LT8331 datasheet(25/34 Pages) AD

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LT8357

Rev. 0

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The final RSENSE value should be lower than the calculated

RSENSE. A 20% to 30% margin is usually recommended.

Always choose a low ESL current sense resistor.

SEPIC Converter: Power MOSFET Selection

For the SEPIC configuration, choose a MOSFET with a

VDC rating higher than the sum of the output voltage and

input voltage by a safety margin (a 10V safety margin is

usually sufficient).

The power dissipated by the MOSFET in a SEPIC converter

is:

PFET =I2SW(MAX)•RDS(ON)•DMAX +2•

(VIN(MIN)+VOUT)2•IL(MAX)•CRSS•f /1A

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

APPLICATIONS INFORMATION

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.

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, and the peak current is equal to:

ID(PEAK) =(1+

)•IO(MAX)•

1−DMAX

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:

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.

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. Please refer to the Boost Converter, Output

Capacitor Selection and Boost Converter, Input Capacitor

Selection sections.

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:

VCDC > VIN(MAX)

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