LT3782AIUFD datasheet(12/20 Pages) LINER

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LT3782A

3782af

The gate drive voltage is set by the 10V GBIAS regulator.

Consequently, 10V rated MOSFETs are required in most

high voltage LT3782A applications.

Pay close attention to the BVDSS speciﬁcations for the

MOSFETs relative to the maximum actual switch voltage

in the application. The switch node can ring during the

turn-off of the MOSFET due to layout parasitics. Check the

switching waveforms of the MOSFET directly across the

drain and source terminals using the actual PC board layout

(not just on a lab breadboard!) for excessive ringing.

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. 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 speci-

ﬁcations 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 2-phase boost

converter is:

PFET =

IO(MAX)

1– D

(

)

•RDS(ON) •D • T

+k• VO

2 •

IO(MAX)

1– D

(

)

•CRSS •f

The ﬁrst term in the equation above represents the I2R

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

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

related to the gate drive current and has the dimension

of 1/current. The ρT term accounts for the temperature

coefﬁcient of the RDS(ON) of the MOSFET, which is typically

0.4%/°C. Figure 4 illustrates the variation of normalized

RDS(ON) over temperature for a typical power MOSFET.

JUNCTION TEMPERATURE (°C)

–50

1.0

1.5

150

3782A F04

0.5

100

2.0

Figure 4. Normalized RDS(ON) vs Temperature

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