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LT8390A

8390afa

For more information www.linear.com/LT8390A

APPLICATIONS INFORMATION

Power MOSFET Selection

TheLT8390ArequiresfourexternalN-channelpowerMOS-

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).

To achieve 2MHz operation, the power MOSFET selection

is critical. With typical 25ns shoot-through protection

deadtime, high performance power MOSFETs with low

Qg and low RDS(ON) must be used.

Since the gate drive voltage is set by the 5V INTVCC supply,

logic-level threshold MOSFETs must be used in LT8390A

applications.SwitchingfourMOSFETsathigherfrequency

like2MHz,thesubstantialgatechargecurrentfromINTVCC

can be estimated as:

IINTVCC=f• QgA+QgB+QgC+QgD

(

)

where:

f is the switching frequency

QgA,QgB,QgC,QgDarethetotalgatechargesofMOSFETs

A, B, C, D

Make sure the total required INTVCC current not exceed-

ing the INTVCC current limit in the datasheet. Typically,

MOSFETs with less than 10nC Qg are recommended.

The LT8390A uses the VIN/VOUT ratio to transition between

modesandregions.BiggerIRdropinthepowerpathcaused

by improper MOSFET and inductor selection may prevent

the LT8390A from smooth transition. 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,BisthemaximumRDS(ON)ofMOSFETsAorBat25°C

RC,D is the maximum RDS(ON) of MOSFETs C or D at

25°C

Figure 11. Normalized RDS(ON) vs Temperature

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

The RDS(ON) and DCR increase at higher junction

temperaturesandtheprocessvariationhavebeenincluded

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) =

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

of 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 •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

JUNCTION TEMPERATURE (°C)

–50

1.0

1.5

150

8390a F11

0.5

100

2.0

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