VIPER53 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'VIPER53' PDF : 24 Pages View PDF
VIPer53DIP / VIPer53SP
introducing a zero and ensuring a correct phase
margin. This configuration is illustrated in figure 18
Figure 19: Typical Transfer Functions
for the schematic and figure 19 for the error
Gain (dB)
60
50
Rcomp=4.7k
Ccomp=470nF
40
30
20
10
0
-10
1
10
Phase (°)
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
1
10
100
1k
10k
100k
1M
Frequency (Hz)
Rcomp=4.7k
Ccomp=470nF
100
1k
10k
100k
1M
Frequency (Hz)
amplifier transfer function for a typical set of values
for CCOMP and RCOMP. Note that a capacitor of
10 nF (minimum value: 8 nF) should always be
connected to the COMP pin to insure a correct
stability of the internal error amplifier.
The complete converter open loop transfer
function can be built from both power cell and error
amplifier transfer functions. A theoretical example
can be seen in figure 20 for a discontinuous mode
flyback loaded by a simple resistor, regulated from
primary side (no optocoupler, the internal error
amplifier is fully used for regulation). A typical
schematic corresponding to this situation can be
seen on figure 14.
The transfer function of the power cell is
represented as G(s) in figure 20. It exhibits a pole
which depends on the output load and on the
output capacitor value. As the load of a converter
may change, two curves are shown for two
different values of output resistance value, RL1 and
RL2. A zero at higher frequency values then
appears, due to the output capacitor ESR. Note
that the overall transfer function doesn’t depend on
the input voltage, thanks to the current mode
control.
The error amplifier has a fixed behavior, similar to
the one shown in figure 19. Its bandwidth is limited,
in order to avoid injection of high frequency noise
17/24
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