LT8331 datasheet(18/30 Pages) LINER

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LT8331

8331fa

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LLKistheleakageinductanceoftheprimarywinding,which

is usually specified in the transformer characteristics. LLK

canbeobtainedbymeasuringtheprimaryinductancewith

the secondary windings shorted. The snubber capacitor

value(CSN)canbedeterminedusingthefollowingequation:

CSN =

VSN

∆VSN • 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 >

• 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

boardtotheambienttemperatureintheenclosure.TJmust

notexceedthediodemaximumjunctiontemperaturerating.

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

following equation:

IRMS(COUT),CONTINUOUS ≈ IO(MAX) •

DMAX

− DMAX

Flyback Converter: Input Capacitor Selection

Theinputcapacitorinaflybackconverterissubjecttoalarge

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 following equation:

IRMS(CIN),CONTINUOUS ≈

POUT(MAX)

VIN(MIN) • η

•

−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

in continuous conduction mode (CCM).

APPLICATIONS INFORMATION

Figure 8. LT8331 Configured in a SEPIC Topology

VOUT

VIN

FBX

GND

EN/UVLO

LT8331

VIN

VCC

INT

CIN

COUT

CDC

8331 F08

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