LT3757/LT3757A
17
3757afd
applicaTions inForMaTion
For the bulk C component, which also contributes 1% to
the total ripple:
C
OUT
≥
I
O(MAX)
0.01• V
OUT
• f
The output capacitor in a boost regulator experiences high
RMS ripple currents, as shown in Figure 6. The RMS ripple
current rating of the output capacitor can be determined
using the following equation:
I
RMS(COUT)
≥I
O(MAX)
•
D
MAX
1− D
MAX
Multiple capacitors are often paralleled to meet ESR
requirements. Typically, once the ESR requirement is
satisfied, the capacitance is adequate for filtering and has
the required RMS current rating. Additional ceramic capaci-
tors in parallel are commonly used to reduce the effect of
parasitic inductance in the output capacitor, which reduces
high frequency switching noise on the converter output.
Boost Converter: Input Capacitor Selection
The input capacitor of a boost converter is less critical
than the output capacitor, due to the fact that the inductor
is in series with the input, and the input current wave-
form is continuous. The input voltage source impedance
determines the size of the input capacitor, which is typi-
cally in the range of 10µF to 100µF. A low ESR capacitor
is recommended, although it is not as critical as for the
output capacitor.
The RMS input capacitor ripple current for a boost con-
verter is:
I
RMS(CIN)
= 0.3 • ∆I
L
FLYBACK CONVERTER APPLICATIONS
The LT3757 can be configured as a flyback converter
for the applications where the converters have multiple
outputs, high output voltages or isolated outputs. Figure
7 shows a simplified flyback converter.
The flyback converter has a very low parts count for mul-
tiple
outputs, and with prudent selection of turns ratio, can
have high output/input voltage conversion ratios with a
desirable duty cycle. However, it has low efficiency due to
the high peak currents, high peak voltages and consequent
power loss. The flyback converter is commonly used for
an output power of less than 50W.
The flyback converter can be designed to operate either
in continuous or discontinuous mode. Compared to con-
tinuous mode, discontinuous mode has the advantage of
smaller transformer inductances and easy loop compen-
sation, and the disadvantage of higher peak-to-average
current and lower efficiency. In the high output voltage
applications, the flyback converters can be designed
to operate in discontinuous mode to avoid using large
transformers.
Figure 7. A Simplified Flyback Converter
R
SENSE
N
P
:N
S
V
IN
C
IN
C
SN
V
SN
L
P
D
SUGGESTED
RCD SNUBBER
I
D
I
SW
V
DS
3757 F06
GATE
GND
LT3757
SENSE
L
S
M
+
–
+
–
R
SN
D
SN
–
+
C
OUT
