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LM2657 Datasheet(PDF) 11 Page - Texas Instruments

Part # LM2657
Description  LM2657 Dual Synchronous Buck Regulator Controller
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Manufacturer  TI [Texas Instruments]
Direct Link  http://www.ti.com
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LM2657 Datasheet(HTML) 11 Page - Texas Instruments

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LM2657
www.ti.com
SNVS342B – JANUARY 2005 – REVISED MARCH 2013
CH1: HDRV, CH2: LDRV, CH3: SW, CH4: IL (0.2A/div)
Output 1V @ 0.04A, VIN = 10V, FPWM, L = 10µH, f = 300kHz
Figure 8. Normal FPWM Mode Operation at Light Loads
In a conventional converter, as the load is decreased to about 10-30% of maximum load current, DCM
(Discontinuous 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 amplifier struggles to maintain regulation. There are two ways to prevent random
pulse skipping from occuring.
One way is to keep the lower FET ON until the start of the next cycle (as in the LM2657 operated in FPWM
mode). This allows the inductor current to drop to zero and then actually reverse direction (negative direction
through inductor, passing from drain to source of lower FET, see Channel 4 in Figure 8). Now the current can
continue to flow continuously until the end of the switching cycle. This maintains CCM and 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 the switching frequency is set high. Let us check the LM2657 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* 3.3µs = 0.14 µs at a switching frequency of 300kHz (T=3.3 µs). Since
140ns exceeds the minimum ON-time of 30ns of the LM2657, normal constant frequency CCM mode of
operation is assured in FPWM mode at virtually any load.
The second way to prevent random pulse skipping in discontinuous mode is 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 VSW_ZERO, see ELECTRICAL CHARACTERISTICS table). This, however, would still amount to
conventional DCM, with its attendant idiosyncrasies at extremely light loads as described earlier. The LM2657
avoids the random skipping behavior and replaces it with a more consistent 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 LM2657, 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 LM2657 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 amplifier 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 circulating currents and energy associated with the FPWM
Copyright © 2005–2013, Texas Instruments Incorporated
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