LT3474 datasheet(14/28 Pages) AD

14 / 28 page

LT3755/LT3755-1/LT3755-2

Rev. E

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APPLICATIONS INFORMATION

When VCTRL is higher than 1.2V, the LED current is

regulated to:

ILED =

100mV

RLED

The LED current programming feature can increase total

dimming range by a factor of 10. The CTRL pin should

not be left open (tie to VREF if not used). The CTRL pin

can also be used in conjunction with a thermistor to

provide overtemperature protection for the LED load, or

with a resistor divider to VIN to reduce output power and

switching current when VIN is low. The presence of a time

varying differential voltage signal (ripple) across ISP and

ISN at the switching frequency is expected. The amplitude

of this signal is increased by high LED load current, low

switching frequency and/or a smaller value output filter

capacitor. Some level of ripple signal is acceptable: the

compensation capacitor on the VC pin filters the signal so

the average difference between ISP and ISN is regulated

to the user-programmed value. Ripple voltage amplitude

(peak-to-peak) in excess of 20mV should not cause mis-

operation, but may lead to noticeable offset between the

average value and the user-programmed value.

Programming Output Voltage (Constant Voltage

Regulation) or Open LED/Overvoltage Threshold

For a boost application, the output voltage can be set by

selecting the values of R3 and R4 (see Figure 2) according

to the following equation:

VOUT = 1.25 •

R3 + R4

For a boost type LED driver, set the resistor from the output

to the FB pin such that the expected VFB during normal

LT3755

VOUT

37551 F02

Figure 2. Feedback Resistor Connection for

Boost or SEPIC LED Driver

LT3755

VOUT

100k

37551 F03

LED

ARRAY

RSEN(EXT)

COUT

–

operation will not exceed 1.1V. For an LED driver of buck or

a buck-boost configuration, the output voltage is typically

level-shifted to a signal with respect to GND as illustrated

in Figure 3. The output can be expressed as:

VOUT = VBE + 1.25 •

Figure 3. Feedback Resistor Connection for

Buck Mode or Buck-Boost Mode LED Driver

ISP/ISN Short-Circuit Protection Feature (for SEPIC)

The ISP and ISN pins have a protection feature indepen-

dent of the LED current sense feature that operates at

ISN below 3V. The purpose of this feature is to provide

continuous current sensing when ISN is below the LED

current sense common mode range (during start-up or

an output short-circuit fault) to prevent the development

of excessive switching currents that could damage the

power components in a SEPIC converter. The action

threshold (150mV, typ) is above the default LED current

sense threshold, so that no interference will occur over

the ISN voltage range where these two functions overlap.

This feature acts in the same manner as SENSE current

limit — it prevents GATE from going high (switch turn-on)

until the ISP/ISN difference falls below the threshold. If the

load has appreciable series inductance, use of a Schottky

clamp from GND to ISN is recommended for the SEPIC

to prevent excessive current flowing from the ISN pin in

a fault.

Dimming Control

There are two methods to control the current source for

dimmingusingtheLT3755.OnemethodusestheCTRLpin

toadjustthecurrentregulatedintheLEDs.Asecondmethod

uses the PWM pin to modulate the current source between

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