LT3791_15 datasheet(21/28 Pages) LINER

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LT3791

3791fb

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Power MOSFET Selections and Efficiency

Considerations

The LT3791 requires four external N-channel power MOS-

FETs,twoforthetopswitches(switchM1andM4,shownin

Figure 1) and two for the bottom switches (switch M2 and

M3showninFigure1).Importantparametersforthepower

MOSFETs are the breakdown voltage, VBR(DSS), threshold

voltage, VGS(TH), on-resistance, RDS(ON), reverse transfer

capacitance, CRSS, and maximum current, IDS(MAX).

The drive voltage is set by the 5V INTVCC supply. Con-

sequently, logic-level threshold MOSFETs must be used

in LT3791 applications. If the input voltage is expected

to drop below the 5V, then sub-logic threshold MOSFETs

should be considered.

In order to select the power MOSFETs, the power dis-

sipated by the device must be known. For switch M1, the

maximum power dissipation happens in boost operation,

when it remains on all the time. Its maximum power dis-

sipation at maximum output current is given by:

PM1(BOOST) =

ILED • VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

•

ρT •RDS(ON)

where

ρT is a normalization factor (unity at 25°C)

accounting for the significant variation in on-resistance

with temperature, typically 0.4%/°C as shown in Figure

10. For a maximum junction temperature of 125°C, using

a value of

ρT = 1.5 is reasonable.

Switch M2 operates in buck operation as the synchronous

rectifier. Its power dissipation at maximum output current

is given by:

PM2(BUCK) =

VIN – VOUT

VIN

•ILED2 •ρT •RDS(ON)

Switch M3 operates in boost operation as the control

switch. Its power dissipation at maximum current is

given by:

PM3(BOOST) =

VOUT – VIN

(

)•VOUT

VIN2

•ILED2 •ρT •RDS(ON)

+ k • VOUT3 •

ILED

VIN

•CROSS • f

where CRSS is usually specified by the MOSFET manufac-

turers. The constant k, which accounts for the loss caused

by reverse-recovery current, is inversely proportional to

the gate drive current and has an empirical value of 1.7.

For switch M4, the maximum power dissipation happens

in boost operation, when its duty cycle is higher than

50%. Its maximum power dissipation at maximum output

current is given by:

PM4(BOOST) =

VIN

VOUT

•

ILED • VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

•

ρT •RDS(ON)

For the same output voltage and current, switch M1 has

the highest power dissipation and switch M2 has the low-

est power dissipation unless a short occurs at the output.

applicaTions inForMaTion

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