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參數(shù)資料
| 型號： | LM3477AMMX |
| 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)： | 21/23頁 |
| 文件大小： | 815K |
| 代理商： | LM3477AMMX |
Compensation
(Continued)
Choosing C
C1
= 88x10
9
F will set f
Z2
= f
P1
, canceling out
the power pole and insuring a 20dB/decade slope in the
low frequency magnitude response. In other words, the
phase margin below the crossover frequency will always be
higher than the phase margin at the crossover frequency.
If better transient response times are desired, a second
method is to set f
between f
and
1
2
decade before f
, the
target crossover frequency. This trades more low frequency
gain for less phase margin, which translates to faster but
more oscillatory step responses. We pick C
C1
= 49nF (use
47nF).
If the esr zero of the output capacitor (f
) is too low or if
more phase margin is required, additional components may
be added to increase the flexibility of the compensator.
Use C
C2
if f
ESR
<
1
2
f
S
, that is if:
For this example, f
ESR
= 159 kHz, so use C
C2
.
The equations used here for R
C
, C
, and C
are approxi-
mations valid when C
<<
C
. For exact equations, see
Plotting Open Loop Response earlier in this section. In some
cases, the desired inductance is several times higher than
the optimal inductance set by the internal slope compensa-
tion. This results in a Q lower than 0.15, in which case
additional methods of compensating are presented (see
SAMPLING POLE QUALITY FACTOR
section).
Sampling Pole Quality Factor
In a current mode control architecture, there is an inherent
resonace at half the switching frequency. The LM3477/A
internally compensates for this by adding a negative slope to
the PWM control waveform (see
DEFAULT/ADJUSTABLE
SLOPE COMPENSATION
section). The factor in the power
stage equations above, Q, describes how much resonance
will be observed. Q is a function of duty cycle and m
.
Figure
16
shows how the power stage bode plot is affected as Q is
varied from 0.01 to 10. The resonance is caused by two
complex poles at half the switching frequency. If m
is too
low, the resonant peaking could become severe coinciding
with subharmonic oscillations in the inductor current. If m
c
is
too high, the two complex poles split and the converter
begins to act like a voltage mode converter and the compen-
sation scheme used above should be changed.
If Q
>
2, the sampling poles are imaginary and are approach-
ing the right half of the imaginary plane (the system is
becoming unstable). In this case, Q must be decreased by
either increasing the inductance, or more preferably, adding
more slope compensation through the R
resistor (see
DEFAULT/ADJUSTABLE SLOPE COMPENSATION
sec-
tion).
If Q
<
0.15, it means that one of the sampling poles is de-
creasing in frequency towards the dominant power pole, f
p1
.
There are three ways to compensate for this. Decrease the
crossover frequency, add a phase lead network, or use the
output capacitor’s ESR to cancel out the low frequency
sampling pole.
One option is to decrease the crossover frequency so that
the phase margin is not as severely decreased by the sam-
pling pole. Decreasing the crossover frequency to between
1kHz to 10kHz is advisable here. As a result, there will be a
decrease in transient response performance.
Another option is the use of the feed-forward capacitor, Cff.
This will provide a positive phase shift (lead) which can be
used to increase phase margin. However, it is important to
note that the effectiveness of Cff decreases with output
voltage. This is due to the fact that the frequencies of the
zero f
and pole f
pff
get closer together as the output voltage
is reduced.
The frequency of the feed-forward zero and pole are:
200033J3
FIGURE 15. Open Loop Frequency Response for
LM3477 Compensation Design Example
200033J5
FIGURE 16. The Quality Factor Q of the Two Complex
Poles is used to qualify how much resonant peaking is
observed in the Power Stage Bode Plot
L
www.national.com
21
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相關代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM3477AMMX/NOPB | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 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 |
| LM3477EVAL | 功能描述:電源管理IC開發(fā)工具 EVAL KIT FOR LM3477 RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
| LM3477MM | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 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 |
| LM3477MM | 制造商:Texas Instruments 功能描述:SWITCH CONTROLLER SMD 3477 MSOP8 |
| LM3477MM/NOPB | 功能描述:DC/DC 開關控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關頻率: 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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