Obsolete Product(s) - Obsolete Product(s)
13/20
L6911C
Where
Where Z
C
(s) and Z
L
(s) are the output capacitor and inductor impedance respectively.
The expression of Z
I
(s) may be simplified as follow:
Where:
τ
1
= R4×C20,
τ
2
= (R4+R3)×C20 and
τ
d
= Rd×C25.
The regulator transfer function became now:
Figure 8 shows a method to select the regulator components (please note that the frequencies f
EC
and f
CC
cor-
responds to the singularities introduced by additional ceramic capacitors in parallel to the output main electro-
lytic capacitor).
■
To obtain a flat frequency response of the output impedance, the droop time constant
τ
d
has to be equal
to the inductor time constant (see the note at the end of the section):
■
To obtain a constant -20dB/dec Gloop(s) shape the singularity f
1
and f
2
are placed in proximity of f
CE
and f
LC
respectively. This implies that:
■
To obtain a Gloop bandwidth of f
C
, results:
Note.
To understand the reason of the previous assumption, the scheme in figure 9 must be considered.
In this scheme, the inductor current has been substituted by the load current, because in the frequencies range
of interest for the Droop function these current are substantially the same and it was supposed that the droop
network don't represent a charge for the inductor.
Gloop s
()
Av s
()
Rs
()⋅
Av s
()
Zf s
()
Zi s
()
--------------
⋅==
Av s
()
Vin
V
osc
∆
----------------
Z
C
s
()
Z
C
s
()
Z
L
s
()+
-------------------------------------
⋅=
Z
I
s
()
Rd
1
s
---
C25
⋅⋅
Rd
1
s
---+
C25
⋅
----------------------------------
R4
1
s
---
C20
⋅+
R3
⋅
R4
1
s
---
C20
⋅+
R3
+
------------------------------------------------------+
Rd 1 s
τ
1
τ
d
+()
s
2
R3
R
d
--------
τ
1
τ
d
⋅⋅⋅+⋅+
1s
τ
2
⋅+()
1s
τ
d
⋅+()⋅
---------------------------------------------------------------------------------------------------
=
==
Rd
1s
R3
R
d
-------- τ
d
⋅+
1s
τ
1
⋅+()⋅
1s
τ
2
⋅+()
1s
τ
d
⋅+()⋅
---------------------------------------------------------------------
=
Rs
()
1s
τ
2
⋅+()
1s
τ
d
⋅+()⋅
sC18R
d
1s
R3
R
d
--------
τ
d
⋅+
1s
τ
1
⋅+()⋅⋅⋅ ⋅
--------------------------------------------------------------------------------------------------------
≈
τ
d
R
d
C25
⋅
L
R
L
------- τ
L
C25
⇒
L
R
L
R
d
⋅()
-----------------------=== =
f
2
f
1
----
f
LC
f
CE
---------
R4
⇒
R3
f
LC
f
CE
---------
1
–
⋅==
f1 f
CE
C20
1
2
--- π
R4 f
CE
⋅⋅⋅=⇒=
G
0
f
LC
⋅ 1f
C
G
0
⇒⋅ A
0
R
0
⋅
VIN
Vosc∆
------------------
C20 // C25
C18
-----------------------------
⋅
f
C
f
LC
--------
C18⇒
VIN
Vosc∆
------------------
C20 C25⋅
C20 C25
+
-----------------------------
f
LC
f
C
--------
⋅⋅
=== ==
Obsolete Product(s) - Obsolete Product(s)
