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LT8331 Datasheet(HTML) 19 Page - Analog Devices

Part No. LT8331
Description  Low IQ Boost/SEPIC/Flyback/Inverting Converter with 0.5A, 140V Switch
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LT8331 Datasheet(HTML) 19 Page - Analog Devices

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LT8331
19
Rev. C
For more information www.analog.com
APPLICATIONS INFORMATION
In a SEPIC converter, no DC path exists between the input
and output. This is an advantage over the boost converter
for applications requiring the output to be disconnected
from the input source when the circuit is in shutdown.
SEPIC Converter: Switch Duty Cycle and Frequency
For a SEPIC converter operating in CCM, the duty cycle
of the main switch can be calculated based on the output
voltage (VOUT), the input voltage (VIN) and the diode for-
ward voltage (VD).
The maximum duty cycle (DMAX) occurs when the con-
verter operates at the minimum input voltage:
DMAX =
VOUT + VD
VIN(MIN) + VOUT + VD
Conversely, the minimum duty cycle (DMIN) occurs when
the converter operates at the maximum input voltage:
DMIN =
VOUT + VD
VIN(MAX) + VOUT + VD
Be sure to check that DMAX and DMIN obey:
DMAX < 1 – Minimum Off-Time(MAX) • fOSC(MAX)
and
DMIN > Minimum On-Time(MAX) • fOSC(MAX)
where Minimum Off-Time, Minimum On-Time and fOSC
are specified in the Electrical Characteristics table.
SEPIC Converter: The Maximum Output Current
Capability and Inductor Selection
As shown in Figure 8, the SEPIC converter contains two
inductors: L1 and L2. L1 and L2 can be independent,
but can also be wound on the same core, since iden-
tical voltages are applied to L1 and L2 throughout the
switching cycle.
For the SEPIC topology, the current through L1 is the
converter input current. Based on the fact that, ideally, the
output power is equal to the input power, the maximum
average inductor currents of L1 and L2 are:
IL1(MAX)(AVE) =IIN(MAX)(AVE)=IO(MAX)
DMAX
1 − DMAX
IL2(MAX)(AVE)= IO(MAX)
In a SEPIC converter, the switch current is equal to IL1 +
IL2 when the power switch is on, therefore, the maximum
average switch current is defined as:
ISW(MAX)(AVE) = IL1(MAX)(AVE)+ IL2(MAX)(AVE)
=IO(MAX)
1
1 − DMAX
and the peak switch current is:
ISW(PEAK) = 1 +
c
2
⎟ • IO(MAX)
1
1 − DMAX
The constant c in the preceding equations represents
the percentage peak-to-peak ripple current in the switch,
relative to ISW(MAX)(AVE), as shown in Figure 9. Then, the
switch ripple current ∆ISW can be calculated by:
∆ISW = c • ISW(MAX)(AVE)
The inductor ripple currents ∆IL1 and ∆IL2 are identical:
∆IL1 = ∆IL2 = 0.5 • ∆ISW
Figure 9. The Switch Current Waveform of the SEPIC Converter
8331 F09
∆ISW = χ•ISW(MAX)(AVE)
ISW
t
DTS
ISW(MAX)(AVE)
TS


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