LT3474 datasheet(13/28 Pages) AD

13 / 28 page

LT3755/LT3755-1/LT3755-2

Rev. E

For more information www.analog.com

APPLICATIONS INFORMATION

INTVCC Regulator Bypassing and Operation

The INTVCC pin requires a capacitor for stable operation

and to store the charge for the large GATE switching cur-

rents. Choose a 10V rated low ESR, X7R or X5R ceramic

capacitor for best performance. A 4.7µF capacitor will be

adequate for many applications. Place the capacitor close

to the IC to minimize the trace length to the INTVCC pin

and also to the IC ground.

An internal current limit on the INTVCC output protects

the LT3755 from excessive on-chip power dissipation.

The minimum value of this current should be considered

when choosing the switching NMOS and the operating

frequency.

IINTVCC can be calculated from the following equation:

IINTVCC = QG • fOSC

Careful choice of a lower QG FET will allow higher switch-

ing frequencies, leading to smaller magnetics. The INTVCC

pin has its own undervoltage disable (UVLO) set to 4.1V

(typical) to protect the external FETs from excessive power

dissipation caused by not being fully enhanced. If the

INTVCC pin drops below the UVLO threshold, the GATE

and PWMOUT pins will be forced to 0V and the soft-start

pin will be reset.

If the input voltage, VIN, will not exceed 8V, then the IN-

TVCC pin could be connected to the input supply. Be aware

that a small current (less than 12μA) will load the INTVCC

in shutdown. This action allows the LT3755 to operate

from VIN as low as 4.5V. If VIN is normally above, but

occasionally drops below the INTVCC regulation voltage,

then the minimum operating VIN will be close to 6V. This

value is determined by the dropout voltage of the linear

regulator and the INTVCC undervoltage lockout threshold

mentioned above.

Programming the Turn-On and Turn-Off Thresholds

with the SHDN/UVLO Pin

ThefallingUVLOvaluecanbeaccuratelysetbytheresistor

divider. A small 2.1µA pull-down current is active when

SHDN/UVLO is below the threshold. The purpose of this

currentistoallowtheusertoprogramtherisinghysteresis.

SHDN/UVLO

LT3755

VIN

37551 F01

Figure 1. Resistor Connection to Set

VIN Undervoltage Shutdown Threshold

The following equations should be used to determine the

values of the resistors:

VIN,FALLING = 1.22 •

R1+ R2

IN,RISING = 2.1µA • R1 + VIN,FALLING

LED Current Programming

The LED current is programmed by placing an appropriate

value current sense resistor, RLED, in series with the LED

string. The voltage drop across RLED is (Kelvin) sensed

by the ISP and ISN pins. Typically, sensing of the current

should be done at the top of the LED string. If this option

is not available, then the current may be sensed at the

bottom of the string, but take caution that the minimum

ISN value does not fall below 3V, which is the lower limit of

the LED current regulation function. The CTRL pin should

be tied to a voltage higher than 1.2V to get the full-scale

100mV (typical) threshold across the sense resistor. The

CTRL pin can also be used to dim the LED current to zero,

although relative accuracy decreases with the decreasing

voltage sense threshold. When the CTRL pin voltage is

less than 1V, the LED current is:

ILED =

VCTRL − 100mV

RLED • 10

WhentheCTRLpinvoltageisbetween1Vand1.2VtheLED

current varies with CTRL, but departs from the equation

abovebyanincreasingamountasCTRLvoltageincreases.

Ultimately, above CTRL = 1.2V the LED current no longer

varies with CTRL. At CTRL = 1.1V, the actual value of ILED

is ~98% of the equation’s estimate.

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