LT3755 datasheet(11/24 Pages) LINER

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LT3755/LT3755-1

37551fa

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. The value of the capacitor

is determined primarily by the stability of the regulator

rather than the gate charge, QG, of the switching NMOS—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 4.3V

(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 7V, then the

INTVCC pin should be connected to the input supply. Be

aware that a small current (typically less than 10μA) will

load the INTVCC in shutdown. 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 4.5V (4.3V typical) INTVCC undervoltage

lockout threshold mentioned above.

SHDN/UVLO

LT3755

VIN

37551 F01

Figure 1

Programming the Turn-On and Turn-Off Thresholds

with the SHDN/UVLO Pin

The falling UVLO value can be accurately set by the resis-

tor divider. A small 2μA pull-down current is active when

SHDN/UVLO is below the 1.24V threshold. The purpose

of this current is to allow the user to program the rising

hysteresis. The following equations should be used to

determine the values of the resistors:

VμA

IN FALLING

IN RISING HYST

.•

124

2••R1

LED Current Programming

The LED current is programmed by placing an appropriate

value current sense resistor between 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 1.1V 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 1.1V, the LED

current is:

VmV

LED

CTRL

LED

− 100

•

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