LTC1871 Linear Technology, LTC1871 Datasheet - Page 24

• Manufacturer Part Number: LTC1871
• Description: Wide Input Range/ No RSENSE Current Mode Boost/ Flyback and SEPIC Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1871
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.
24
Figures 17. SEPIC Converter Switching Waveforms
V
17c. DC Coupling Capacitor Current
(AC)
OUT
I
I
I
I
C1
D1
L1
L2
17b. Output Inductor Current
17a. Input Inductor Current
17e. Output Ripple Voltage
SW
ON
17d. Diode Current
U
OFF
V
SW
ESR
U
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
W
V
COUT
I
I
I
I
I
IN
O
IN
O
O
U
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:
where V
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:
The maximum duty cycle of the LTC1871 is typically 92%.
SEPIC Converter: The Peak and Average
Input Currents
The control circuit in the LTC1871 is measuring the input
current (either using the R
by means of a sense resistor in the MOSFET source), so
the output current needs to be reflected 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:
The maximum duty cycle, D
minimum V
The constant ‘ ’ represents the fraction of ripple current in
the inductor relative to its maximum value. For example, if
30% ripple current is chosen, then = 0.30 and the peak
current is 15% greater than the average.
The peak input current is
D
V
O MAX
(
D
I
I
V
IN MAX
IN PEAK
is the forward voltage of the diode. For convert-
IN
)
(
(
V
IN
O
.
V
V
)
O
IN
)
V
D
I
O MAX
V
V
(
1
D
D
1
2
)
:
DS(ON)
–
D
•
MAX
D
•
MAX
1
I
MAX
–
O MAX
D
(
MAX
D
, should be calculated at
of the power MOSFET or
MAX
–
)
V
•
D
1
1
D
–
–
MAX
D
D
1
MAX
MAX