Datasheet

LT3759
22
3759fc
For more information www.linear.com/3759
For maximum efficiency, R
DS(ON)
and C
RSS
should be
minimized. From a known power dissipated in the power
MOSFET, its junction temperature can be obtained using
the following equation:
T
J
= T
A
+ P
FET
• θ
JA
= T
A
+P
FET
• (θ
JC
+ θ
CA
)
T
J
must not exceed the MOSFET maximum junction
temperature rating. It is recommended to measure the
MOSFET temperature in steady state to ensure that absolute
maximum ratings are not exceeded.
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+
χ
2






• I
O(MAX)
•
1
1−D
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
θ
JA
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 rated for the maximum
input voltage:
C
DC
≥ 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 LT3759 can be configured as a dual-inductor inverting
topology, as shown in Figure 9. The V
OUT
to V
IN
ratio is:
V
OUT
– V
D
V
IN
=
D
1−D
In continuous conduction mode (CCM).
APPLICATIONS INFORMATION
Figure 9. A Simplified Inverting Converter
R
SENSE
C
DC
V
IN
C
IN
L1
D1
C
OUT
V
OUT
3759 F09
+
GATE
GND
LT3759
SENSE
L2
M1
+
–
+
–
+
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.