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LM2578 数据表(PDF) 9 Page - National Semiconductor (TI) |
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LM2578 数据表(HTML) 9 Page - National Semiconductor (TI) |
9 / 18 page Applications Information (Continued) REMOTE SHUTDOWN The LM1578A may be remotely shutdown by sinking a greater current from the non-inverting input than from the in- verting input. This may be accomplished by selecting resistor R3 to be approximately one-half the value of R1 and R2 in parallel. EMITTER OUTPUT When the LM1578A 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 oscil- lators 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 1.5V to 2.0V. The signal source must be capable of 1.) driving ca- pacitive loads and 2.) delivering up to 500 µA for each LM1578A. Capacitors C1 thru CN are to be selected for a 20% slower frequency than the synchronization frequency. Typical Applications The LM1578A may be operated in either the continuous or the discontinuous conduction mode. The following applica- tions (except for the Buck-Boost Regulator) are designed for continuous conduction operation. That is, the inductor cur- rent is not allowed to fall to zero. This mode of operation has higher efficiency and lower EMI characteristics than the dis- continuous mode. BUCK REGULATOR The buck configuration is used to step an input voltage down to a lower level. Transistor Q1 in Figure 14 chops the input DC voltage into a squarewave. This squarewave is then con- verted back into a DC voltage of lower magnitude by the low pass filter consisting of L1 and C1. The duty cycle, D, of the squarewave relates the output voltage to the input voltage by the following equation: V out = DxVin = Vin x(ton)/(ton +toff). DS008711-23 FIGURE 10. Duty Cycle Adjustment DS008711-24 FIGURE 11. Shutdown Occurs when V L is High DS008711-30 FIGURE 12. D1 Prevents Output Transistor from Improperly Turning ON due to D2’s Forward Voltage DS008711-25 FIGURE 13. Synchronizing Devices 9 www.national.com |
类似零件编号 - LM2578 |
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类似说明 - LM2578 |
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