User manual

page 52
Analog System Lab Kit PRO
experiment 9
The goal of the experiment is to design a high-ecient DC-DC converter using
a general purpose OP-Amp and a comparator and study its characteristics.
We also aim to study the characteristics of a DC-DC converter IC, and for
this purpose we selected the wide-input non synchronous buck DC/DC
controller TPS40200 from Texas Instruments. This IC is included in ASLK
PRO as evaluation module.
Thereaderwillbenetfromviewingtherecordedlectureat[24]. Also refer to the
applicationnote,DesignConsiderationsforClass-DAudioPowerAmpliers[15].
Function generator is the basic block for DC-DC converter. The triangular output
of the function generator with peak amplitude
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
and frequency
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
is fed to the
comparator whose other input is connected to the reference voltage Vref. The output
of this comparator is the PWM (Pulse width modulation) waveform whose duty cycle
is given by
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
, where T is time period of triangular wave and is
equal to
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
. This duty cycle is directly proportional to reference voltage Vref.
Ifweconnectthelosslesslow-passlter(LClter)attheoutputofthecomparator
as shown in Figure 9.1, it is possible to get stable DC voltage
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
withhigheciency
Design a DC-DC converter which has 10 kHz oscillator whose triangular wave output
with peak amplitude Vp is fed to a comparator whose other input is connected to
Vref (reference voltage).
Obtain the time response of the system and plot
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
versus
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
.
Obtain the
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
versus
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
characteristics.
9.1 Brief theory and motivation
9.2Specications
9.3 Measurements to be taken
Goal of the experiment
L
Vref
Function
Generator
C
RL
Vg VO
Vav
+VSS
-VSS
Figure 9.1: DC-DC Converter and PWM waveform
9.3.1 Time response
9.3.2 Transfer function
between
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
depending upon the value of
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
. Hence circuit becomes SMPS system
where
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
.
IfwereplaceLClterwithMOSFET,andapplyaudioinputas
V
f
V
T
VV
T
Tf
V
V
VVVV
T
V
T
2
1
1
1
p
ref
refp
av
ss
av refssp
ref
!
$
x
x
x
=
=
=
-
-
_i
to the comparator
thenatoutputoftheMOSFETampliedaudiooutputisobtained,thisisClassD
PowerAmplieroperation.