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LT3755/LT3755-1

37551fa

APPLICATIONS INFORMATION

Table 3. MOSFET Manufacturers

VENDOR

PHONE

WEB

Vishay Siliconix

402-563-6866

www.vishay.com

Fairchild

972-910-8000

www.fairchildsemi.com

International Rectiﬁer

310-252-7105

www.irf.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

increases with the temperature, from the output during the

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

PHONE

WEB

On Semiconductor

888-743-7826

www.onsemi.com

Diodes, Inc.

805-446-4800

www.diodes.com

Central Semiconductor

631-435-1110

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 LT3755. 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 ≤

007

ILED

For buck-boost, select a resistor according to:

RSENSE,BUCK-BOOST ≤

()

LED LED

•.

007

For boost, select a resistor according to:

RSENSE,BOOST ≤

LED

•.

•

007

The placement of RSENSE should be close to the source of

the NMOS FET and GND of the LT3755. The SENSE input

to LT3755 should be a Kelvin connection to the positive

terminal of RSENSE.

Inductor Selection

The inductor used with the LT3755 should have a saturation

current rating appropriate to the maximum switch current

selected with the RSENSE resistor. Choose an inductor value

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 (TSW = 1/fOSC):

BUCK

SENSE

LED

()

••

–

•.

002

LBUCK-BOOST =

()

SENSE

LED

••

•

•0..02V

BOOST

SENSE

LED

()

••

–

•.

002

Table 5 provides some recommended inductor vendors.

Table 5. Inductor Manufacturers

VENDOR

PHONE

WEB

Sumida

408-321-9660

www.sumida.com

Würth Elektronik

605-886-4385

www.we-online.com

Coiltronics

561-998-4100

www.cooperet.com

Vishay

402-563-6866

www.vishay.com

Coilcraft

847-639-6400

www.coilcraft.com

Loop Compensation

The LT3755 uses an internal transconductance error ampli-

ﬁer whose VC output compensates the control loop. The

external inductor, output capacitor and the compensation

resistor and capacitor determine the loop stability.

The inductor and output capacitor are chosen based on

performance, size and cost. The compensation resistor

and capacitor at VC are selected to optimize control loop

response and stability. For typical LED applications, a

2.2nF compensation capacitor at VC is adequate, and

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