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LM1949M датащи(PDF) 7 Page - National Semiconductor (TI) |
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LM1949M датащи(HTML) 7 Page - National Semiconductor (TI) |
7 / 10 page POWER DISSIPATION The power dissipation of the system shown in Figure 1 is dependent upon several external factors including the fre- quency and duty cycle of the input waveform to Pin 1 Cal- culations are made more difficult since there are many dis- continuities and breakpoints in the power waveforms of the various components most notably at the peak-to-hold tran- sition Some generalizations can be made for normal opera- tion For example in a typical cycle of operation the majori- ty of dissipation occurs during the hold state The hold state is usually much longer than the peak state and in the peak state nearly all power is stored as energy in the magnetic field of the injector later to be dumped mostly through the zener While this assumption is less accurate in the case of low battery voltage it nevertheless gives an unexpectedly accurate set of approximations for general operation The following nomenclature refers to Figure 1 Typical val- ues are given in parentheses RS e Sense Resistor (01X) VH e Sense Input Hold Voltage (094V) Vp e Sense Input Peak Voltage (385V) VZ e Z1 Zener Breakdown Voltage (33V) VBATT e Battery Voltage (14V) L1 e Injector Inductance (002H) R1 e Injector Resistance (1X) n e Duty Cycle of Input Voltage of Pin 1 (0 to 1) f e Frequency of Input (10Hz to 200Hz) Q1 Power Dissipation PQ n VBATT VH RS Watts Zener Dissipation PZ VZ L1 f (VP2 a VH2) ((VZ-VBATT) RS2) Watts Injector Dissipation PI n R1 VH2 RS2 Watts Sense Resistor PR n VH2 RS2 Watts PR (worst case) n VP2 RS2 Watts SWITCHING INJECTOR DRIVER CIRCUIT The power dissipation of the system and especially of Q1 can be reduced by employing a switching injector driver cir- cuit Since the injector load is mainly inductive transistor Q1 can be rapidly switched on and off in a manner similar to switching regulators The solenoid inductance will naturally integrate the voltage to produce the required injector cur- rent while the power consumed by Q1 will be reduced A note of caution The large amplitude switching voltages that are present on the injector can and do generate a tremen- dous amount of radio frequency interference (RFI) Because of this switching circuits are not recommended The extra cost of shielding can easily exceed the savings of reduced power In systems where switching circuits are mandatory extensive field testing is required to guarantee that RFI can- not create problems with engine control or entertainment equipment within the vicinity The LM1949 can be easily modified to function as a switch- er Accomplished with the circuit of Figure 7 the only addi- tional components required are two external resistors RA and RB Additionally the zener needs to be reconnected as shown to RS The amount of ripple on the hold current is easily controlled by the resistor ratio of RA to RB RB is kept small so that sense input bias current (typically 03 mA) has negligible effect on VH Duty cycle and frequency of oscilla- tion during the hold state are dependent on the injector characteristics RA RB and the zener voltage as shown in the following equations Hold Current VH RS Minimum Hold Current VHbRBR A VZ J RS Ripple or DI Hold RB RA VZ 1 RS fo RS L1 RA RB VBATT VZ 1bVBATT VZ J fo e Hold State Oscillation Frequency Duty Cycle of fo VBATT VZ Component Power Dissipation PQ n 1bVBATT VZ J VSAT RS VH VSAT e Q1 Saturation Volt E 1 Amp (15V) PZ n VBATT VH RS PRA VB VZ R1 As shown the power dissipation by Q1 in this manner is substantially reduced Measurements made with a thermo- couple on the bench indicated better than a fourfold reduc- tion in power in Q1 However the power dissipation of the zener (which is independent of the zener voltage chosen) is increased over the circuit of Figure 1 TLH5062 – 9 FIGURE 6 Switching Waveforms 7 |
Аналогичный номер детали - LM1949M |
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Аналогичное описание - LM1949M |
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