Datasheet

LT3574
17
3574f
applications inForMation
Figure 9. RCD Snubber in a Flyback Converter Figure 10. Typical Switch Node Waveform
3574 F09
L
S
D
R
•
C
•
V
IN
V
C
NV
OUT
t
SP
3574 F10
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
L I f
V V N V
R
S PK SW
C C OUT
=
−( • )
The snubber resistor affects the spike amplitude VC 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 finish the flyback power stage
design.
5. Select the feedback resistor for proper output
voltage.
Using the resistor Tables 1-4, select the feedback resis-
tor 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
filters out the noise
during the voltage spike, however, the capacitor should
limit to 10pF. A large capacitor causes distortion on volt-
age 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 1nF, 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 flyback converter.
Select the UVLO resistor divider for intended input opera-
tion range. These equations are aforementioned.