SC2446AEVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2446AEVB Datasheet PDF : 27 Pages
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SC2446A
POWER MANAGEMENT
Application Information (Cont.)
Ro2 and the current sensing gain, such that the converter
output voltage is regulated with satisfactory dynamic
performance.
With the output voltage Vo known, the feedback gain h
and the feedback resistor values are determined using
the equations given in the “Output Voltage Setting”
section with
h
=
0.5
Vo
.
For the rated output current Io, the current sensing gain
k is first estimated as
k
=
Io .
2.1
From the transfer function from the voltage error
amplifier output vc to the converter output vo is
Vo (s)
Vc (s)
:=
Gvc (s)
=
kRo
1+
1+
s
s z1
s
sp1
.
where, the single dominant pole is
sp1
=
(Ro
1
+ Roesr )Co
,
The loop transfer function is then
T(s)=Gvc(s)C(s).
To simplify design, we assume that C3<<C2, Roesr<<Ro,
selects Sp1=Sz2 and specifies the loop crossover
frequency fc. It is noted that the crossover frequency
determines the converter dynamic bandwidth. With these
assumptions, the controller parameters are determined
as following.
C2
=
gmhkR o
2πfc
,
and
R2
=
RoCo
C2
,
C3
=
R oesr C o
R2
K,
with a constant K.
For example, if Vo=2.5V, Io=15A, fs=300kHz, Co=1.68mF,
Roesr=4.67mΩ, one can calculate that
Ro
=
Vo
Io
= 167mΩ,
and the zero due to the output capacitor ESR is
s z1
=
1
R oesr C o
.
The dominant pole moves as output load varies.
The controller transfer function (from the converter
output vo to the voltage error amplifier output vc) is
C(s)
=
gmh
s(C2 + C3 )
1+
1+
s
sz2
s
sp2
,
where
and
sz2
=
1
R2C2
,
sp2
=
R2
1
C2C3
C2 + C3
.
h
=
0.5
Vo
=
0.2,
and
k
=
Io
2.1
=
7.14.
If the converter crossover frequency is set around 1/10
of the switching frequency, fc = 30kHz, the controller
parameters then can be calculated as
C2
=
gmhkRo
2πfc
≈ 0.328nF.
where, gm is the error amplifier transconductance gain
(260 µΩ−1).
If we use C2 = 0.33 nF,
R2
=
RoCo
C2
≈ 848.5kΩ,
2005 Semtech Corp.
18
www.semtech.com
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