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參數(shù)資料
| 型號： | LM27241 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Synchronous Buck Regulator Controller for Mobile Systems |
| 中文描述： | 同步降壓穩(wěn)壓控制器，用于移動通信系統(tǒng) |
| 文件頁數(shù)： | 11/27頁 |
| 文件大?。?/td> | 1336K |
| 代理商： | LM27241 |
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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 (Discon-
tinuous Conduction Mode) occurs. In this condition the in-
ductor 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 mini-
mum ON-time achievable by the converter (controller +
FETs). Then a sort of random skipping behavior occurs as
the error amplifier struggles to maintain regulation. This is
not the most desirable type of behavior. There are two
modes of operation that address this issue.
The first method is to keep the lower FET ON until the start
of the next cycle (as in the LM27241 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 2
). Now the current can continue to flow
continuously till the end of the switching cycle. This main-
tains CCM and therefore the duty cycle does not start to
pinch off as in typical DCM. Nor does it lead to the undesir-
able 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. The LM27241 can exhibit skip pulsing in
FPWM when the Duty-Cycle is very low, and the switching
frequency is high (minimum ON-Time). Let us check the
LM27241 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
LM27241, normal constant frequency CCM mode of opera-
tion is assured in FPWM mode, at virtually any load.
Another way out of the problematic spontaneous DCM op-
eration is the second operating mode of the LM27241, the
Pulse-skip (SKIP) Mode. In SKIP Mode, a zero-cross detec-
tor 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 would however still amount to
conventional DCM, with its associated problems at ex-
tremely light loads as described earlier. The LM27241 how-
ever 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 ex-
plained previously. So it would start with the CCM duty cycle
value (at the CCM-DCM boundary), but as the load de-
creases, the duty cycle would try to shrink to zero. However,
in the LM27241, 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
LM27241 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
leads to a reduction in the average switching frequency.
Switching losses and FET driver losses, both of which are
proportional to switching 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 mode. See
Figure 3
for a typical plot of SKIP
mode at very light loads. Note the bunching of several
fixed-width pulses followed by skipped pulses. The average
frequency can actually 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. 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 LM27241 or
the associated converter waveforms based on the voltage
applied on the FPWM pin. The differences show up only at
light loads.
Under startup, the peak current through the inductor is
higher than the steady state peak current. This is due to the
output capacitors being charged up to the regulated output
voltage. 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 wave-
forms 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
. Duty cycle remains fairly con-
stant in CCM, therefore V = L x
I /
t . It can be seen that I
(or I
pp
in
Figure 4
) must remain constant for any load,
including zero load current. 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 power dissipation.
20120111
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
www.national.com
11
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM27241MTC | 制造商:Texas Instruments 功能描述:PWM CONTROLLER SMD 27241 TSSOP20 |
| LM27241MTC/NOPB | 功能描述:IC REG CTRLR BUCK PWM VM 20TSSOP RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標(biāo)準(zhǔn)包裝:4,000 系列:- PWM 型:電壓模式 輸出數(shù):1 頻率 - 最大:1.5MHz 占空比:66.7% 電源電壓:4.75 V ~ 5.25 V 降壓:是 升壓:無 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-40°C ~ 85°C 封裝/外殼:40-VFQFN 裸露焊盤 包裝:帶卷 (TR) |
| LM27241MTCX | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Synchronous Buck Regulator Controller for Mobile Systems |
| LM27241MTCX/NOPB | 功能描述:IC REG CTRLR BUCK PWM VM 20TSSOP RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標(biāo)準(zhǔn)包裝:2,500 系列:- PWM 型:電流模式 輸出數(shù):1 頻率 - 最大:500kHz 占空比:100% 電源電壓:8.2 V ~ 30 V 降壓:無 升壓:無 回掃:是 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:是 工作溫度:0°C ~ 70°C 封裝/外殼:8-DIP（0.300"，7.62mm） 包裝:管件 產(chǎn)品目錄頁面:1316 (CN2011-ZH PDF) |
| LM27243A E WAF | 制造商:Texas Instruments 功能描述: |
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