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Os motores de busca de Datasheet de Componentes eletrônicos |
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Os motores de busca de Datasheet de Componentes eletrônicos |
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LM1949M Folha de dados(PDF) 6 Page - National Semiconductor (TI) |
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LM1949M Folha de dados(HTML) 6 Page - National Semiconductor (TI) |
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6 / 10 page  Timer Function (Continued) amps or 1 amp below the normal threshold Since continu- ous operation at 3 amps may overheat the injectors the timer function on the IC will force the transition into the hold state after one time constant (the time constant is equal to RTCT) The timer is reset at the end of each input pulse For systems where the timer function is not needed it can be disabled by grounding Pin 8 For systems where the initial peak state is not required (ie where the solenoid current rises immediately to the hold level) the timer can be used to disable the peak function This is done by setting the time constant equal to zero (ie CT e 0) Leaving RT in place is recommended The timer will then complete its time-out and disable the peak condition before the solenoid current has had a chance to rise above the hold level The actual range of the timer in injection systems will proba- bly never vary much from the 39 milliseconds shown in Figure 1 However the actual useful range of the timer ex- tends from microseconds to seconds depending on the component values chosen The useful range of RT is ap- proximately 1k to 240k The capacitor CT is limited only by stray capacitances for low values and by leakages for large values The capacitor reset time at the end of each controller pulse is determined by the supply voltage and the capacitor value The IC resets the capacitor to an initial voltage (VBE)by discharging it with a current of approximately 15 mA Thus a 01 mF cap is reset in approximately 25 ms COMPENSATION Compensation of the error amplifier provides stability for the circuit during the hold state External compensation (from Pin 2 to Pin 3) allows each design to be tailored for the characteristics of the system andor type of Darlington pow- er device used In the vast majority of designs the value or type of the compensation capacitor is not critical Values of 100 pF to 01 mF work well with the circuit of Figure 1 The value shown of 01 mF (disc) provides a close optimum in choice between economy speed and noise immunity In some systems increased phase and gain margin may be acquired by bypassing the collector of Q1 to ground with an appropriately rated 01 mF capacitor This is however rarely necessary FLYBACK ZENER The purpose of zener Z1 is twofold Since the load is induc- tive a voltage spike is produced at the collector of Q1 any- time the injector current is reduced This occurs at the peak- to-hold transition (when the current is reduced to one fourth of its peak value) and also at the end of each input pulse (when the current is reduced to zero) The zener provides a current path for the inductive kickback limiting the voltage spike to the zener value and preventing Q1 from damaging voltage levels Thus the rated zener voltage at the system peak current must be less than the guaranteed minimum breakdown of Q1 Also even while Z1 is conducting the majority of the injector current during the peak-to-hold tran- sition (see Figure 4 ) Q1 is operating at the hold current level This fact is easily overlooked and as described in the following text can be corrected if necessary Since the error amplifier in the IC demands 94 mV across RS Q1 will be biased to provide exactly that Thus the safe operating area (SOA) of Q1 must include the hold current with a VCE of Z1 volts For systems where this is not desired the zener an- ode may be reconnected to the top of RS as shown in Fig- ure 5 Since the voltage across the sense resistor now ac- curately portrays the injector current at all times the error TLH5062 – 7 FIGURE 4 Circuit Waveforms amplifier keeps Q1 off until the injector current has decayed to the proper value The disadvantage of this particular con- figuration is that the ungrounded zener is more difficult to heat sink if that becomes necessary The second purpose of Z1 is to provide system transient protection Automotive systems are susceptible to a vast array of voltage transients on the battery line Though their duration is usually only milliseconds long Q1 could suffer permanent damage unless buffered by the injector and Z1 This is one reason why a zener is preferred over a clamp diode back to the battery line the other reason being long decay times TLH5062 – 8 FIGURE 5 Alternate Configuration for Zener Z1 6 |
Nº de peça semelhante - LM1949M |
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Descrição semelhante - LM1949M |
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