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LM2647MTCX Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM2647MTCX Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 25 page Operation Descriptions (Continued) In a conventional converter, as the load is decreased to about 10-30% of maximum load current, DCM (Discontinu- ous Conduction Mode) occurs. In this condition the inductor current falls to zero during the OFF-time, and stays there until the start of the next switching cycle. In this mode, if the load is decreased further, the duty cycle decreases (pinches off), and ultimately may decrease to the point where the required pulse width becomes less than the minimum ON- time achievable by the converter (controller + FETs). Then a sort of random skipping behavior occurs as the error ampli- fier struggles to maintain regulation. This is not the most desirable type of behavior. There are two ways out of this problem. One way is to keep the lower FET ON until the start of the next cycle (as in the LM2647 operated in FPWM mode). This allows the inductor current to drop to zero and then actually reverse direction (negative direction through inductor, pass- ing from Drain to Source of lower FET, see Channel 4 in Figure 2). Now the current can continue to flow continuously till the end of the switching cycle. This maintains CCM and so the duty cycle does not start to pinch off as in typical DCM. Nor does it lead to the undesirable random skipping described above. Note that the pulse width (duty cycle) for CCM is virtually constant for any load and therefore does not usually run into the minimum ON-time restriction. But it can happen, especially when the application consists of a very high input voltage, a low output voltage rail, and also the switching frequency is set high. Let us check the LM2647 to rule out this remote possibility. For example, with an input of 24V, an output of 1V, the duty cycle is 1/24 = 4.2%. This leads to a required ON-time of 0.042* 3.3 = 0.14 µs at a switching frequency of 300kHz (T=3.3 µs). Since 140ns exceeds the minimum ON-time of 30ns of the LM2647, normal constant frequency CCM mode of operation is as- sured in FPWM mode, at virtually any load. The second way out of the problems of discontinuous mode is the second operating mode of the LM2647, the Pulse-skip (SKIP) Mode. In SKIP Mode, a zero-cross detector at the SW pin turns off the bottom FET when the inductor current decays to zero (actually at V SW_ZERO, see Electrical Char- acteristics table). This would however still amount to conven- tional DCM, with its attendant problems at extremely light loads as described earlier. The LM2647 however avoids the random skipping behavior described earlier, and replaces it with a more defined or formal SKIP mode. In conventional DCM, a converter would try to reduce its duty cycle from the CCM value as the load decreases, as explained previously. So it would start with the CCM duty cycle value (at the CCM-DCM boundary), but as the load decreases, the duty cycle would try to shrink to zero. However, in the LM2647, the DCM duty cycle is not allowed to fall below 85% of the CCM value. So when the theoretically required DCM duty cycle value falls below what the LM2647 is allowed to deliver (in this mode), pulse-skipping starts. It will be seen that several of these excess pulses may be delivered, until the output capacitors charge up enough to notify the error am- plifier and cause its output to reverse. Thereafter several pulses could be skipped entirely until the output of the error amplifier again reverses. The SKIP mode therefore leads to a reduction in the average switching frequency. Switching losses and FET driver losses, both of which are proportional to frequency, are significantly reduced at very light loads and efficiency is boosted. SKIP mode also reduces the circulat- ing currents and energy associated with the FPWM mode. See Figure 3 for a typical plot of SKIP mode at very light loads. Note the bunching of several fixed-width pulses fol- lowed by skipped pulses. The average frequency can actu- ally fall very low at very light loads. Note however that when this happens the inductor core is seeing only very mild flux excursions, and so no significant audible noise is created. But if EMI is a particularly sensitive issue for the particular application, the user can simply opt for the slightly less efficient, though constant frequency FPWM mode. The SKIP mode is enabled when the FPWM pin is held low (or left floating). Note that at higher loads, and under steady state conditions (above CCM-DCM boundary), there will be absolutely no difference in the behavior of the LM2647 or the associated converter waveforms based on the voltage ap- plied on the FPWM pin. The differences show up only at light loads. Under startup too, since the currents are high until the output capacitors have charged up, there will be no observable difference in the shape of the ramp-up of the output rails in either SKIP mode or FPWM mode. The design has thus forced the startup waveforms to be identical irrespective of whether the FPWM mode or the SKIP mode has been selected. The designer must realize that even at zero load condition, there is circulating current when operated in FPWM mode. This is illustrated in Figure 4. Since duty cycle is the same as for conventional CCM, fromV=L* ∆I/ ∆t it can be seen that ∆I (or Ipp in Figure 4) must remain constant for any load, including zero. At zero load, the average current through the inductor is zero, so the geometric center of the sawtooth waveform (the center being always equal to load current) is along the x-axis. At critical conduction (boundary between conventional CCM and what should have been DCM were it not in FPWM mode), the load current is equal to Ipp/2. Note that excessively low values of inductance will produce much higher current ripple and this will lead to higher circulating currents and dissipation. 20056311 CH1: HDRV, CH2: LDRV, CH3: SW, CH4: IL (0.2A/div) Output 1V @ 0.04A, VIN = 10V, SKIP, L = 10µH, f = 300kHz FIGURE 3. Normal SKIP Mode Operation at Light Loads www.national.com 11 |
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