LT3592IMSE datasheet(10/24 Pages) LINER

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LT3592

3592fa

Oscillator

The frequency of operation is programmed by an external

resistor from RT to ground. Table 1 shows RT values for

commonly used oscillator frequencies, and refer to the Typi-

cal Performance Characteristics curve for other values.

APPLICATIONS INFORMATION

The BRIGHT mode current is given by:

IBRIGHT = 200mV/RSENSE

The DIM mode current is 10% of the BRIGHT mode value.

The maximum allowed DC value of the BRIGHT mode cur-

rent is 500mA. When the recommended component values

are used in a 900kHz 2 LED application, the transient from

switching between BRIGHT and DIM currents will be less

than 50μs in duration.

The sense resistor used should exhibit a low TC to keep

the LED current from drifting as the operating temperature

changes.

The BRIGHT pin can tolerate voltages as high as 36V and

can be safely tied to VIN even in high voltage applications,

but it also has a low threshold voltage (~0.7V) that allows

it to interface to logic level control signals.

Input Voltage Range

The maximum allowed input voltage for the LT3592 is

36V. The minimum input voltage is determined by either

the LT3592’s minimum operating voltage of 3.6V or by

its maximum duty cycle. The duty cycle is the fraction of

time that the internal switch is on and is determined by

the input and output voltages:

OUT + VD

IN –VSW + VD

where VD is the forward voltage drop of the catch diode

(~0.4V) and VSW is the voltage drop of the internal switch

Table 2. Inductor Vendor Information

SUPPLIER

PHONE

FAX

WEBSITE

Panasonic

(800) 344-2112

www.panasonic.com/industrial/components/components.html

Vishay

(402) 563-6866

(402) 563-6296

www.vishay.com/resistors

Coilcraft

(847) 639-6400

(847) 639-1469

www.coilcraft.com

CoEv Magnetics

(800) 227-7040

(650) 361-2508

www.circuitprotection.com/magnetics.asp

Murata

(814) 237-1431

(800) 831-9172

(814) 238-0490

www.murata.com

Sumida

USA: (847) 956-0666

Japan: 81(3) 3607-5111

USA: (847) 956-0702

Japan: 81(3) 3607-5144

www.sumida.com

TDK

(847) 803-6100

(847) 803-6296

www.component.tdk.com

TOKO

(847) 297-0070

(847) 699-7864

www.tokoam.com

FB Resistor Network

The output voltage limit is programmed with a resistor

divider between the output and the VFB pin. This is the

voltage that the output will be clamped to in case the LED

goes open circuit. Choose the resistors according to

R1 = R2([VOUT/1.21V] – 1)

Be sure to choose VOUT such that it does not interfere with

the operation of the current control loop; it should be set

at least 10% above the maximum expected LED voltage

for the selected BRIGHT output current. R2 should be 20k

or less to avoid bias current errors. An optional phase-

lead capacitor of 22pf between VOUT and VFB reduces

light-load ripple.

Output Current Selection

The output current levels are programmed by the value of

the external current sense resistor between CAP and OUT.

Table 1. RT Values for Selector Oscillator Frequencies

fOSC

400kHz

357k

900kHz

140k

2.2MHz

48.7k

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