LT3757IDD#PBF Linear Technology, LT3757IDD#PBF Datasheet - Page 17

• Manufacturer Part Number: LT3757IDD#PBF
• Description: IC CTRLR DC/DC ADJ 10-DFN
• Manufacturer: Linear Technology
• Type: Step-Up (Boost), Inverting, Flyback, Sepicr

Specifications of LT3757IDD#PBF

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Frequency - Switching: 100kHz ~ 1MHz
• Voltage - Input: 2.9 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-DFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Voltage - Output: -
• Power - Output: -

applicaTions inForMaTion
For the bulk C component, which also contributes 1% to
the total ripple:
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:
Multiple capacitors are often paralleled to meet ESR require-
ments. Typically, once the ESR requirement is satisfied, the
capacitance is adequate for filtering and has the required
RMS current rating. Additional ceramic capacitors in par-
allel 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
I
C
RMS(CIN)
RMS COUT
OUT
(
≥
0 01
= 0.3 • ∆I
. •
)
I
≥
O MAX
(
I
O MAX
V
(
OUT
)
L
•
)
f
•
− 1
D
MAX
D
MAX
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.
V
IN
+
C
IN
Figure 7. A Simplified Flyback Converter
V
SN
–
+
RCD SNUBBER
SUGGESTED
C
3757 F06
SN
LT3757
SENSE
GATE
R
D
GND
SN
SN
L
P
N
P
M
R
:N
SENSE
I
SW
S
+
–
V
L
DS
S
I
LT3757
D
D
+

+
–
C
OUT
3757fb