LT3595 datasheet(13/32 Pages) LINER

13 / 32 page

LT3760

3760fc

In addition, the current drive required for GATE switching

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

(see“ThermalConsiderations”intheApplicationsInforma-

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 LT3760 current mode boost converter controls peak

currentintheinductorbycontrollingpeakMOSFETcurrent

ineachswitchingcycle.TheLT3760monitorscurrentinthe

external N-channel power MOSFET by sensing the voltage

acrossasenseresistor(RS)connectedbetweenthesource

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:

≤

52mV • 0.7

IL(PEAK)

applicaTions inForMaTion

where

IL(PEAK) =

−D

• 8 •ILEDx • 1+

0.5

= MOSFET duty cycle = 1−

VIN(MIN)

VOUT(MAX)

VOUT(MAX) = N • VF(MAX)

(

)+1V

= number of LEDs in each string,

VF(MAX) = maximum LED forward voltage drop,

VIN(MIN) = minimum input voltage to the inductor,

ILED = current in each LED pin,

andthe0.5termrepresentsaninductorpeak-to-peakripple

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 52mV sense

threshold, ILEDx, RS and circuit efficiency.

Example: For a 12W LED driver application requiring 8

strings of 10 LEDs each driven with 40mA, and choosing

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

= 40mA, the value for RS is chosen as:

≤

52mV • 0.7

IL(PEAK)

≤

52mV • 0.7

• 8 • 0.04











 • 1+ 0.25

(

)

≤

52mV • 0.7

2.05

≤ 17.7 mΩ

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