LTC4366 datasheet(18/32 Pages) AD

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LTC7862

Rev 0

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APPLICATIONS INFORMATION

Power MOSFET Selection

Two external power MOSFETs must be selected for the

LTC7862 controller: one N-channel MOSFET for the top

(main) switch, and one N-channel MOSFET for the bottom

(synchronous) switch.

The peak-to-peak drive levels are set by the DRVCC volt-

age. This voltage can be 6V or 9V depending on configu-

ration of the DRVUV pin. Therefore, both logic-level and

standard-level threshold MOSFETs can be used in most

applications depending on the programmed DRVCC volt-

age. Pay close attention to the BVDSS specification for the

MOSFETs as well.

Selection criteria for the power MOSFET for normal pass-

through operation include the on-resistance RDS(ON),

input voltage and maximum output current.

The MOSFET power dissipation at maximum output cur-

rent are given by:

PMAIN = (IOUT(MAX))2 (1+ )RDS(ON)

PSYNC = 0 (Synchronous FET is OFF during normal pass-

through operation)

where  is the temperature dependency of RDS(ON).

The term (1 + ) is generally given for a MOSFET in the

form of a normalized RDS(ON) vs temperature curve, but

 = 0.005/°C can be used as an approximation for low

voltage MOSFETs.

Selection criteria for the power MOSFETs during timer-

enabled switching operation include the on-resistance

RDS(ON), Miller capacitance CMILLER, input voltage and

maximum output current. Miller capacitance, CMILLER, can

be approximated from the gate charge curve usually pro-

vided on the MOSFET manufacturers’ data sheet. CMILLER

is equal to the increase in gate charge along the horizontal

axis while the curve is approximately flat divided by the

specified change in VDS. This result is then multiplied

by the ratio of the application applied VDS to the gate

charge curve specified VDS. When the IC is switching in

continuous mode the duty cycles for the top and bottom

MOSFETs are given by:

MAIN SWITCH DUTY CYCLE =

VOUT

VIN

SYNCHRONOUS SWITCH DUTY CYCLE

VIN − VOUT

VIN

The MOSFET power dissipations at maximum output cur-

rent are given by:

PMAIN =

VOUT

VIN

IOUT(MAX)

(

)2 1+δ

(

)RDS(ON) +

(VIN)2

IOUT(MAX)

⎛

⎝⎜

⎞

⎠⎟

(RDR)(CMILLER)•

VDRVCC − VTHMIN

VTHMIN

⎡

⎣

⎢

⎤

⎦

⎥(f)

PSYNC =

VIN − VOUT

VIN

IOUT(MAX)

(

)2 1+δ

(

)RDS(ON)

where δ is the temperature dependency of RDS(ON) and

RDR (approximately 2Ω) is the effective driver resistance

at the MOSFET’s Miller threshold voltage. VTHMIN is the

typical MOSFET minimum threshold voltage.

Both MOSFETs have I2R losses while the main N-channel

equations include an additional term for transition losses,

which are highest at high input voltages. For VIN < 20V

the high current efficiency generally improves with larger

MOSFETs, while for VIN > 20V the transition losses rap-

idly increase to the point that the use of a higher RDS(ON)

device with lower CMILLER actually provides higher effi-

ciency. The synchronous MOSFET losses are greatest at

high input voltage when the top switch duty factor is low

or during a short-circuit when the synchronous switch is

on close to 100% of the period.

The MOSFET’s temperature rise due to power dissipation

must be considered. Refer to Thermal Considerations sec-

tion for more details.

LTC4366 Datasheet(PDF) 18 Page - Analog Devices

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