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LT3754

3754f

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. To

avoid false detection of faults during the initial converter

startup when VOUT is low, the LT3754 ignores low LED

pin voltages until VOUT reaches 90% of its maximum

allowed 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

A short circuit fault between the positive terminal of an

LED 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.

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 datasheet which cover a

range of VIN, VOUT, fOSC, output power and inductor current

ripple values. For application requirements which deviate

from the circuits shown in the datasheet 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 VC pin components 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 VCpinwilladdtoVOUT

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

The internal power dissipation of the LT3754 comes from 3

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

always 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

IQTOTAL represents the VIN quiescent current for the LT3754

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

in the Typical Characteristics Graphs – plus the base

currents 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).

LT3754 Datasheet(PDF) 21 Page - Linear Technology

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