L6911C View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6911C' PDF : 20 Pages View PDF
L6911C
G loo p(s) = Av(s) ⋅ R( s) = Av(s) ⋅ Z-Z----fi--((--ss---)-) Where Av(s) = ∆----VV----io--n-s---c- ⋅ Z----C----(---sZ---)-C---+-(--s--Z--)-L----(--s---)-
Where ZC(s) and ZL(s) are the output capacitor and inductor impedance respectively.
The expression of ZI(s) may be simplified as follow:
ZI(s)
=
-R-----d-----⋅---1-s------⋅---C-----2---5---
+
R
4
+
1-s-
⋅
C 20
⋅
R3
-----------------------------------------------------
Rd + 1-s- ⋅ C25 R 4 + 1-s- ⋅ C20 + R3
=
R-----d-------1-----+---(-s-1----⋅-+--(--τ-s--1---⋅-+--τ---2τ---)d---⋅)---(-+-1----s-+--2---s-⋅---R--R⋅------τ--3d----d---)⋅---τ---1----⋅---τ---d----
=
= R d ---1--(---1+----+-s---R--Rs--------3-d-⋅----τ-⋅--2-τ--)-d--⋅---(--⋅-1--(--1-+----+s-----⋅s---τ--⋅-d--τ-)-1----)
Where: τ1 = R4×C20, τ2 = (R4+R3)×C20 and τd = Rd×C25.
The regulator transfer function became now:
R(
s)
≈
-----------------------(--1-----+-----s----⋅----τ--2---)----⋅---(--1-----+-----s-----⋅---τ--d----)----------------------
s
⋅
C 18
⋅
Rd
⋅
1
+
s
-R----3--
Rd
⋅
τd
⋅
(1
+
s
⋅
τ1)
Figure 8 shows a method to select the regulator components (please note that the frequencies fEC and fCC cor-
responds to the singularities introduced by additional ceramic capacitors in parallel to the output main electro-
lytic capacitor).
s To obtain a flat frequency response of the output impedance, the droop time constant τd has to be equal
to the inductor time constant (see the note at the end of the section):
τd
=
Rd ⋅ C 25
=
--L----
RL
=
τL
⇒ C25 = (---R----L----L-⋅---R----d---)-
s To obtain a constant -20dB/dec Gloop(s) shape the singularity f1 and f2 are placed in proximity of fCE
and fLC respectively. This implies that:
f--2-
f1
=
f-f--CL---CE--
⇒
R4
=
R3
⋅
-f--L---C--
fCE
–
1
f1= fCE ⇒ C20 = 12-- ⋅ π ⋅ R 4 ⋅ fCE
s To obtain a Gloop bandwidth of fC, results:
G0 ⋅ fLC = 1 ⋅ fC
⇒
G0 = A0 ⋅ R0 = ∆----VV-----oI--N--s---c- ⋅ C-----2---0-C-----/1--/--8-C----2----5-- = f--fL--C-C-- ⇒ C 18 = ∆----VV-----Io--N--s---c- ⋅ C-C----2-2--0-0----+-⋅---C-C----2-2--5-5-- ⋅ f--fL--C-C--
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.
13/20
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