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LT8331 Datasheet(HTML) 18 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) 18 Page - Analog Devices

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LT8331
18
Rev. C
For more information www.analog.com
LLK is the leakage inductance of the primary winding,
which is usually specified in the transformer character-
istics. LLK can be obtained by measuring the primary
inductance with the secondary windings shorted. The
snubber capacitor value (CSN) can be determined using
the following equation:
CSN =
VSN
ΔVSN • RSN • fOSC
where ∆VSN is the voltage ripple across CSN. A reasonable
∆VSN is 5% to 10% of VSN. The reverse voltage rating of
DSN should be higher than the sum of VSN and VIN(MAX).
Flyback Converter: Output Diode Selection
The output diode in a flyback converter is subject to large
RMS current and peak reverse voltage stresses. A fast
switching diode with a low forward drop and a low reverse
leakage is desired. Schottky diodes are recommended if
the output voltage is below 100V.
Approximate the required peak repetitive reverse voltage
rating VRRM using:
VRRM >
NS
NP
• VIN(MAX) + VOUT
The power dissipated by the diode is:
PD = IO(MAX) • VD
and the diode junction temperature is:
TJ = TA + PD • RθJA
The RθJA to be 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 tem-
perature rating.
Flyback Converter: Output Capacitor Selection
The output capacitor of the flyback converter has a similar
operation condition as that of the boost converter. Refer to
the Boost Converter: Output Capacitor Selection section
for the calculation of COUT and ESRCOUT.
The RMS ripple current rating of the output capacitors in
continuous operation can be determined using the fol-
lowing equation:
IRMS(COUT),CONTINUOUS ≈ IO(MAX)
DMAX
1 − DMAX
Flyback Converter: Input Capacitor Selection
The input capacitor in a flyback converter is subject to
a large RMS current due to the discontinuous primary
current. To prevent large voltage transients, use a low
ESR input capacitor sized for the maximum RMS current.
The RMS ripple current rating of the input capacitors in
continuous operation can be determined using the fol-
lowing equation:
IRMS(CIN),CONTINUOUS
POUT(MAX)
VIN(MIN) • η
1−DMAX
DMAX
SEPIC CONVERTER APPLICATIONS
The LT8331 can be configured as a SEPIC (single-ended
primary inductance converter), as shown in Figure 8. This
topology allows for the input to be higher, equal, or lower
than the desired output voltage. The conversion ratio as
a function of duty cycle is:
VOUT + VD
VIN
=
D
1 − D
in continuous conduction mode (CCM).
APPLICATIONS INFORMATION
Figure 8. LT8331 Configured in a SEPIC Topology
L1
L2
VOUT
VIN
SW
FBX
GND
EN/UVLO
LT8331
VIN
VCC
INT
D1
CIN
COUT
CDC
8331 F08


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