LT3754_15 datasheet(21/30 Pages) LINER

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LT3754

3754fc

applicaTions inForMaTion

LED Open Circuit and PWM Dimming Ratios

The LT3754 monitors each LED pin voltage to determine if

the LED string has an open fault (LED pin voltage < 0.5V).

If an open LED fault occurs, the FAULT flag is pulled low.

Toavoidfalsedetectionoffaultsduringtheinitialconverter

start-up when VOUT is low, the LT3754 ignores low LED

pin voltages until VOUT reaches 90% of its maximum al-

lowed OVP level. Once this condition is met, the LT3754

monitors all LED pins for open LED faults. To avoid false

detection of faults during PWM dimming edges (where

LED pins can possibly ring and trip fault detection levels)

the LT3754 only monitors/updates fault conditions during

PWM high (and only after a blank duration of 2µs following

each PWM rising edge).

LED Short Circuit

AshortcircuitfaultbetweenthepositiveterminalofanLED

string (VOUT) and the negative terminal of the LED string

(LEDx pin) causes the channel to be disabled in order to

protect the internal current source. A resistive short is

allowed as long as (VOUT-VLEDx) < 6V. During the short,

however, cable inductance can cause the LED pin voltage

to overshoot past VOUT voltage. To avoid LED pin voltage

exceeding its absolute maximum rated voltage, a diode

may be required to clamp the LED pin. The anode of a

1N4148WS diode should be connected to the LED pin and

the cathode of the diode connected to the VOUT pin. Keep

the traces as short as possible. A Schottky diode should

not be used due to high reverse bias leakage currents.

Loop Compensation

Be sure to check the stability of the loop with the LEDs

connected (LED regulation loop) and disconnected

(Overvoltage Protection (OVP) regulation loop). Various

application circuits are shown in the data sheet which

coverarangeofVIN,VOUT,fOSC,outputpowerandinductor

current ripple values. For application requirements which

deviate from the circuits shown in the data sheet be sure

to check the stability of the final application over the full

VIN range, LED current range (if analog dimming) and

temperature range. Be aware that if the VCpincomponents

represent the dominant pole for the converter loop and

they have been adjusted to achieve stability, the VC pin

might move more slowly during load transient conditions

such as an all-LEDs-open fault. A slower moving VC pin

will add to VOUT overshoot during an all-LEDs-open fault.

An alternative compensation approach is to place the

dominant pole of the converter loop at the output. This

requires an increased output capacitor value but will allow

a much reduced Vc capacitor. The combination will allow

VC to move more quickly and VOUT to move more slowly

resulting in less overshoot during an all-LEDs-open fault.

Thermal Considerations

TheinternalpowerdissipationoftheLT3754comesfrom3

main sources: VIN quiescent current (IQ total), VIN current

for GATE switching (IGATE) and the LT3754 LED current

sources. Since the maximum operational VIN voltage is

40V, care should be taken when selecting the switching

frequency and type of external power MOSFET since the

current required from VIN for GATE switching is given by,

IGATE = fOSC • Qg

where Qg is the gate charge (at VGS = INTVCC) specified

for the MOSFET and fOSC is the programmed switching

frequency for the LT3754. A low Qg MOSFET should al-

ways be used when operating the LT3754 from high VIN

voltages. The internal junction temperature of the LT3754

can be estimated as:

TJ=TA+[VIN•(IQTOTAL+(fOSC•Qg))+(16•I(LEDX)•1.1V)]

•

θJA

where, TA is the ambient temperature for the LT3754

IQTOTALrepresentstheVINquiescentcurrentfortheLT3754

(not switching, PWM = 1.5V and CTRL = 0.1V) - illustrated

in the Typical Characteristics Graphs – plus the base cur-

rents of active channels (typically 16 • I(LED)/75).

θJA is

the thermal resistance of the package (34°C/W for the

5mm × 5mm QFN package).

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