LT8331 datasheet(18/30 Pages) AD

18 / 30 page

LT8331

Rev. C

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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 >

• 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

1 − 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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