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參數(shù)資料
| 型號： | LM3477AMM |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | High Efficiency High-Side N-Channel Controller for Switching Regulator |
| 中文描述： | 1 A SWITCHING CONTROLLER, 575 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | MSOP-8 |
| 文件頁數(shù)： | 12/23頁 |
| 文件大?。?/td> | 815K |
| 代理商： | LM3477AMM |
Functional Description
(Continued)
Default/Adjustable Slope Compensation
The LM3477/A uses a current mode control scheme. There
are many advantages in a current mode architecture includ-
ing inherent cycle-by-cycle current limiting and simple com-
pensation of the control loop. However, there are conse-
quences to using current mode control that one must be
aware of while selecting circuit components. One of these
consequences is the inherent possibility of subharmonic os-
cillations in the inductor current. This is a form of instability
and should be avoided.
As a brief explanation, consider
Figure 4
. A lot of information
is shown here. The top portion shows a schematic of the
current sensing loop. The bottom portion shows the pulse
width modulation (PWM) comparator waveforms for two
switching cycles. The two solid waveforms shown are the
waveforms compared at the internal pulse width modulator,
used to generate the MOSFET drive signal. The top wave-
form with the slope S
is the internally generated control
waveform V
. The bottom waveform with slopes S
and S
is
the sensed inductor current waveform V
. These signals
are compared at the PWM comparator. There is a feedback
loop involved here. The inductor current is sensed and fed
back to the PWM comparator, where it is compared to V
C
.
The output of the comparator in combination with the R/S
latch determine if the MOSFET is on or off, which effectively
controls the amount of current the inductor receives. While
V
C
is higher than V
SEN
, the PWM comparator outputs a high
signal, driving the external power MOSFET on. When MOS-
FET is on, the inductor current rises at a constant slope,
generating the sensed voltage V
. When V
equals V
C
,
the PWM comparator signals to drive the MOSFET off, and
the sensed inductor current decreases with a slope S
. The
process begins again when R
S
latch is set by an internal
oscillator.
The subharmonic oscillation phenomenon is realized when a
load excursion is experienced. The way it is analyzed is to
calculate how the inductor current settles after such an
excursion. Take for example the case when the inductor
current experiences a step increase in its average current,
shown as the dotted line in
Figure 4
. In the switching period
that the excursion occurs, the inductor current will change by
I
. In the following switching period, the inductor current will
have a difference
I
from its original starting value. The
original excursion is being propagated each switching cycle.
What is desired is to find out if this propagation is converging
or diverging. It is apparent that the difference in the inductor
current from one cycle to the next is a function of S
n
, S
f
, and
S
e
, as follows:
Hence, if the quantity (S
f
- S
e
)/(S
n
+ S
e
)is greater than 1, the
inductor current diverges and subharmonic oscillations re-
sult. Notice that as S
e
increases, the factor decreases. Also,
when the duty cycle is greater than 50%, as the inductance
become less, the factor increases.
The LM3477/A internally generates enough slope compen-
sation S
e
to allow for the use of reasonable inductances. The
height of the compensation slope ramp V
SL
can be found in
the
ELECTRICAL
CHARACTERISTICS
LM3477/A incorporates a patented scheme to increase S
if
there is need to use a smaller inductor. With the use of a
single resistor R
, Se can be increased indefinitely. R
SL
increases the compensation slope Se by the amount:
section.
The
Therefore,
When excursions of the inductor current are divergent, the
current sensing control loop is unstable and produces a
subharmonic oscillation in the inductor current. This oscilla-
tion is viewed as a resonance in the outer voltage control
loop at half the switching frequency. In the inductor section,
200033J6
FIGURE 3. The Feedback Voltage Experiences an
Oscillation if the Input Voltage Crosses the 7.2V
Internal Bias Threshold
200033C2
FIGURE 4. The Current Sensing Loop and
Corresponding Waveforms
L
www.national.com
12
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| LM3477AMM/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(fēng)格: 封裝 / 箱體:CPAK |
| LM3477AMMX | 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(fēng)格: 封裝 / 箱體:CPAK |
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