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參數資料
| 型號： | LM3075 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | High Efficiency, Synchronous Current Mode Buck Controller |
| 中文描述： | 高效率，電流模式同步降壓控制器 |
| 文件頁數： | 16/18頁 |
| 文件大小： | 857K |
| 代理商： | LM3075 |
Loop Compensation
(Continued)
The control-output corner frequencies, and thus the desired
compensation corner frequencies, can be determined ap-
proximately by the following equations:
(20)
(21)
Since f
p
is determined by the output network, it shifts with
loading (R
o
) and duty cycle. First determine the range of
frequencies (f
) of the pole across the expected load
range and then place the first compensation zero within that
range.
Example
: R
= 20m
, C
o
= 100uF, R
omax
= 5V/
100mA=50
, R
omin
= 5V/5A = 1
, L = 8 μH.
(22)
(23)
(24)
Once the f
p
range is determined, R
c1
should be calculated
using:
(25)
Where B is the desired gain in V/V at f
(f
z1
), gm is the
transconductance of the error amplifier, and R
1
and R
2
are
the feedback resistors.
A gain value around 10dB (3.3V/V) is generally a good
starting point.
Example
: B = 3.3 V/V, gm = 0.650 μmho, R
1
= 20 k
, R
2
=
60.4 k
:
(26)
Bandwidth varies proportionally to the value of R
c1
. Next,
C
c1
can be determined with the following equation :
(27)
Example
: fpmin = 313Hz, R
c1
= 20k
:
(28)
The value of C
should be within the range determined by
f
. A higher value generally provides a more stable
loop, but too high a value slows the transient response time.
The compensation network also introduces a low frequency
pole that is close to 0Hz. A second pole should also be
placed at f
. This pole can be created with a single capacitor
C
and a shorted R
(see Figure 9). The minimum value for
this capacitor can be calculated by:
(29)
C
c2
may not be necessary, however it does create a more
stable control loop. This is especially important with high
load currents and in current sharing mode.
Example: f
z
= 36kHz, R
c1
= 20k
:
20162331
FIGURE 6. Control-Output Transfer Function
20162332
FIGURE 7. Output-Control Transfer Function
L
www.national.com
16
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