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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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LM2647 Fiches technique(PDF) 10 Page - National Semiconductor (TI) |
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LM2647 Fiches technique(HTML) 10 Page - National Semiconductor (TI) |
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10 / 25 page  Operation Descriptions GENERAL The LM2647 provides two identical synchronously switched buck regulator channels that operate 180˚ out of phase. A voltage-mode control topology was selected to provide fixed- frequency PWM regulation at very low duty cycles, in pref- erence to current-mode control, because the latter has in- herent limitations in being able to achieve low pulse widths due to blanking time requirements. Because of a minimum pulse width of about 30ns for the LM2647, very low duty cycles (low output, high input) are possible. The main ad- vantage of current-mode control is the fact that the slope of its ramp (derived from the switch current), automatically increases with increase in input voltage. This leads to im- proved line rejection and fast response to line variations. In typical voltage-mode control, the ramp is derived from the clock, not from the switch current. But by using the input voltage together with the clock signal to generate the ramp as in the LM2647, this advantage of current-mode control can in fact be completely replicated. The technique is called line feedforward. In addition, the LM2647 features a user- selectable Pulse-skip mode that significantly improves effi- ciency at light loads by reducing switching losses, and driver consumption, both of which are proportional to switching frequency. INPUT VOLTAGE FEEDFORWARD The feedforward circuit of the LM2647 adjusts the slope of the internal PWM ramp in proportion to the regulator input voltage. See Figure 1 for an illustration of how the duty cycle changes as a result of the change in the slope of the ramp, even though the error amplifier output has not had time to react to the line disturbance. The almost instantaneous duty cycle correction provided by the feedforward circuit signifi- cantly improves line transient rejection. FORCED-PWM MODE AND PULSE-SKIP MODE Forced-PWM mode (FPWM) leads to Continuous Conduc- tion Mode (CCM) even at very light loads. It is one of two user-selectable modes of operation provided by the LM2647. When FPWM is chosen (FPWM pin high), the bottom FET will always be turned ON whenever the top FET is OFF. See Figure 2 for a typical FPWM plot. 20056309 FIGURE 1. Voltage Feedforward 20056310 CH1: HDRV, CH2: LDRV, CH3: SW, CH4: IL (0.2A/div) Output 1V @ 0.04A, VIN = 10V, FPWM, L = 10µH, f = 300kHz FIGURE 2. Normal FPWM Mode Operation at Light Loads www.national.com 10 |
Numéro de pièce similaire - LM2647 |
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Description similaire - LM2647 |
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