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LT8392

Rev. 0

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APPLICATIONS INFORMATION

Power MOSFET Selection

The LT8392 requires four external N-channel power

MOSFETs, 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 param-

eters for the power MOSFETs are the breakdown voltage

VBR(DSS), threshold voltage VGS(TH), on-resistance

RDS(ON), reverse transfer capacitance CRSS and maximum

current IDS(MAX).

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

supply, logic-level threshold MOSFETs must be used in

LT8392 applications. Switching four MOSFETs at higher

frequency like 2MHz, the substantial gate charge current

from INTVCC can be estimated as:

IINTVCC = f • QgA + QgB + QgC + QgD

(

)

where:

f is the switching frequency

QgA, QgB, QgC, QgD are the total gate charges of

MOSFETs A, B, C, D

Make sure the total required INTVCC current not exceeding

the INTVCC current limit in the data sheet.

The LT8392 uses 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 LT8392 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,B is the maximum RDS(ON) of MOSFETs A or B

at 25°C

Figure 11. Normalized RDS(ON) vs Temperature

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

at 25°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

⎛

⎝⎜

⎞

⎠⎟

•

ρT •RDS(ON)

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

ing for the significant variation in on-resistance with tem-

perature, 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)

JUNCTION TEMPERATURE (°C)

–50

1.0

1.5

150

8392 F11

0.5

100

2.0

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