LT3757(RevB) 查看數據表（PDF） - Linear Technology
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LT3757 Datasheet PDF : 36 Pages
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LT3757
Applications Information
SEPIC Converter: Output Diode Selection
To maximize efficiency, a fast switching diode with a low
forward drop and low reverse leakage is desirable. The
average forward current in normal operation is equal to
the output current, and the peak current is equal to:
ID(PEAK)
=
1+
c
2
• IO(MAX )
•
1
1− DMAX
It is recommended that the peak repetitive reverse voltage
rating VRRM is higher than VOUT + VIN(MAX) by a safety
margin (a 10V safety margin is usually sufficient).
The power dissipated by the diode is:
PD = IO(MAX) • VD
and the diode junction temperature is:
TJ = TA + PD • RθJA
The RθJA used in this equation normally includes the RθJC
for the device, plus the thermal resistance from the board,
to the ambient temperature in the enclosure. TJ must not
exceed the diode maximum junction temperature rating.
SEPIC Converter: Output and Input Capacitor Selection
The selections of the output and input capacitors of the
SEPIC converter are similar to those of the boost converter.
Please refer to the Boost Converter, Output Capacitor
Selection and Boost Converter, Input Capacitor Selection
sections.
CDC has nearly a rectangular current waveform. During
the switch off-time, the current through CDC is IIN, while
approximately –IO flows during the on-time. The RMS
rating of the coupling capacitor is determined by the fol-
lowing equation:
IRMS(CDC) >IO(MAX) •
VOUT + VD
VIN(MIN)
A low ESR and ESL, X5R or X7R ceramic capacitor works
well for CDC.
Inverting Converter Applications
The LT3757 can be configured as a dual-inductor invert-
ing topology, as shown in Figure 10. The VOUT to VIN
ratio is:
VOUT −
VIN
VD
=
−
D
1− D
in continuous conduction mode (CCM).
VIN +
CIN
L1
LT3757
GATE
SENSE
GND
CDC
L2
+–
–
M1
D1 COUT
+ VOUT
RSENSE
+
3757 F09
SEPIC Converter: Selecting the DC Coupling Capacitor
The DC voltage rating of the DC coupling capacitor (CDC,
as shown in Figure 1) should be larger than the maximum
input voltage:
VCDC > VIN(MAX)
Figure 10. A Simplified Inverting Converter
3757fb
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