LT3797 datasheet(22/36 Pages) LINER

22 / 36 page

LT3797

3797f

For more information www.linear.com/LT3797

APPLICATIONS INFORMATION

Power MOSFET Selection

The selection criteria for the power MOSFET includes the

drain-source breakdown voltage (BVDSS), the threshold

voltage (VGS(TH)), the on-resistance (RDS(ON)), the total

gatecharge(QG),themaximumdraincurrent(ID(MAX))and

the MOSFET’s thermal resistances (RθJC and RθJA), etc.

The required power MOSFET BVDSS rating of different

topologiescanbeestimatedusingthefollowingequations.

Add a diode forward voltage, and any additional ringing

across its drain-to-source during its off-time.

BVDSS_BOOST > VLED

BVDSS_BUCK > VIN(MAX)

BVDSS_BUCK-BOOST > VIN(MAX) + VLED

BVDSS_SEPIC > VIN(MAX) + VLED

The power dissipated by the MOSFET in a boost, buck

mode, or buck-boost mode LED driver is:

PFET = IL(MAX)2 • RDS(ON) • DMAX + 2 • VSW(PEAK) •

IL(MAX) • CRSS • f/1.5A

The power dissipated by the MOSFET in a SEPIC LED

driver is:

PFET = (IL1(MAX) + IL2(MAX))2 • RDS(ON) • DMAX + 2 •

VSW(PEAK) • (IL1(MAX) + IL2(MAX)) • CRSS • f/1.5A

The first terms in the preceding equations represent the

conductionlossesinthedevices,andthesecondterms,the

switching losses. CRSSisthereversetransfercapacitance,

which is usually specified in the MOSFET characteristics.

For maximum efficiency, RDS(ON) and QG should be

minimized. From a known power dissipated in the power

MOSFET, its junction temperature can be obtained using

the following equation:

TJ = TA + PFET • θJA = TA + PFET • (θJC + θCA)

TJ must not exceed the MOSFET maximum junction

temperature rating. It is recommended to measure the

MOSFETtemperatureinsteadystatetoensurethatabsolute

maximum ratings are not exceeded.

Schottky Rectifier Selection

The power Schottky diode conducts current during the

interval when the switch is turned off. In an LT3797 LED

driver, the Schottky diode should have the same voltage

ratingasthepowerN-channelMOSFETinthesamechannel.

Refer to the power MOSFET BVDSS rating in the previous

sectionforthepeakreversevoltageratingselection.Ifusing

the PWM feature for dimming, it is important to consider

diodeleakage,whichincreaseswiththetemperature,from

theoutputduringthePWMlowinterval.ChooseaSchottky

diode with sufficiently low leakage current.

The power dissipated by the diode in a boost, buck, or

buck-boost converter in CCM is:

PD = IL_AVG(MAX) • VD • (1 – DMAX)

where VD is the diode forward voltage drop.

The power dissipated by the diode in a SEPIC converter is:

PD = (IL1_AVG(MAX) + IL2_AVG(MAX)) • VD • (1 – DMAX)

and the diode junction temperature is:

TJ = TA + PD • (θJC + θCA)

TJ must not exceed the diode maximum junction tem-

perature rating.

LT3797 Datasheet(PDF) 22 Page - Linear Technology

LT3797 Datasheet(HTML) 22 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 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL