LT3741 datasheet(20/32 Pages) LINER

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LT8390

8390fa

For more information www.linear.com/LT8390

APPLICATIONS INFORMATION

The maximum current sense RSENSE in boost region is:

RSENSE(BOOST) =

2 • 50mV • VIN(MIN)

2 •IOUT(MAX) • VOUT + ∆IL(BOOST) • VIN(MIN)

The maximum current sense RSENSE in buck region is

RSENSE(BUCK) =

2 • 50mV

2 •IOUT(MAX) + ∆IL(BUCK)

The final RSENSE value should be lower than the calculated

RSENSE in both buck and boost regions. A 20% to 30%

margin is usually recommended. Always choose a low

ESL current sense resistor.

Power MOSFET Selection

The LT8390 requires four external N-channel power MOS-

FETs, two for the top switches (switches A and D shown

in Figure 1) and two for the bottom switches (switches B

and C shown in Figure 1). Important parameters for the

power MOSFETs are the breakdown voltage VBR(DSS),

threshold voltage VGS(TH), on-resistance RDS(ON), reverse

transfercapacitanceCRSSandmaximumcurrentIDS(MAX).

The drive voltage is set by the 5V INTVCC supply. Conse-

quently, logic-level threshold MOSFETs must be used in

LT8390 applications.

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) =

IOUT(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

ρT = 1.5 is reasonable.

Figure 11. Normalized RDS(ON) vs Temperature

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 •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

VIN

•IOUT(MAX)

2 •ρT

•RDS(ON) + k • VOUT

3 •

IOUT(MAX)

VIN

• CRSS • f

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 •ρT •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.

JUNCTION TEMPERATURE (°C)

–50

1.0

1.5

150

8390 F11

0.5

100

2.0

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