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LTC3704

sn3704 3704fs

Another method of choosing which power MOSFET to use

is to check the maximum output current for a given

RDS(ON), since MOSFET on-resistances are generally

available in discrete values.

O MAX

SENSE MAX

MAX

DS ON

()

–•

–

••









ρΤ

For the case where a conventional sense resistor is used,

SENSE

SENSE MAX

MAX

O MAX









()

•

–

•

Sense resistors are generally low TC and are available with

different ranges of tolerance depending on price. The

power dissipated in the sense resistor is:

SENSE

SW PEAK

SENSE

MAX

() ••

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)).

As a result, some iterative calculation is normally 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 tempera-

ture), and for the worst-case specifications for VSENSE(MAX)

and the RDS(ON) of the MOSFET listed in the manufacturer’s

data sheet.

The power dissipated by the MOSFET in a positive-to-

negative converter is:

FET

OMAX

MAX

DS ON

MAX

OMAX

MAX

RSS













–

••

•

•(

–

)

•

–

••

()

185

where IO(MAX) and VO are negative numbers.

The first term in the equation above represents the

I2R losses in the device, and the second term, the switch-

ing losses. The constant, k = 1.7, is an empirical factor

inversely 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)

Figure 12. Normalized RDS(ON) vs Temperature

Figure 11. Maximum SENSE Threshold Voltage vs Duty Cycle

APPLICATIO S I FOR ATIO

DUTY CYCLE

100

150

0.8

3704 F11

0.2

0.4

0.5

1.0

200

JUNCTION TEMPERATURE (

°C)

–50

1.0

1.5

150

3704 F12

0.5

100

2.0

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