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LTC1871X

1871xf

For more information www.linear.com/LTC1871X

APPLICATIONS INFORMATION

Another method of choosing which power MOSFET to

use is to check what the maximum output current is for a

given RDS(ON),sinceMOSFETon-resistancesareavailable

in discrete values.

IO(MAX) = VSENSE(MAX) •

1– DMAX

⎛

⎝⎜

⎞

⎠⎟

•RDS(ON) •ρT

It is worth noting that the 1 – DMAX relationship between

IO(MAX) and RDS(ON) can cause boost converters with a

wide input range to experience a dramatic range of maxi-

mum input and output current. This should be taken into

consideration in applications where it is important to limit

the maximum current drawn from the input supply.

Calculating Power MOSFET Switching and Conduction

Losses and Junction Temperatures

In order to calculate the junction temperature of the

power MOSFET, the power dissipated by the device must

be known. This power dissipation is a function of the

duty cycle, the load current and the junction temperature

itself (due to the positive temperature coefficient of its

RDS(ON)).Asaresult,someiterativecalculationisnormally

required to determine a reasonably accurate value. Since

the controller is using the MOSFET as both a switching

and a sensing element, care should be taken to ensure

that the converter is capable of delivering the required

load current over all operating conditions (line voltage

and temperature), and for the worst-case specifications

for VSENSE(MAX) and the RDS(ON) of the MOSFET listed in

the manufacturer’s data sheet.

ThepowerdissipatedbytheMOSFETinaboostconverteris:

PFET =

IO(MAX)

1– DMAX

⎛

⎝⎜

⎞

⎠⎟

• RDS(ON) •DMAX •ρT

k • VO1.85 •

IO(MAX)

1– DMAX

(

)

• CRSS • f

The first term in the equation above represents the I2R

losses in the device, and the second term, the switching

losses.Theconstant,k=1.7,isanempiricalfactorinversely

related to the gate drive current and has the dimension

of 1/current.

From a known power dissipated in the power MOSFET, its

junction temperature can be obtained using the following

formula:

TJ = TA + PFET • RTH(JA)

The RTH(JA) to be used in this equation normally includes

the RTH(JC) for the device plus the thermal resistance from

the case to the ambient temperature (RTH(CA)). This value

of TJ can then be compared to the original, assumed value

used in the iterative calculation process.

Boost Converter: Output Diode Selection

To maximize efficiency, a fast switching diode with low

forwarddropandlowreverseleakageisdesired.Theoutput

diode in a boost converter conducts current during the

switch off-time. The peak reverse voltage that the diode

must withstand is equal to the regulator output voltage.

The average forward current in normal operation is equal

to the output current, and the peak current is equal to the

peak inductor current.

ID(PEAK) =IL(PEAK) = 1+

⎛

⎝⎜

⎞

⎠⎟

•

IO(MAX)

1– DMAX

The power dissipated by the diode is:

PD = IO(MAX) • VD

and the diode junction temperature is:

TJ = TA + PD • RTH(JA)

The RTH(JA) to be used in this equation normally includes

the RTH(JC) for the device plus the thermal resistance from

the board to the ambient temperature in the enclosure.

Remember to keep the diode lead lengths short and to

observe proper switch-node layout (see Board Layout

Checklist) to avoid excessive ringing and increased dis-

sipation.

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