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LM2618 Arkusz danych(PDF) 10 Page - National Semiconductor (TI) |
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LM2618 Arkusz danych(HTML) 10 Page - National Semiconductor (TI) |
10 / 15 page Circuit Operation (Continued) PWM Operation The LM2618 can be set to current-mode PWM operation by connecting the SYNC/MODE pin to VDD. While in PWM (Pulse Width Modulation) mode, the output voltage is regu- lated by switching at a constant frequency and then modu- lating the energy per cycle to control power to the load. Energy per cycle is set by modulating the PFET switch on-time pulse-width to control the peak inductor current. This is done by controlling the PFET switch using a flip-flop driven by an oscillator and a comparator that compares a ramp from the current-sense amplifier with an error signal from a voltage-feedback error amplifier. At the beginning of each cycle, the oscillator sets the flip-flop and turns on the PFET switch, causing the inductor current to ramp up. When the current sense signal ramps past the error amplifier signal, the PWM comparator resets the flip-flop and turns off the PFET switch, ending the first part of the cycle. The NFET synchronous rectifier turns on until the next clock pulse or the inductor current ramps to zero. If an increase in load pulls the output voltage down, the error amplifier output increases, which allows the inductor current to ramp higher before the comparator turns off the PFET switch. This in- creases the average current sent to the output and adjusts for the increase in the load. Before going to the PWM comparator, the current sense signal is summed with a slope compensation ramp from the oscillator for stability of the current feedback loop. During the second part of the cycle, a zero crossing detector turns off the NFET synchronous rectifier if the inductor current ramps to zero. 20036401 FIGURE 2. Simplified Functional Diagram PWM Mode Switching Waveform PFM Mode Switching Waveform 20036410 20036411 FIGURE 3. www.national.com 10 |
Podobny numer części - LM2618 |
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Podobny opis - LM2618 |
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