LT8253EUFDM datasheet(14/20 Pages) AD

14 / 20 page

LT8253/LT8253A

Rev. 0

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APPLICATIONS INFORMATION

The LT8253/LT8253A use the VIN/VOUT ratio to transition

between modes and regions. Bigger IR drop in the power

path caused by improper MOSFET and inductor selection

may prevent the LT8253/LT8253A from smooth transi-

tion. To ensure smooth transitions between buck, buck-

boost, and boost modes of operation, choose low RDS(ON)

MOSFETs and low DCR inductor to satisfy:

IOUT(MAX) �

0.025 • VOUT

RA,B +RC,D +RSENSE +RL

where:

RA,B isthemaximumRDS(ON) ofMOSFETsAorBat25°C

RC,D isthemaximumRDS(ON) ofMOSFETsCorDat25°C

RL is the maximum DCR resistor of inductor at 25°C

The RDS(ON) and DCR increase at higher junction

temperatures 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

maximum power dissipation happens in boost region,

when it remains on all the time. Its maximum power dis-

sipation at maximum output current is given by:

PA(BOOST) =

IOUT(MAX) • VOUT

VIN

�

•

�T • 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 7. For

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

ρT = 1.5 is reasonable.

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

• IOUT(MAX)

2 • �T •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

VIN

• IOUT(MAX)

2 • �T

• RDS(ON) + k • VOUT

3 •

IOUT(MAX)

VIN

• CRSS • f

JUNCTION TEMPERATURE (°C)

–50

1.0

1.5

150

8253A F07

0.5

100

2.0

Figure 7. 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

• IOUT(MAX)

2 • �T •RDS(ON)

For the same output voltage and current, switch A has the

highest power dissipation and switch B has the lowest

power dissipation unless a short occurs at the output.

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)

includes 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.

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