LT8331 datasheet(17/34 Pages) AD

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LT8357

Rev. 0

For more information www.analog.com

The peak and RMS inductor current are:

IL(PEAK) =IL(MAX)•(1+

)

IL(RMS) =IL(MAX)• 1+

χ2

Based on these equations, the user should choose the

inductors having sufficient saturation and RMS current

ratings.

Set the sense voltage across the sense resistor RSENSE at

IL(PEAK) to be less than the maximum SENSE current limit

threshold of 60mV (typical). The sense resistor value can

then be calculated as:

RSENSE =

60mV

IL(PEAK)

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.

Boost Converter: Power MOSFET Selection

Important parameters for the power MOSFET include the

drain-source voltage rating (VDS), the threshold voltage

(VGS(TH)), the on-resistance (RDS(ON)), the gate to source

and gate to drain charges (QGS and QGD), the maximum

drain current (ID(MAX)) and the MOSFET’s thermal resis-

tances (RθJC and RθJA).

The power MOSFET will see full output voltage, plus a

diode forward voltage, and any additional ringing across

its drain-to-source during its off-time. It is recommended

to choose a MOSFET whose breakdown voltage BVDSS is

higher than VOUT by a safety margin (a 10V safety margin

is usually sufficient).

The power dissipated by the MOSFET in a boost converter is:

PFET =I2L(MAX)•RDS(ON)•DMAX +2•V2OUT •IL(MAX)•CRSS•f /1A

The first term in this equation represents the conduction

losses in the device, and the second term represents the

APPLICATIONS INFORMATION

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 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 substantial 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.

Boost Converter: Output Diode Selection

To maximize efficiency, a fast switching diode with low

forward voltage drop and low reverse leakage is desirable.

The peak reverse voltage that the diode must withstand is

equal to the regulator output voltage plus any additional

ringing across its anode-to-cathode during the on-time.

The average forward current in normal operation is equal

to the output current, and the peak current is equal to:

ID(PEAK) =IL(PEAK) =IL(MAX)•(1+

)

It is recommended that the peak repetitive reverse voltage

rating VRRM is higher than VOUT by a safety margin (a 10V

safety margin is usually sufficient).

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