by trimming the input voltage Current Driven Meter TL H 5263 – 7 Deflection e 54R2IIN (degrees) Inputs of 0 to 100 mA deflect the meter 0 to 270 Full scale deflection is adjusted by trimming R2 7 Typical Applications (Continued) Level Shifted Voltage Driven Meter TL H 5263 – 8 Deflection e 54VIN (degrees) Inputs of 0 to 5 65V deflect the meter through a range of 0 to 305 Full scale deflection is adjusted by trimming the input voltage 8 Physical Dimensions inches (millimeters) 14-Lead (0 150 Wide) Molded Small Outline Package JEDEC Order Number LM1819M NS Package Number M14A 9 LM1819 Air-Core Meter Driver Physical Dimensions inches (millimeters) (Continued) Molded Dual-In-Line Package (N) Order Number LM1819N NS Package Number N14A 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 1 Life support devices or systems are devices or systems which (a) are intended for surgical implant into the body or (b) support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user National Semiconductor Corporation 1111 West Bardin Road Arlington TX 76017 Tel 1(800) 272-9959 Fax 1(800) 737-7018 2 A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness National Semiconductor Europe Fax (a49) 0-180-530 85 86 Email cnjwge tevm2 nsc com Deutsch Tel (a49) 0-180-530 85 85 English Tel (a49) 0-180-532 78 32 Fran ais Tel (a49) 0-180-532 93 58 Italiano Tel (a49) 0-180-534 16 80 National Semiconductor Hong Kong Ltd 13th Floor Straight Block Ocean Centre 5 Canton Rd Tsimshatsui Kowloon Hong Kong Tel (852) 2737-1600 Fax (852) 2736-9960 National Semiconductor Japan Ltd Tel 81-043-299-2309 Fax 81-043-299-2408 National does not assume any responsibility for use of any circuitry described no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and 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