LT8393EFE datasheet(21/32 Pages) AD

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LT8393

Rev. A

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APPLICATIONS INFORMATION

The RDS(ON) and DCR increase at higher junction tem-

peratures and the process variation have been included

in the calculation above.

In order to select the power MOSFETs, the power dis-

sipated by the device must be known. For switch A, the

maximumpowerdissipationhappensinboostregion,when

it remains on all the time. Its maximum power dissipation

at maximum output current is given by:

PA(BOOST) =

ILED(MAX)•VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟

•ρT •RDS(ON)

where ρT is a normalization factor (unity at 25°C) ac-

counting for the significant variation in on-resistance with

temperature, typically 0.4%/°C as shown in Figure 11. For

a maximum junction temperature of 125°C, using a value

of ρT = 1.5 is reasonable.

Figure 11. Normalized RDS(ON) vs Temperature

where CRSS is usually specified by the MOSFET manufac-

turers. The constant k, which accounts for the loss caused

by reverse recovery current, is inversely proportional to

the gate drive current and has an empirical value of 1.7.

For switch D, the maximum power dissipation happens in

boost region, when its duty cycle is higher than 50%. Its

maximum power dissipation at maximum output current

is given by:

PD(BOOST) =

VOUT

VIN

•ILED(MAX)

2•ρ

T •RDS(ON)

For the same output voltage and current, typically switch

A has the highest power dissipation in buck region at

VIN(MAX) and switch C has the highest power dissipation

in boost region at VIN(MIN).

From a known power dissipated in the power MOSFET, its

junction temperature can be obtained using the following

formula:

TJ = TA + P • RTH(JA)

The junction-to-ambient thermal resistance RTH(JA) in-

cludes the junction-to-case thermal resistance RTH(JC)

and the case-to-ambient thermal resistance RTH(CA). This

value of TJ can then be compared to the original, assumed

value used in the iterative calculation process.

Optional Schottky Diode (DB, DD) Selection

The optional Schottky diodes DB (in parallel with switch B)

and DD (in parallel with switch D) conduct during the

dead time between the conduction of the power MOSFET

switches. They are intended to prevent the body diode of

synchronousswitchesBandDfromturningonandstoring

charge during the dead time. In particular, DB significantly

reduces reverse recovery current between switch B turn-

off and switch A turn-on, and DD significantly reduces

reverse recovery current between switch D turn-off and

switch C turn-on. They improve converter efficiency and

reduce switch voltage stress. In order for the diode to be

effective, the inductance between it and the synchronous

switch must be as small as possible, mandating that these

components be placed adjacently.

JUNCTION TEMPERATURE (°C)

–50

1.0

1.5

150

8393 F11

0.5

100

2.0

Switch B operates in buck region as the synchronous

rectifier. Its power dissipation at maximum output cur-

rent is given by:

PB(BUCK) =

VIN−VOUT

VIN

•ILED(MAX)

2•ρT•RDS(ON)

Switch C operates in boost region as the control switch.

Its power dissipation at maximum current is given by:

PC(BOOST) =

(VOUT−VIN)•VOUT

VIN2

•ILED(MAX)

2•ρT

•RDS(ON)+k•VOUT

3•

ILED(MAX)

VIN

•CRSS•f

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