SC2446ITETRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC2446ITETRT' PDF : 38 Pages View PDF
SC2446
POWER MANAGEMENT
Application Information (Cont.)
The transconductance error amplifier (in the SC2446)
has a gain g of 260µA/V. The target of the compensation
m
design is to select the compensation network consisting
of C2, C3 and R2, along with the feedback resistors Ro1,
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
and
sz2
=
1
R2C2
,
sp2
=
R2
1
C2C3
C2 + C3
.
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.
From Figure 17, 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
,
and the zero due to the output capacitor ESR is
and
C2
=
gmhkR o
2πfc
,
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
s z1
=
1
R oesr C o
.
Ro
=
Vo
Io
= 167mΩ,
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
,
and
h
=
0.5
Vo
=
0.2,
k
=
Io
2.1
=
7.14.
where
2004 Semtech Corp.
23
www.semtech.com
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