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參數(shù)資料
| 型號： | LM2744 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller with External Reference |
| 中文描述： | 低壓N溝道MOSFET同步降壓穩(wěn)壓控制器與外部參考 |
| 文件頁數(shù)： | 17/22頁 |
| 文件大?。?/td> | 663K |
| 代理商： | LM2744 |
Application Information
(Continued)
point is one-fifth of the switching frequency, or 60kHz for this
example. The generic equation for the error amplifier transfer
function is:
In this equation the variableA
is a ratio of the values of the
capacitance and resistance of the compensation compo-
nents, arranged as shown in
Figure 12
. A
is selected to
provide the desired bandwidth. A starting value of 80,000 for
A
should give a conservative bandwidth. Increasing the
value will increase the bandwidth, but will also decrease
phase margin. Designs with 45-60 are usually best because
they represent a good tradeoff between bandwidth and
phase margin. In general, phase margin is lowest and gain
highest (worst-case) for maximum input voltage and mini-
mum output current. One method to select A
EA
is to use an
iterative process beginning with these worst-case conditions.
1.
Increase A
EA
2.
Check overall bandwidth and phase margin
3.
Change V
IN
to minimum and recheck overall bandwidth
and phase margin
4.
Change I
to maximum and recheck overall bandwidth
and phase margin
The process ends when the both bandwidth and the phase
margin are sufficiently high. For this example input voltage
can vary from 3.0 to 3.6V and output current can vary from 0
to 4A, and after a few iterations a moderate gain factor of
110,000 is used.
The error amplifier of the LM2744 has a unity-gain band-
width of 9MHz. In order to model the effect of this limitation,
the open-loop gain can be calculated as:
The new error amplifier transfer function that takes into
account unity-gain bandwidth is:
The gain and phase of the error amplifier are shown in
Figure 13
.
In VM regulators, the top feedback resistor R
FB2
forms a part
of the compensation. Setting R
to 10k
±
1% usually
gives values for the other compensation resistors and ca-
pacitors that fall within a reasonable range. (Capacitances
>
1pF, resistances
<
1M
) C
, C
, C
, R
, and R
are
selected to provide the poles and zeroes at the desired
frequencies, using the following equations:
20106074
20106075
FIGURE 13. Error Amp Gain and Phase
L
www.national.com
17
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