LT3754 datasheet(13/28 Pages) LINER

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LT3754

3754f

In addition, the current drive required for GATE switching

should also be kept low in the case of high VIN voltages

(see “Thermal Considerations” in the Applications Informa-

tion section). The RDS(ON) of the MOSFET will determine

d.c. power losses but will usually be less signiﬁcant

compared to switching losses. Be aware of the power

dissipation within the MOSFET by calculating d.c. and

switching losses and deciding if the thermal resistance

of the MOSFET package causes the junction temperature

to exceed maximum ratings.

Table 4. MOSFET Manufacturers

MANUFACTURER

PHONE NUMBER

WEB

Vishay Siliconix

402-563-6866

www.vishay.com

International Rectiﬁer

310-252-7105

www.irf.com

Fairchild

972-910-8000

www.fairchildsemi.com

Power MOSFET: Current Sense Resistor

The LT3754 current mode boost converter controls peak

current in the inductor by controlling peak MOSFET current

in each switching cycle. The LT3754 monitors current in the

external N-channel power MOSFET by sensing the voltage

across a sense resistor (RS) connected between the source

of the FET and the power ground in the application. The

length of these tracks should be minimized and a Kelvin

sense should be taken from the top of RS to the sense

pin. A 52mV sense pin threshold combined with the value

of RS sets the maximum cycle-by-cycle peak MOSFET

current. The low 52mV threshold improves efﬁciency and

determines the value for RS given by:

APPLICATIONS INFORMATION

where

LPEAK

≤

−

0 7

•.

()

LEDx

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

••

•

MOS

SFET duty cycle

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

OUT MAX

IN MIN

()

OUT(MAX) =

()+

N number

FMAX

•

()

ff LEDs in each string,

Vimum

FMAX

() max

LED forward voltage drop,

IN MIN

() min

mum input voltage to the inductor,

and the 0.5 term represents an inductor peak-to-peak ripple

current of 50% of average inductor current.

The scale factor of • 0.7 ensures the boost converter

can meet the peak inductor requirements of the loop by

accounting for the combined errors of the 50mV sense

threshold, ILEDx, RS and circuit efﬁciency.

Example: For a 12W LED driver application requiring 16

strings of 10 LEDs each driven with 20mA, and choosing

VIN(MIN) = 8V, VOUT(MAX) = (4V • 10)+1V = 41V and ILEDx

= 20mA, the value for RS is chosen as:

LPEAK

≤≤

⎛

⎝

⎜

⎞

0 7

16 0 02

•.

•• .

()

⎠⎠

⎟

()

≤≤

•.

10 25

0 7

205

17.7 m

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