Datasheet

LT3757/LT3757A
24
3757afe
For more information www.linear.com/LT3757
applicaTions inForMaTion
Figure10. A Simplified Inverting Converter
R
SENSE
C
DC
V
IN
C
IN
L1
D1
C
OUT
V
OUT
3757 F10
+
GATE
GND
LT3757
SENSE
L2
M1
+
+
+
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:
I
D(PEAK)
= 1+
c
2
I
O(MAX)
1
1D
MAX
It is recommended that the peak repetitive reverse voltage
rating V
RRM
is higher than V
OUT
+ V
IN(MAX)
by a safety
margin (a 10V safety margin is usually sufficient).
The power dissipated by the diode is:
P
D
= I
O(MAX)
V
D
and the diode junction temperature is:
T
J
= T
A
+ P
D
R
θJA
The R
θJA
used in this equation normally includes the R
θJC
for the device, plus the thermal resistance from the board,
to the ambient temperature in the enclosure. T
J
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.
SEPIC Converter: Selecting the DC Coupling Capacitor
The DC voltage rating of the DC coupling capacitor (C
DC
,
as shown in Figure1) should be larger than the maximum
input voltage:
V
CDC
> V
IN(MAX)
C
DC
has nearly a rectangular current waveform. During
the switch off-time, the current through C
DC
is I
IN
, while
approximately I
O
flows during the on-time. The RMS
rating of the coupling capacitor is determined by the fol-
lowing equation:
I
RMS(CDC)
>I
O(MAX)
V
OUT
+ V
D
V
IN(MIN)
A low ESR and ESL, X5R or X7R ceramic capacitor works
well for C
DC
.
INVERTING CONVERTER APPLICATIONS
The LT3757 can be configured as a dual-inductor invert
-
ing topology, as shown in Figure10. The V
OUT
to V
IN
ratio is:
V
OUT
V
D
V
IN
=
D
1D
in continuous conduction mode (CCM).
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