參數(shù)資料
型號: L6917D
英文描述: Analog IC
中文描述: 模擬IC
文件頁數(shù): 19/27頁
文件大小: 411K
代理商: L6917D
19/27
L6917
Figure 12. Current loop Gain (left) and Voltage loop Gain (right)
The R
F
C
F
network may be designed considering the desired crossover frequency
ω
TI
as follow:
I
Assuming that
ω
F
=
ω
LC
, and that Z
F
=R
F
if
ω
>
ω
F
, it can be observed that:
I
Given the cross-over frequency
ω
TI
, it results:
I
Since
ω
F
=
ω
LC
:
Since the device works in current mode and then the control loop acts to control the current, the highest is
ω
TI
,
the fastest is the device to react after a load transient. The placement of this singularity must consider the worst
case for the load and precisely the maximum output current (minimum output resistance R
OUT
).
The voltage loop gain may be expressed by the following relationship:
Assuming that G
LOOP
I>>1 and after substitution and simplification the final expression for GLOOPV is given by:
1
s ESR C
+
1
s R
OUT
C
OUT
+
Where the additional pole placed at the current loop
ω
TI
must be inserted to consider that the current loop gain
is not always G
LOOPI
>>1. The LC resonance disappears thanks to the average current mode control and the
system is automatically stable if ESR is small enough.
Since all the above modeling are valid at frequencies much lower than the switching frequency, the highest is
this, the highest may be the converter’s loop bandwidth (both current and voltage). In this way the converter is
able to fast react after a load transient following, with the current delivered by the inductors, the current required
by the load minimizing the number of the output capacitor required.
The average current mode compensation network is then designed as follow:
ω
OUT
ω
TI
ω
ESR
ω
dB
ω
TV
FB
R
Rs
Rg
Ro
ω
OUT
ω
F
ω
LC
ω
dB
Z
F
Ac
G
LOOPI
ω
TI
G
LOOPI
ω
=
ω
LC
(
)
V
R
R
Ro
Vosc
Rg
-------------------------------------------
ω
ω
O
----------
=
R
F
Rs
-------
-------------------------------
Vin
ω
ω
ω
LC
2
--------------------
Rs
-------
-------------------------------
Vin
L C
ω
Ro C
OUT
-------------------------------------
-------------------------------------
Rs
-------
Vin
------------------
2
ω
TI
----------------
=
=
=
C
F
C
OUT
R
F
L
2
--
---------------------------
=
G
LOOPV
s
( )
V
V
COMP
-------------------
V
V
OUT
-------------------
Av s
1
G
LOOPI
+
------------------
-------------------------------
1
Z
s
( )
R
FB
---------------
=
=
G
LOOPV
s
( )
...
{
}
----------------------------------------------------
R
Rg
Rs R
FB
----------------------------
1
TI
--------
+
-------------------
=
=
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