lt3573 Linear Technology Corporation, lt3573 Datasheet - Page 17

• Manufacturer Part Number: lt3573
• Description: Isolated Flyback Converter Without An Opto-coupler
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
and 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.
The snubber resistor affects the spike amplitude V
duration t
t
to the output voltage sensing.
The previous steps fi nish the fl yback power stage de-
sign.
5. Select the feedback resistor for proper output
Using the resistor Tables 1-4, select the feedback resis-
tor R
SP
voltage.
2
1
is about 150ns. Prolonged t
FB
L I
S PK SW
, and program the output voltage to 5V. Adjust the
Figure 9. RCD Snubber in a Flyback Converter
2
SP
+
–
, the snubber resistor is adjusted such that
f
C
V V
C
R
D
(
C
L
S
R
3573 F09
N V
•
SP
OUT
may cause distortion
)
C
and
R
voltage. R
A small capacitor in parallel with R
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
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
Use the current limit resistor R
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.
TC
transient performance.
resistor divider
resistor for temperature compensation of the output
V
IN
REF
Figure 10. Typical Switch Node Waveform
V
C
is selected as 6.04k.
t
SP
NV
OUT
LIM
REF
to lower the current
fi lters out the noise
LT3573
3573 F10
17
3573f