LT3956 datasheet(13/20 Pages) LINER

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LT3956

3956f

applicaTions inForMaTion

will be enabled after the device temperature drops 10°C.

This function is intended to protect the device during

momentary overload conditions.

The major contributors to internal power dissipation are

the current in the linear regulator to drive the switch, and

the ohmic losses in the switch. The linear regulator power

is proportional to VIN and switching frequency, so at high

VIN the switching frequency should be chosen carefully

to ensure that the IC does not exceed a safe junction

temperature. The internal junction temperature of the IC

can be estimated by:

TJ = TA + [VIN • (IQ + fSW • 7nC) + ISW2 • 0.14Ω • DSW]

•

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 (43°C/W for the 5mm

× 6mm

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

85°C, VIN(MAX)=60V,fSW=400kHz,andhavinganaverage

switching current of 2.5A at 70% duty cycle, the maximum

IC junction temperature will be approximately:

TJ = 85°C + [(2.5A)2 • 0.14Ω • 0.7 + 60V •

(1.7mA + 400kHz • 7nC)] • 43°C/W= 123°C

The Exposed Pads on the bottom of the package must be

solderedtoaplane.Theseshouldthenbeconnectedtointer-

nal copper planes with thermal vias placed directly under

the package to spread out the heat dissipated by the IC.

Open LED Detection

The LT3956 provides an open-drain status pin, VMODE,

that pulls low when the FB pin is within ~50mV of its 1.25V

regulated voltage. If the open LED clamp voltage is pro-

grammedcorrectlyusingtheFBpin,thentheFBpinshould

neverexceed1.1VwhenLEDsareconnected,therefore,the

only way for the FB pin to be within 50mV of the regulation

voltage is for an open LED event to have occurred.

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:

µF

A µs

IN µF

LED A

OUT

SW µs

( )

•

Therefore, a 4.7µF capacitor is an appropriate selection

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

and 1A load.

WiththesameVINvoltagerippleof100mV,theinputcapaci-

tor for a buck converter can be estimated as follows:

µF

A µs

IN µF

LED A

SW µs

( )

•

4 7

A 10µF input capacitor is an appropriate selection for a

400kHz buck mode converter with a 1A load.

In the buck mode configuration, the input capacitor has

large pulsed currents due to the current returned through

the Schottky diode when the switch is off. In this buck

converter case it is important to place the capacitor as

close as possible to the Schottky diode and to the PGND

return of the switch. It is also important to consider the

ripple current rating of the capacitor. For best reliability,

this capacitor should have low ESR and ESL and have an

adequate ripple current rating. The RMS input current for

a buck mode LED driver is:

IIN(RMS) =

( )

D D

LED •

–

•

where D is the switch duty cycle.

Table 2. Recommended Ceramic Capacitor Manufacturers

MANUFACTURER

WEB SITE

TDK

www.tdk.com

Kemet

www.kemet.com

Murata

www.murata.com

Taiyo Yuden

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