LT3496 datasheet(17/26 Pages) AD

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LT3756/LT3756-1/LT3756-2

Rev. C

For more information www.analog.com

Schottky Rectifier Selection

The power Schottky diode conducts current during the

interval when the switch is turned off. Select a diode rated

for the maximum SW voltage. If using the PWM feature for

dimming, it is important to consider diode leakage, which

increaseswiththetemperature,fromtheoutputduringthe

PWM low interval. Therefore, choose the Schottky diode

with sufficiently low leakage current. Table 4 has some

recommended component vendors.

Table 4. Schottky Rectifier Manufacturers

VENDOR

WEB

On Semiconductor

www.onsemi.com

Diodes, Inc.

www.diodes.com

Central Semiconductor

www.centralsemi.com

Sense Resistor Selection

The resistor, RSENSE, between the source of the exter-

nal NMOS FET and GND should be selected to provide

adequate switch current to drive the application without

exceeding the 108mV (typical) current limit threshold on

the SENSE pin of LT3756. For buck mode applications,

select a resistor that gives a switch current at least 30%

greater than the required LED current. For buck mode,

select a resistor according to:

RSENSE,BUCK ≤

0.07V

ILED

For buck-boost, select a resistor according to:

RSENSE,BUCK-BOOST ≤

VIN • 0.07V

VIN + VLED

(

)ILED

For boost, select a resistor according to:

RSENSE,BOOST ≤

VIN • 0.07V

VLED • ILED

The placement of RSENSE should be close to the source of

the NMOS FET and GND of the LT3756. The SENSE input

to LT3756 should be a Kelvin connection to the positive

terminal of RSENSE.

These equations provide an estimate of the sense resistor

value based on reasonable assumptions about induc-

tor current ripple during steady state switching. Lower

values of sense resistor may be required in applications

where inductor ripple current is higher. Examples include

applications with current limited operation at high duty

cycle, and those with discontinuous conduction mode

(DCM) switching. It is always prudent to verify the peak

inductor current in the application to ensure the sense

resistor selection provides margin to the SENSE current

limit threshold.

Inductor Selection

TheinductorusedwiththeLT3756shouldhaveasaturation

current rating appropriate to the maximum switch current

selectedwiththeRSENSEresistor.Chooseaninductorvalue

based on operating frequency, input and output voltage to

provide a current mode ramp on SENSE during the switch

on-time of approximately 20mV magnitude. The following

equations are useful to estimate the inductor value for

continuous conduction mode operation:

LBUCK =

RSENSE • VLED VIN – VLED

(

)

VIN • 0.02V • fOSC

LBUCK-BOOST =

RSENSE • VLED • VIN

VLED + VIN

(

) • 0.02V • fOSC

LBOOST =

RSENSE • VIN VLED – VIN

(

)

VLED • 0.02V • fOSC

Table 5 provides some recommended inductor vendors.

Table 5. Inductor Manufacturers

VENDOR

WEB

Sumida

www.sumida.com

Würth Elektronik

www.we-online.com

Coiltronics

www.cooperet.com

Vishay

www.vishay.com

Coilcraft

www.coilcraft.com

APPLICATIONS INFORMATION

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