Datasheet

LTC1871-7
24
18717fd
applicaTions inForMaTion
9. For applications with multiple switching power convert-
ers connected to the same input supply, make sure
that the input filter capacitor for the LTC1871-7 is not
shared with other converters. AC input current from
another converter could cause substantial input volt-
age ripple, and this could interfere with the operation
of the LTC1871-7. A few inches of PC trace or wire (L
≈ 100nH) between the C
IN
of the LTC1871-7 and the
actual source V
IN
should be sufficient to prevent current
sharing problems.
SEPIC Converter Applications
The LTC1871-7 is also well suited to SEPIC (single-ended
primary inductance converter) converter applications. The
SEPIC converter shown in Figure 20 uses two inductors.
The advantage of the SEPIC converter is the input voltage
may be higher or lower than the output voltage, and the
output is short-circuit protected.
The first inductor, L1, together with the main switch,
resembles a boost converter. The second inductor, L2,
together with the output diode D1, resembles a flyback or
buck-boost converter. The two inductors L1 and L2 can be
independent but can also be wound on the same core since
identical voltages are applied to L1 and L2 throughout the
switching cycle. By making L1 = L2 and winding them on
the same core the input ripple is reduced along with cost
and size. All of the SEPIC applications information that
follows assumes L1 = L2 = L.
SEPIC Converter: Duty Cycle Considerations
For a SEPIC converter operating in a continuous conduc-
tion mode (CCM), the duty cycle of the main switch is:
D =
V
O
+ V
D
V
IN
+ V
O
+ V
D






where V
D
is the forward voltage of the diode. For convert-
ers where the input voltage is close to the output voltage
the duty cycle is near 50%.
Figure 20. SEPIC Topology and Current Flow Figure 21. SEPIC Converter Switching Waveforms
+
+
+
•
•
•
•
SW L2
C
OUT
R
L
V
OUT
V
IN
C1
D1
L1
20a. SEPIC Topology
+
+
+
•
R
L
V
OUT
18717 F20
V
IN
D1
20c. Current Flow During Switch Off-Time
+
+
+
•
R
L
V
OUT
V
IN
V
IN
V
IN
20b. Current Flow During Switch On-Time
21a. Input Inductor Current
I
IN
I
L1
SW
ON
SW
OFF
21b. Output Inductor Current
I
O
I
L2
21c. DC Coupling Capacitor Current
I
O
I
IN
I
C1
21e. Output Ripple Voltage
V
OUT
(AC)
ΔV
ESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
ΔV
COUT
21d. Diode Current
I
O
18717 F21
I
D1