LT3474 datasheet(16/28 Pages) AD

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LT3755/LT3755-1/LT3755-2

Rev. E

For more information www.analog.com

APPLICATIONS INFORMATION

important when operating at high ambient temperatures.

The majority of the power dissipation in the IC comes from

the supply current needed to drive the gate capacitance of

the external power MOSFET. This gate drive current can

be calculated as:

IGATE = fSW • QG

A low QG power MOSFET should always be used when op-

erating at high input voltages, and the switching frequency

should also be chosen carefully to ensure that the IC does

not exceed a safe junction temperature. The internal junc-

tion temperature of the IC can be estimated by:

TJ = TA + [VIN (IQ + fSW • QG) • θJA]

where TA is the ambient temperature, IQ is the quiescent

current of the part (maximum 1.7mA) and θJA is the

package thermal impedance (68°C/W for the 3mm × 3mm

QFN package). For example, an application with TA(MAX)

= 85°C, VIN(MAX) = 40V, fSW = 400kHz, and having a FET

with QG = 20nC, the maximum IC junction temperature

will be approximately:

TJ = 85°C + [40V (1.7mA + 400kHz • 20nC) • 68°C/W]

= 111°C

The Exposed Pad on the bottom of the package must be

soldered to a ground plane. This ground should then be

connected to an internal copper ground plane with thermal

vias placed directly under the package to spread out the

heat dissipated by the IC.

If LT3755 junction temperature reaches 165°C, the GATE

and PWMOUT pins will be driven to GND and the soft-

start (SS) pin will be discharged to GND. Switching will

be enabled after device temperature is reduced 10°C. This

functionisintendedtoprotectthedeviceduringmomentary

thermal overload conditions.

Frequency Synchronization (LT3755-1 Only)

The LT3755-1 switching frequency can be synchronized to

anexternalclockusingtheSYNCpin.Forproperoperation,

the RTresistorshouldbechosenforaswitchingfrequency

20% lower than the external clock frequency. The SYNC

pin is disabled during the soft-start period.

Observation of the following guidelines about the SYNC

waveform will ensure proper operation of this feature.

Driving SYNC with a 50% duty cycle waveform is always

a good choice, otherwise, maintain the duty cycle between

20% and 60%. When using both PWM and SYNC features,

the PWM signal rising edge should occur at least 200ns

before the SYNC rising edge (VIH) for optimal PWM

performance. If the SYNC pin is not used, it should be

connected to GND.

Open LED Detection (LT3755 and LT3755-2)

The LT3755 and LT3755-2 provide an open-drain status

pin, OPENLED, that pulls low when the FB pin is within

~50mV of its 1.25V regulated voltage. If the open LED

clamp voltage is programmed correctly using the FB pin,

then the FB pin should never exceed 1.1V when LEDs are

connected,therefore,theonlywayfortheFBpintobewithin

50mV of the regulation voltage is for an open LED event to

have occurred. The key difference between the LT3755 and

LT3755-2 is the behavior of the OPENLED pin when the FB

pin crosses and re-crosses the FB overvoltage threshold

(1.31V typ). The LT3755-2 asserts/de-asserts OPENLED

freely when crossing the 1.31V threshold. The LT3755,

by comparison, de-asserts OPENLED when FB exceeds

1.31V and is prevented from re-asserting OPENLED until

the FB pin falls below the 1.2V (typ) open LED threshold

and clears the fault. The LT3755-2 has the more general

purpose behavior and is recommended for applications

using OPENLED.

Input Capacitor Selection

The input capacitor supplies the transient input current for

the power inductor of the converter and must be placed

and sized according to the transient current requirements.

Theswitchingfrequency,outputcurrentandtolerableinput

voltage ripple are key inputs to estimating the capacitor

value. An X7R type ceramic capacitor is usually the best

choice since it has the least variation with temperature and

DC bias. Typically, boost and SEPIC converters require a

lower value capacitor than a buck mode converter. As-

suming that a 100mV input voltage ripple is acceptable,

the required capacitor value for a boost converter can be

estimated as follows:

CIN(µF) = ILED(A) •

VOUT

VIN

• tSW(µs) •

µF

A • µs

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