LT3755 datasheet(20/32 Pages) AD

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LT8393

Rev. A

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APPLICATIONS INFORMATION

RSENSE Selection and Maximum Output Current

RSENSE is chosen based on the required output current.

The duty cycle independent maximum current sense

thresholds (50mV in peak-buck and 50mV in peak-boost)

set the maximum inductor peak current in buck region,

buck-boost region, and boost region.

In boost region, the lowest maximum average load current

happens at VIN(MIN) and can be calculated as:

IOUT(MAX_BOOST) =

50mV

RSENSE

−

ΔIL(BOOST)

⎛

⎝⎜

⎞

⎠⎟

•

VIN(MIN)

VOUT

where ∆IL(BOOST) is peak-to-peak inductor ripple current

in boost region and can be calculated as:

ΔIL(BOOST) =

VIN(MIN)•(VOUT− VIN(MIN))

•L•VOUT

In buck region, the lowest maximum average load current

happens at VIN(MAX) and can be calculated as:

IOUT(MAX_BUCK) =

50mV

RSENSE

−

ΔIL(BUCK)

⎛

⎝⎜

⎞

⎠⎟

where ∆IL(BUCK) is peak-to-peak inductor ripple current in

buck region and can be calculated as:

ΔIL(BUCK) =

VOUT•(VIN(MAX)− VOUT)

•L•VIN(MAX)

The maximum current sense RSENSE in boost region is:

RSENSE(BOOST) =

•50mV•VIN(MIN)

•ILED(MAX)•VOUT+ ΔIL(BOOST)•VIN(MIN)

The maximum current sense RSENSE in buck region is

RSENSE(BUCK) =

•50mV

•ILED(MAX)+ ΔIL(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.

Power MOSFET Selection

The LT8393 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).

To achieve high frequency operation, the power MOSFET

selection is critical. With typical 60ns shoot-through

protection dead time, high performance power MOSFETs

with low Qg and low RDS(ON) must be used.

Since the gate drive voltage is set by the 5V INTVCCsupply,

logic-level threshold MOSFETs must be used in LT8393

applications.SwitchingfourMOSFETsathigherfrequency,

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,QgDarethetotalgatechargesofMOSFETs

A, B, C, D at 5V VGS

Make sure the total required INTVCC current not exceeding

the INTVCC current limit in the data sheet.

The LT8393 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 LT8393 from smooth transition. Make sure

that low RDS(ON) MOSFETs and low DCR inductor are

used to satisfy:

ILED(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

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

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