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參數(shù)資料
| 型號： | LM2647LQX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Dual Synchronous Buck Regulator Controller |
| 中文描述： | DUAL SWITCHING CONTROLLER, 345 kHz SWITCHING FREQ-MAX, QCC28 |
| 封裝： | LLP-28 |
| 文件頁數(shù)： | 11/25頁 |
| 文件大?。?/td> | 763K |
| 代理商： | LM2647LQX |
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, from V = 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: I
L
(0.2A/div)
Output 1V
@
0.04A, VIN = 10V, SKIP, L = 10μH, f = 300kHz
FIGURE 3. Normal SKIP Mode Operation at Light
Loads
L
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM2647LQX/NOPB | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
| LM2647MTC | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
| LM2647MTC/NOPB | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
| LM2647MTCX | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
| LM2647MTCX/NOPB | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風格: 封裝 / 箱體:CPAK |
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