LTC4449 datasheet(22/38 Pages) LINER

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LTC3774

3774fc

For more information www.linear.com/LTC3774

APPLICATIONS INFORMATION

The initial slope is the effect of the gate-to-source and

the gate-to-drain capacitance. The flat portion of the

curve is the result of the Miller multiplication effect of the

drain-to-gate capacitance as the drain drops the voltage

across the current source load. The upper sloping line is

due to the drain-to-gate accumulation capacitance and

the gate-to-source capacitance. The Miller charge (the

increase in coulombs on the horizontal axis from a to b

while the curve is flat) is specified for a given VDS drain

voltage, but can be adjusted for different VDS voltages by

multiplying the ratio of the application VDS to the curve

specified VDS values. A way to estimate the CMILLER term

is to take the change in gate charge from points a and b

on a manufacturer’s data sheet and divide by the stated

VDS voltage specified. CMILLER is the most important se-

lection criteria for determining the transition loss term in

the top MOSFET but is not directly specified on MOSFET

data sheets. CRSS and COS are specified sometimes but

definitions of these parameters are not included. When the

controller is operating in continuous mode the duty cycles

for the top and bottom MOSFETs are given by:

Main SwitchDuty Cycle =

VOUT

VIN

Synchronous SwitchDuty Cycle =

VIN – VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟

The power dissipation for the main and synchronous

MOSFETs at maximum output current are given by:

PMAIN =

VOUT

VIN

IMAX

( )

2 1+δ

(

)RDS(ON) +

VIN

( )

2 IMAX

⎛

⎝

⎜

⎞

⎠

⎟ RDR

(

) CMILLER

(

) •

VINTVCC – VTH(MIN)

VTH(MIN)

⎡

⎣

⎢

⎤

⎦

⎥

• f

PSYNC =

VIN – VOUT

VIN

IMAX

( )

2 1+δ

(

)RDS(ON)

where δ is the temperature dependency of RDS(ON), RDR

is the effective top driver resistance (approximately 2Ω at

VGS = VMILLER), VIN is the drain potential and the change

in drain potential in the particular application. VTH(MIN)

is the data sheet specified typical gate threshold voltage

specified in the power MOSFET data sheet at the specified

drain current. CMILLER is the calculated capacitance using

the gate charge curve from the MOSFET data sheet and

the technique described above.

BothMOSFETshaveI2RlosseswhilethetopsideN-channel

equation includes an additional term for transition losses,

which peak at the highest input voltage. For VIN < 20V,

the high current efficiency generally improves with larger

MOSFETs, while for VIN>20V,thetransitionlossesrapidly

increase to the point that the use of a higher RDS(ON)device

withlowerCMILLERactuallyprovideshigherefficiency.The

synchronous MOSFET losses are greatest at high input

voltage when the top switch duty factor is low or during

a short-circuit when the synchronous switch is on close

to 100% of the period.

The term (1 + δ ) is generally given for a MOSFET in the

form of a normalized RDS(ON) vs temperature curve, but

δ = 0.005/°C can be used as an approximation for low

voltage MOSFETs.

An optional Schottky diode across the synchronous

MOSFET conducts during the dead time between the con-

ductionofthetwolargepowerMOSFETs.Thispreventsthe

bodydiodeofthebottomMOSFETfromturningon,storing

chargeduringthedeadtimeandrequiringareverse-recovery

period which could cost as much as several percent in effi-

ciency.A2Ato8ASchottkyisgenerallyagoodcompromise

for both regions of operation due to the relatively small

averagecurrent.Largerdiodesresultinadditionaltransition

loss due to their larger junction capacitance.

MOSFET Driver Selection

GatedriverICs,DrMOSsandpowerblockswithaninterface

compatible with the LTC3774's three-state PWM outputs

or the LTC3774's PWM/PWMEN outputs can be used.

Always enable the power stage first, before the LTC3774

is enabled.

CIN and COUT Selection

Incontinuousmode,thesourcecurrentofthetopMOSFET

is a square wave of duty cycle (VOUT)/(VIN). To prevent

large voltage transients, a low ESR capacitor sized for the

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