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LM2657MTC bảng dữ liệu(PDF) 11 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
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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. 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, pass-
ing from drain to source of lower FET, see Channel 4 in
Figure 2). 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 de-
scribed 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 switch-
ing 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 discon-
tinuous 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 V
SW-
_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 re-
places 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 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. When this happens the
inductor core is seeing only very mild flux excursions, and no
significant audible noise is created. But if EMI is a particu-
larly sensitive issue for the particular application, the user
can simply opt for the slightly less efficient, constant fre-
quency FPWM mode.
The SKIP mode is enabled when the FPWM pin is held low
(or left floating). At higher loads, and under steady state
conditions (above CCM-DCM boundary), there will be abso-
lutely no difference in the behavior of the LM2657 or the
associated converter waveforms based on the voltage ap-
plied on the FPWM pin. The differences show up only at light
loads.
Also, under startup, since the currents are high until the
output capacitors have charged up, there will be no observ-
able 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 operating 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.
20134711
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
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