поискавой системы для электроныых деталей |
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LM2722 датащи(PDF) 7 Page - National Semiconductor (TI) |
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LM2722 датащи(HTML) 7 Page - National Semiconductor (TI) |
7 / 8 page Typical Waveforms Application Information Minimum Pulse Width In order for the shoot-through prevention circuitry in the LM2722 to work properly, the pulses into the PWM_IN pin must be longer than 55ns. The internal logic waits until the first FET is off plus 20ns before turning on the opposite FET. If, after a falling edge, a rising edge occurs sooner than the specified time, t off_min, the IC may intermittently fail to turn on the top gate when the bottom gate is off. As the rising edge occurs sooner and sooner, the driver may start to ignore the pulse and produce no output. This condition results in the PWM_IN pin in a high state and neither FET turned on. To get out of this state, the PWM_IN pin must see a low signal for greater than 55ns, before the rising edge. This will also assure that the gate drive bias voltage has been restored by forcing the top FET source and C boot to ground first. Then the internal circuitry is reset and normal operation will resume. Conversely, if, after a rising edge, a falling edge occurs sooner than the specified miniumum pulse width, t on_min, the IC may intermittently fail to turn on the bottom FET. As the falling edge occurs sooner and sooner, the driver will start to ignore the pulse and produce no output. This will result in the t off inductor current taking a path through a diode provided for non-synchronous operation. The circuit will resume syn- chronous operation when the rising PWM pulses exceed 55ns in duration. High Input Voltages or High Output Currents At input voltages above twice the output voltage and at higher power levels, the designer may find snubber networks and gate drive limiting useful in reducing EMI and preventing injurious transients. A small resistor, 1 Ω to 5Ω, between the driver outputs and the MOSFET gates will slightly increase the rise time and fall time of the output stage and reduce switching noise. The trade-off is 1% to 2% in efficiency. A series R-C snubber across in parallel with the bottom FET can also be used to reduce ringing. Values of 10nF and 10 Ω to 100 Ω are a good starting point. 20028913 FIGURE 1. Switching Waveforms of Test Circuit 20028914 FIGURE 2. When Input Goes High 20028915 FIGURE 3. When Input Goes Low 20028916 FIGURE 4. Minimum Positive Pulse www.national.com 7 |
Аналогичный номер детали - LM2722 |
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Аналогичное описание - LM2722 |
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