Datasheet

LTC1871-1
24
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APPLICATIONS INFORMATION
SEPIC Converter Applications
The LTC1871-1 is also well suited to SEPIC (single-ended
primary inductance converter) converter applications. The
SEPIC converter shown in Figure 16 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 fi rst inductor, L1, together with the main switch,
resembles a boost converter. The second inductor, L2,
together with the output diode D1, resembles a fl yback 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 conduction
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%.
The maximum output voltage for a SEPIC converter is:
V
O(MAX)
= V
IN
+ V
D
()
D
MAX
1–D
MAX
–V
D
1
1–D
MAX
The maximum duty cycle of the LTC1871-1 is typically
92%.
SEPIC Converter: The Peak and Average
Input Currents
The control circuit in the LTC1871-1 is measuring the input
current (either using the R
DS(ON)
of the power MOSFET
or by means of a sense resistor in the MOSFET source),
so the output current needs to be refl ected back to the
input in order to dimension the power MOSFET properly.
Based on the fact that, ideally, the output power is equal
to the input power, the maximum input current for a SEPIC
converter is:
I
IN(MAX)
= I
O(MAX)
•
D
MAX
1–D
MAX
The peak input current is:
I
IN(PEAK)
= 1+

2






•I
O(MAX)
•
D
MAX
1–D
MAX
The maximum duty cycle, D
MAX
, should be calculated at
minimum V
IN
.
Figure 17. SEPIC Converter Switching Waveforms
17a. Input Inductor Current
I
IN
I
L1
SW
ON
SW
OFF
17b. Output Inductor Current
I
O
I
L2
17c. DC Coupling Capacitor Current
I
O
I
IN
I
C1
17e. Output Ripple Voltage
V
OUT
(AC)
ΔV
ESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
ΔV
COUT
17d. Diode Current
I
O
I
D1