VIPER53 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'VIPER53' PDF : 24 Pages View PDF
VIPer53DIP / VIPer53SP
Figure 20: Complete Converter Transfer Function
G(S)
3.2 ⋅
--P----M-----A----X----
PO U T1
3.2 ⋅ --P----M------A----X---
POUT 2
π-----⋅----R----L---1----1-⋅---C----O-----U-----T-
π-----⋅----R----L---2---1--⋅---C----O-----U-----T-
F
1
------------------------1-----------------------
2 ⋅ π ⋅ ESR ⋅ COUT
F(S)
Gm ⋅ RCOMP
1
F(S).G(S)
2-----⋅---π-----⋅---R----C----O------M--1---P-----⋅---C----C----O-----M------P--
FC
F
FBW2
F
1
FBW1
in the current mode section. A zero due to the
RCOMP-CCOMP network is set at the same value as
the maximum load RL2 pole.
The total transfer function is shown as F(s).G(s) at
the bottom of figure 20. For maximum load (plain
line), the load pole is exactly compensated by the
zero of the error amplifier, and the result is a
perfect first order decreasing slope until it reaches
the zero of the output capacitor ESR. The error
amplifier cut off then prevents definitely any further
spurious noise or resonance from disturbing the
regulation loop.
The point where the complete transfer function has
a unity gain is known as the regulation bandwidth
and has a double interest:
– The higher it is the faster will be the reaction to
an eventual load change, and the smaller will be
the output voltage change.
– The phase shift in the complete system at this
point has to be less than 135 ° to ensure a good
stability. Generally, a first order gives 90 ° of
phase shift, and 180 ° for a second order.
In figure 20, the unity gain is reached in a first order
slope, so the stability is ensured.
The dynamic load regulation is improved by
increasing the regulation bandwidth, but some
limitations have to be respected: As the transfer
function above the zero due the capacitor ESR is
not reliable (The ESR itself is not well specified,
and other parasitic effects may take place), the
bandwidth should always be lower than the
minimum of FC and ESR zero.
As the highest bandwidth is obtained with the
highest output power (Plain line with RL2 load in
figure 20), the above criteria will be checked for
this condition and allows to define the value of
RCOMP, as the error amplifier gain depends only
on this value for this frequency range. The
following formula can be derived:
RCOMP =
P----O----U----T----2-
PMAX
⋅
F----B----W-----2----⋅---R----L---2----⋅---C----O----U----T--
Gm
With:
POUT2
=
V----O2----U----T--
RL2
And:
PMAX
=
12--
⋅
LP
⋅
2
ILIM
⋅
FSW
:
The lowest load gives another condition for
stability: The frequency FBW1 must not encounter
the second order slope generated by the load pole
and the integrator part of the error amplifier. This
condition can be met by adjusting the CCOMP
value:
CCOMP
>
--------R----L---1----⋅---C-----O----U----T---------
6.3 ⋅ Gm ⋅ RC2 OMP
⋅
P----O----U-----T---1-
PMAX
With:
POUT1
=
V----O2-----U----T-
RL1
The above formula gives a minimum value for
CCOMP. It can be then increased to provide a
natural soft start function as this capacitor is
charged by the error amplifier current capacity
ICOMPhi at start-up.
18/24
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