LT3756 datasheet(13/20 Pages) LINER

13 / 20 page

LT3756/LT3756-1

37561f

APPLICATIONS INFORMATION

Thermal Considerations

The LT3756 and LT3756-1 are rated to a maximum input

voltage of 100V. Careful attention must be paid to the

internal power dissipation of the IC at higher input volt-

ages to ensure that a junction temperature of 125°C is not

exceeded. This junction limit is especially 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 1.5mA) 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) = 60V, fSW = 400kHz, and having a FET

with QG = 20nC, the maximum IC junction temperature

will be approximately:

TJ = 85°C + [60V (1.5mA + 400kHz • 20nC) • 68°C/W]

= 124°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.

Frequency Synchronization (LT3756-1 Only)

The LT3756-1 switching frequency can be synchronized to

an external clock using the SYNC pin. For proper operation,

the RT resistor should be chosen for a switching frequency

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 (LT3756 Only)

The LT3756 provides 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, the only way for the FB pin to be within 50mV

of the 1.24V regulation voltage is for an open LED event to

have occurred. When an open LED fault occurs, the output

may initially overshoot the FB regulation point by several

percent, due to slew rate limitations on VC and the absence

of any load on the output. In order to ensure the voltage

on switching components remains below programmed

limits, and to guarantee accurate reporting of the open

LED fault, adding a silicon diode between OPENLED and

SS is recommended, as well as a 10k resistor in series

with the soft-start capacitor, if one is used.

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.

The switching frequency, output current and tolerable input

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:

IN F

LED A

OUT

()

( )

(

)

••

•

μμ

Therefore, a 4.7μF capacitor is an appropriate selection

for a 400kHz boost regulator with 12V input, 48V output

and 1A load.

LT3756 Datasheet(PDF) 13 Page - Linear Technology

LT3756 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL