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LT3791-1

37911fa

For more information www.linear.com/LT3791-1

Soft-Start

Soft-startreducestheinputpowersources’surgecurrents

by gradually increasing the controller’s current limit (pro-

portional to an internally buffered clamped equivalent of

VC). The soft-start interval is set by the soft-start capacitor

selection according to the following equation

tSS =

1.2V

14µA

• CSS

A 100k resistor must be placed between SS and VREF for

theLT3791-1.This100kresistoralsocontributestheextra

SS charge current. Make sure CSS is large enough when

there is loading during start-up.

Loop Compensation

The LT3791-1 uses an internal transconductance error

amplifier whose VC output compensates the control loop.

Theexternalinductor,outputcapacitorandthecompensa-

tion resistor and capacitor determine the loop stability.

The inductor and output capacitor are chosen based on

performance,sizeandcost.Thecompensationresistorand

capacitor at VC are set to optimize control loop response

andstability.Fortypicalapplications,a10nFcompensation

capacitor at VC is adequate, and a series resistor should

always be used to increase the slew rate on the VC pin to

maintain tighter regulation of output current during fast

transients on the input supply of the converter.

Power MOSFET Selections and Efficiency

Considerations

The LT3791-1 requires four external N-channel power

MOSFETs, two for the top switches (switch M1 and M4,

showninFigure1)andtwoforthebottomswitches(switch

M2 and M3 shown in Figure 1). Important parameters for

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

thresholdvoltage,VGS(TH),on-resistance,RDS(ON),reverse

transfercapacitance,CRSS,andmaximumcurrent,IDS(MAX).

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

sequently, logic-level threshold MOSFETs must be used

in LT3791-1 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

withtemperature,typically0.4%/°CasshowninFigure 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

• CRSS • 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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