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LT8331 Datasheet(HTML) 20 Page - Analog Devices

Part No. LT8331
Description  Low IQ Boost/SEPIC/Flyback/Inverting Converter with 0.5A, 140V Switch
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com

LT8331 Datasheet(HTML) 20 Page - Analog Devices

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Rev. C
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The inductor ripple current has a direct effect on the
choice of the inductor value. Choosing smaller values of
∆IL requires large inductances and reduces the current
loop gain (the converter will approach voltage mode).
Accepting larger values of ∆IL allows the use of low
inductances, but results in higher input current ripple and
greater core losses. It is recommended that c falls in the
range of 0.5 to 0.8.
Due to the current limit of its internal power switch, the
LT8331 should be used in a SEPIC converter whose maxi-
mum output current (IO(MAX)) is:
IO(MAX) < (1 – DMAX) • (0.5A – 0.5 • ∆ISW ) • η
where η (< 1.0) is the converter efficiency. Minimum
possible inductor value and switching frequency should
also be considered since they will increase inductor ripple
current ∆ISW.
Given an operating input voltage range, and having cho-
sen ripple current in the inductor, the inductor value (L1
and L2 are independent) of the SEPIC converter can be
determined using the following equation:
L1 = L2 =
0.5 • ΔISW • fOSC
For most SEPIC applications, the equal inductor values
will fall in the range of 4.7µH to 220µH.
By making L1 = L2, and winding them on the same core,
the value of inductance in the preceding equation is
replaced by 2L, due to mutual inductance:
L =
This maintains the same ripple current and energy storage
in the inductors. The peak inductor currents are:
IL1(PEAK) = IL1(MAX) + 0.5 • ∆IL1
IL2(PEAK) = IL2(MAX) + 0.5 • ∆IL2
The maximum RMS inductor currents are approximately
equal to the maximum average inductor currents.
Based on the preceding equations, the user should choose
the inductors having sufficient saturation and RMS cur-
rent ratings.
SEPIC Converter: Output Diode Selection
To maximize efficiency, a fast switching diode with a low
forward drop and low reverse leakage is desirable. The
average forward current in normal operation is equal to
the output current.
It is recommended that the peak repetitive reverse voltage
rating VRRM is higher than VOUT + VIN(MAX) by a safety
margin (a 10V safety margin is usually sufficient).
The power dissipated by the diode is:
where VD is diode’s forward voltage drop, and the diode
junction temperature is:
TJ = TA + PD • RθJA
The RθJA used in this equation normally includes the RθJC
for the device, plus the thermal resistance from the board,
to the ambient temperature in the enclosure. TJ must not
exceed the diode maximum junction temperature rating.
SEPIC Converter: Output and Input Capacitor Selection
The selections of the output and input capacitors of the
SEPIC converter are similar to those of the boost converter.
SEPIC Converter: Selecting the DC Coupling Capacitor
The DC voltage rating of the DC coupling capacitor (CDC,
as shown in Figure 10) should be larger than the maxi-
mum input voltage:

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