LT3782AIFE(RevC) データシートの表示（PDF） - Linear Technology

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LT3782AIFE
(Rev.:RevC)

2-Phase Step-Up DC/DC Controller

Linear Technology 

LT3782A

APPLICATIONS INFORMATION

Sense Resistor Selection

During the switch on-time, the control circuit limits the

maximum voltage drop across the sense resistor to about

63mV. The peak inductor current is therefore limited to

63mV/R. The relationship between the maximum load

current, duty cycle and the sense resistor RSENSE is:

R

≤

VSENSE(MAX )

•

1–

1.2 •

DMAX

IO(MAX )

2

Power MOSFET Selection

Important parameters for the power MOSFET include the

drain-to-source breakdown voltage (BVDSS), the threshold

voltage (VGS(TH)), the on-resistance (RDS(ON)) versus gate-

to-source voltage, the gate-to-source and gate-to-drain

charges (QGS and QGD, respectively), the maximum drain

current (ID(MAX)) and the MOSFET’s thermal resistances

(RTH(JC) and RTH(JA)).

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(M2AX )

(1– D)

⎞

⎟

⎠

2

• RDS(ON)

•D

•

ρT

+k

•

VO 2

•

⎛⎝⎜ IO(M2AX )

(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.

2.0

1.5

1.0

0.5

0

–50

0

50

100

150

JUNCTION TEMPERATURE (°C)

3782A F04

Figure 4. Normalized RDS(ON) vs Temperature

For more information www.linear.com/LT3782A

3782afc

11

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