Datasheet

LT3758/LT3758A
23
3758afd
ApplicAtions inForMAtion
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+
c
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
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 LT3758 can be configured as a dual-inductor inverting
topology, as shown in Figure 10. The V
OUT
to V
IN
ratio is:
V
OUT
− V
D
V
IN
= −
D
1− D
in continuous conduction mode (CCM).
Figure 10. A Simplified Inverting Converter
R
SENSE
C
DC
V
IN
C
IN
L1
D1
C
OUT
V
OUT
3758 F10
+
GATE
GND
LT3758
SENSE
L2
M1
+
–
+
–
+
Inverting Converter: Switch Duty Cycle and Frequency
For an inverting converter operating in CCM, the duty
cycle of the main switch can be calculated based on the
negative output voltage (V
OUT
) and the input voltage (V
IN
).
The maximum duty cycle (D
MAX
) occurs when the converter
has the minimum input voltage:
D
MAX
=
V
OUT
− V
D
V
OUT
− V
D
− V
IN(MIN)
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