Datasheet

LT3575
17
3575f
APPLICATIONS INFORMATION
Zener clamps. Among them, RCD is widely used. Figure 9
shows the RCD snubber in a fl yback converter.
A typical switch node waveform is shown in Figure 10.
During switch turn-off, the energy stored in the leakage
inductance is transferred to the snubber capacitor, and
eventually dissipated in the snubber resistor.
1
2
2
LI f
VV NV
R
SPKSW
C C OUT
=
−(•)
The snubber resistor affects the spike amplitude V
C
and
duration t
SP
, the snubber resistor is adjusted such that
t
SP
is about 150ns. Prolonged t
SP
may cause distortion
to the output voltage sensing.
The previous steps fi nish the fl yback power stage design.
5. Select the feedback resistor for proper output voltage.
Using the resistor Tables 1-4, select the feedback resistor
R
FB
, and program the output voltage to 5V. Adjust the
R
TC
resistor for temperature compensation of the output
voltage. R
REF
is selected as 6.04k.
A small capacitor in parallel with R
REF
fi lters out the
noise during the voltage spike, however, the capacitor
should limit to 10pF. A large capacitor causes distortion
on voltage sensing.
6. Optimize the compensation network to improve the
transient performance.
The transient performance is optimized by adjusting the
compensation network. For best ripple performance, select
a compensation capacitor not less than 1.5nF, and select
a compensation resistor not greater than 50k.
7. Current limit resistor, soft-start capacitor and UVLO
resistor divider
Use the current limit resistor R
LIM
to lower the current
limit if a compact transformer design is required. Soft-start
capacitor helps during the start-up of the fl yback converter .
Select the UVLO resistor divider for intended input opera-
tion range. These equations are aforementioned.
Figure 9. RCD Snubber in a Flyback Converter Figure 10. Typical Switch Node Waveform
3575 F09
L
S
D
R
C
V
IN
V
C
NV
OUT
t
SP
3575 F10