LT8331 datasheet(22/34 Pages) AD

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LT8357

Rev. 0

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The first term in this equation represents the conduction

losses in the device, and the second term represents the

switching loss. CRSS is the reverse transfer capacitance of

the MOSFET. f is the switching frequency. For maximum

efficiency, RDS(ON) and CRSS should be minimized.

From a known power dissipated in the power MOSFET, its

junction temperature can be obtained using the following

equation:

TJ =TA +PFET •θJA =TA +PFET •(θJC+θCA)

TJ must not exceed the MOSFET maximum junction

temperature rating. It is recommended to measure the

MOSFET temperature in steady state to ensure that abso-

lute maximum ratings are not exceeded.

To achieve high switching frequency (such as 2MHz)

operation, QG and RDS(ON) of the power MOSFET must

be carefully selected. High performance power MOSFETs

with low QG and low RDS(ON) must be used. Since the gate

drive voltage is set by the 5V INTVCC supply, logic-level

threshold MOSFETs must be used in LT8357 applications.

When switching at high frequency like 2MHz, the substan-

tial gate charge current from INTVCC can be estimated as:

IINTVCC = f • QG

Make sure the total required INTVCC current not exceed-

ing the INTVCC current limit in the data sheet. Typically,

MOSFETs with less than 10nC QG are recommended.

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

APPLICATIONS INFORMATION

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 discontinuous operation can be determined using the

following equation:

IRMS(COUT),DISCONTINOUS ≥IO(MAX)•

4−(3•D2)

3•D2

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 capaci-

tor sized for the maximum RMS current. The RMS ripple cur-

rent rating of the input capacitors in discontinuous operation

can be determined using the following equation:

IRMS(CIN),DISCONTINUOUS ≥

POUT(MAX)

VIN(MIN)•η

•

4−(3•DMAX)

3•DMAX

SEPIC CONVERTER APPLICATIONS

The LT8357 can be configured as a SEPIC (single-ended

primary inductance converter), as shown in Figure 10.

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).

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