LTC1871-7_15 Datasheet, PDF (24/34 Pages) Linear Technology – High Input Voltage, Current Mode Boost, Flyback and SEPIC Controller

LTC1871-7
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
L1
•
V_IN
SW
C1
L2
D1
V_OUT
+
C_OUT
R_L
•
20a. SEPIC Topology
•
V_IN
V_IN
V_OUT
+
R_L
•
20b. Current Flow During Switch On-Time
•
V_IN
V_IN
D1
V_OUT
+
R_L
•
18717 F20
20c. Current Flow During Switch Off-Time
Figure 20. SEPIC Topology and Current Flow
24
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_IN +
+ V_D
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%.
I_L1
SW
SW
I_IN
ON OFF
21a. Input Inductor Current
I_L2
I_O
21b. Output Inductor Current
I_IN
I_C1
I_O
21c. DC Coupling Capacitor Current
I_D1
I_O
21d. Diode Current
V_OUT
(AC)
ΔV_ESR
ΔV_COUT
18717 F21
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
21e. Output Ripple Voltage
Figure 21. SEPIC Converter Switching Waveforms
18717fd