SC2441AEVB(2006) View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2441AEVB
(Rev.:2006)
(Rev.:2006)
Semtech Corporation
SC2441AEVB Datasheet PDF : 37 Pages
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SC2441A
POWER MANAGEMENT
Applications Information
function from the voltage error amplifier output v to
c
the converter output vo can be derived from Figure 24.
1+ s
Vo (s)
Vc (s)
:=
Gvc
(s)
=
kRo
1+
sz1
s
,
sp1
where the single dominant-pole is
sp1
=
(Ro
1
+ Roesr
)Co
and the zero associated with the output capacitor ESR
is
s z1
=
1
R oesr Co
.
The dominant-pole changes with the converter output
load.
The controller transfer function (from the converter
output vo to the voltage error amplifier output vc) is
1+ s
C(s) = gmh
sz2 ,
s(C2 + C3 ) 1+ s
sp2
P2 is a pole for suppressing high-frequency switching
noise. So P >> Z . To simplify design, one usually assumes
2
2
that C <<C , R <<R S =S and specifies the loop
3
2 oesr
o, p1 z2
crossover frequency fc. The loop crossover frequency
determines the converter dynamic response. With these
assumptions, the controller parameters are determined
as follows
C2
=
gmhkR o
2pfc
,
and
R2
=
RoCo
C2
C3
=
R oesr C o
R2
.
For example, if Vin=3.3V, Vo=1.2V, Io=4A, fs=500kHz,
Co=390µF, Roesr=16mΩ, one can calculate that
Ro
=
Vo
Io
= 300mW,
h = 0.5 = 0.42
Vo
and
where
and
sz2
=
1
R2C2
sp2
=
R2
1
C2C3
C2 + C3
.
The loop transfer function is then
T(s)=Gvc(s)C(s).
k = DIo = 2.60.
DVc
If the converter crossover frequency is set around 1/10
of the switching frequency, fc=50kHz, the controller
parameters then can be calculated.
C2
=
gmhkRo
2pfc
»
0.326nF,
use C2=0.33nF.
R2
=
RoCo
C2
» 354.5kW,
use R2=357kΩ.
It is further calculated that
C3
=
Roesr Co
R2
» 17.48pF,
use C3=22pF. The Bode plots of the loop transfer function
(magnitude and phase) are shown in Fig. 25.
2006 Semtech Corp.
34
www.semtech.com
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