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LM3578A bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI) |
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LM3578A bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI) |
10 / 20 page Applications Information (Continued) DUTY CYCLE ADJUSTMENT When manual or mechanical selection of the output transis- tor’s duty cycle is needed, the cirucit shown below may be used. The output will turn on with the beginning of each oscillator cycle and turn off when the current sunk by R2 and R3 from the non-inverting terminal becomes greater than the current sunk from the inverting terminal. With the resistor values as shown, R3 can be used to adjust the duty cycle from 0% to 90%. When the sum of R2 and R3 is twice the value of R1, the duty cycle will be about 50%. C1 may be a large electrolytic capacitor to lower the oscillator frequency below 1 Hz. REMOTE SHUTDOWN The LM2578A may be remotely shutdown by sinking a greater current from the non-inverting input than from the inverting input. This may be accomplished by selecting re- sistor R3 to be approximately one-half the value of R1 and R2 in parallel. EMITTER OUTPUT When the LM2578A output transistor is in the OFF state, if the Emitter output swings below the ground pin voltage, the output transistor will turn ON because its base is clamped near ground. The Collector Current with Emitter Output Be- low Ground curve shows the amount of Collector current drawn in this mode, vs temperature and Emitter voltage. When the Collector-Emitter voltage is high, this current will cause high power dissipation in the output transistor and should be avoided. This situation can occur in the high-current high-voltage buck application if the Emitter output is used and the catch diode’s forward voltage drop is greater than 0.6V. A fast- recovery diode can be added in series with the Emitter output to counter the forward voltage drop of the catch diode (see Figure 2). For better efficiency of a high output current buck regulator, an external PNP transistor should be used as shown in Figure 16. SYNCHRONIZING DEVICES When several devices are to be operated at once, their oscillators may be synchronized by the application of an external signal. This drive signal should be a pulse waveform with a minimum pulse width of 2 µs. and an amplitude from 00871121 FIGURE 9. Maximum Duty Cycle Limiting 00871123 FIGURE 10. Duty Cycle Adjustment 00871124 FIGURE 11. Shutdown Occurs when V L is High 00871130 FIGURE 12. D1 Prevents Output Transistor from Improperly Turning ON due to D2’s Forward Voltage www.national.com 10 |
Số phần tương tự - LM3578A |
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Mô tả tương tự - LM3578A |
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