ltc1871ems-trpbf Linear Technology Corporation, ltc1871ems-trpbf Datasheet - Page 24

• Manufacturer Part Number: ltc1871ems-trpbf
• Description: Wide Input Range, No Rsense Current Mode Boost, Flyback And Sepic Controller
• Manufacturer: Linear Technology Corporation

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
LTC1871
APPLICATIONS INFORMATION
SEPIC Converter Applications
The LTC1871 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.
24
that the input fi lter capacitor for the LTC1871 is not
shared with other converters. AC input current from
another converter could cause substantial input voltage
ripple, and this could interfere with the operation of the
LTC1871. A few inches of PC trace or wire (L ≈ 100nH)
between the C
V
problems.
IN
V
V
V
should be suffi cient to prevent current sharing
IN
IN
IN
Figures 16. SEPIC Topology and Current Flow
+
+
+
16b. Current Flow During Switch On-Time
16c. Current Flow During Switch Off-Time
•
•
•
L1
IN
SW
16a. SEPIC Topology
of the LTC1871 and the actual source
+
+
+
V
V
C1
IN
IN
L2
•
•
•
D1
D1
C
OUT
+
+
+
V
V
V
OUT
OUT
OUT
R
R
R
L
L
L
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:
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
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:
The maximum duty cycle, D
minimum V
The constant ‘χ’ represents the fraction of ripple current in
the inductor relative to its maximum value. For example, if
30% ripple current is chosen, then χ = 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)
IN
is the forward voltage of the diode. For convert-
V
= V
IN
+ V
O
(
.
+ V
O
IN
=I
+ V
= 1+
D
+ V
O(MAX)
D
D
)
1– D
2
D
•
DS(ON)
MAX
•I
1– D
MAX
MAX
D
O(MAX)
MAX
, should be calculated at
MAX
– V
of the power MOSFET
•
D
1– D
1– D
D
MAX
1
MAX
MAX
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