LT8705 datasheet(15/24 Pages) LINER

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LT3840

3840fa

For more information www.linear.com/LT3840

applicaTions inForMaTion

MOSFET Selection

Two external N-channel MOSFETs are used with the

LT3840 controller, one top (main) switch, and one bot-

tom (synchronous) switch. The gate drive levels are set

by the INTVCC voltage. Therefore, standard or logic level

threshold MOSFETs can be used.

Selection criteria for the power MOSFETs include break-

down voltage (BVDSS), maximum current (IOUTMAX), on-

resistance (RDSON) and gate charge.

First select a MOSFET with a BVDSS greater than VIN. Next

consider the package and current rating of the device. The

maximumcurrentratingofthedevicetypicallycorresponds

to a particular package. The RMS current of each device

is calculated below:

TopSwitchDuty Cycle(DCTOP)=

VOUT

VIN

TopSwitchRMS Current = DCTOP •IOUTMAX

BottomSwitchDuty Cycle(DCBOT)=

VIN – VOUT

VIN

BottomSwitchRMS Current = DCBOT •IOUTMAX

Selectadevicethathasacontinuouscurrentratinggreater

than the calculated RMS current.

Lastly,considertheRDSONandgatechargeoftheMOSFET.

These two parameters are considered together because

theyaretypicallyinverselyproportionaltooneanother.The

RDSON determines the conduction losses of the MOSFET

and the gate charge determines the switching losses.

The switching and conduction losses of each MOSFET

can be calculated as follows:

PCOND(TOP)=IOUT(MAX)2•

VOUT

VIN

•RDS(ON)

PCOND(BOT)=IOUT(MAX)2•

VIN – VOUT

VIN

•RDS(ON)

Note that RDSON has a large positive temperature depen-

dence. The MOSFET manufacturer’s data sheet contains

a curve, RDSON vs Temperature. In the main MOSFET,

transition losses are proportional to VIN2 and can be con-

siderably large in high voltage applications (VIN > 20V).

Calculate the maximum transition losses:

PTRAN(TOP)= VIN•IOUT • fSW •

QGSW

IDRIVE

where QGSW can be found in the MOSFET specification

or calculated by:

QGSW =QGD+

QGS

and IDRIVE = 1A

ThetotalmaximumpowerdissipationsoftheMOSFETare:

PTOP(TOTAL) = PCOND(TOP) + PTRAN(TOP)

PBOT(TOTAL) = PCOND(BOT)

Complete a thermal analysis to ensure that the MOSFET’s

junction temperatures are not exceeded.

TJ = TA + P(TOTAL) • θJA

where

θJA is the package thermal resistance and TA is the

ambient temperature. Keep the calculated TJ below the

maximumspecifiedjunctiontemperature,typically150°C.

Note that when VIN is high and fSW is high, the transition

losses may dominate. A MOSFET with higher RDSON and

lowergatechargemayprovidehigherefficiency.MOSFETs

with a higher voltage BVDSS specification usually have

higher RDSON and lower gate charge.

A Schottky diode can be inserted in parallel with the

synchronous MOSFET to conduct during the dead time

between the conduction of the two power MOSFETs. This

prevents the body diode of the bottom MOSFET from turn-

ing on, storing charge during the dead time and requiring

a reverse recovery period.

Input Capacitor Selection

Alocalinputbypasscapacitorisrequiredforbuckconvert-

ersbecausetheinputcurrentispulsedwithfastriseandfall

times. The input capacitor selection criteria are based on

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