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LM1949N Arkusz danych(PDF) 5 Page - National Semiconductor (TI) |
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LM1949N Arkusz danych(HTML) 5 Page - National Semiconductor (TI) |
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5 / 10 page  Typical Performance Characteristics (Continued) Sense Input Hold Voltage vs Junction Temperature Normalized Timer Function vs Junction Temperature LM1949N Junction Temperature Rise Above Ambient vs Supply Voltage TLH5062 – 5 Application Hints The injector driver integrated circuits were designed to be used in conjunction with an external controller The LM1949 derives its input signal from either a control oriented proces- sor (COPSTM) microprocessor or some other system This input signal in the form of a square wave with a variable duty cycle andor variable frequency is applied to Pin 1 In a typical system input frequency is proportional to engine RPM Duty cycle is proportional to the engine load The cir- cuits discussed are suitable for use in either open or closed loop systems In closed loop systems the engine exhaust is monitored and the air-to-fuel mixture is varied (via the duty cycle) to maintain a perfect or stochiometric ratio INJECTORS Injectors and solenoids are available in a vast array of sizes and characteristics Therefore it is necessary to be able to design a drive system to suit each type of solenoid The purpose of this section is to enable any system designer to use and modify the LM1949 and associated circuitry to meet the system specifications Fuel injectors can usually be modeled by a simple RL circuit Figure 3 shows such a model for a typical fuel injector In actual operation the value of L1 will depend upon the status of the solenoid In other words L1 will change depending TLH5062 – 6 FIGURE 3 Model of a Typical Fuel Injector upon whether the solenoid is open or closed This effect if pronounced enough can be a valuable aid in determining the current necessary to open a particular type of injector The change in inductance manifests itself as a breakpoint in the initial rise of solenoid current The waveforms on Page 2 at the sense input show this occurring at approximately 130 mV Thus the current necessary to overcome the constric- tive forces of that particular injector is 13 amperes PEAK AND HOLD CURRENTS The peak and hold currents are determined by the value of the sense resistor RS The driver IC when initiated by a logic 1 signal at Pin 1 initially drives Darlington transistor Q1 into saturation The injector current will rise exponentially from zero at a rate dependent upon L1 R1 the battery volt- age and the saturation voltage of Q1 The drop across the sense resistor is created by the solenoid current and when this drop reaches the peak threshold level typically 385 mV the IC is tripped from the peak state into the hold state The IC now behaves more as an op amp and drives Q1 within a closed loop system to maintain the hold reference voltage typically 94 mV across RS Once the injector current drops from the peak level to the hold level it remains there for the duration of the input signal at Pin 1 This mode of operation is preferable when working with solenoids since the current required to overcome kinetic and constriction forces is often a factor of four or more times the current necessary to hold the injector open By holding the injector current at one fourth of the peak current power dissipation in the sole- noids and Q1 is reduced by at least the same factor In the circuit of Figure 1 it was known that the type of injec- tor shown opens when the current exceeds 13 amps and closes when the current then falls below 03 amps In order to guarantee injector operation over the life and tempera- ture range of the system a peak current of approximately 4 amps was chosen This led to a value of RS of 01X Divid- ing the peak and hold thresholds by this factor gives peak and hold currents through the solenoid of 385 amps and 094 amps respectively Different types of solenoids may require different values of current The sense resistor RS may be changed accordingly An 8-amp peak injector would use RS equal to 05X etc Note that for large currents above one amp IR drops within the component leads or printed circuit board may create substantial errors unless appropriate care is taken The sense input and sense ground leads (Pins 4 and 5 respec- tively) should be Kelvin connected to RS High current should not be allowed to flow through any part of these traces or connections An easy solution to this problem on double-sided PC boards (without plated-through holes) is to have the high current trace and sense trace attach to the RS lead from opposite sides of the board TIMER FUNCTION The purpose of the timer function is to limit the power dissi- pated by the injector or solenoid under certain conditions Specifically when the battery voltage is low due to engine cranking or just undercharged there may not be sufficient voltage available for the injector to achieve the peak cur- rent In the Figure 2 waveforms under the low battery condi- tion the injector current can be seen to be leveling out at 3 5 |
Podobny numer części - LM1949N |
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Podobny opis - LM1949N |
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