LT3782A LINER [Linear Technology], LT3782A Datasheet - Page 12

• Manufacturer Part Number: LT3782A
• Description: 2-Phase Step-Up DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LT3782A
APPLICATIONS INFORMATION
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 BV
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 coeffi cient of its R
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
12
DSS
specifi cations for the
Figure 4. Normalized R
2.0
1.5
1.0
0.5
0
–50
DS(ON)
JUNCTION TEMPERATURE (°C)
). As a
0
50
DS(ON)
voltage and temperature), and for the worst-case speci-
fi cations for V
listed in the manufacturer’s data sheet.
The power dissipated by the MOSFET in a 2-phase boost
converter is:
The fi rst term in the equation above represents the I
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 ρ
coeffi cient of the R
0.4%/°C. Figure 4 illustrates the variation of normalized
R
DS(ON)
P
FET
100
vs Temperature
=
+k • V
over temperature for a typical power MOSFET.
3782A F04
I
O(MAX)
(
150
1– D
SENSE(MAX)
2
O
2
)
•
DS(ON)
T
I
2
(
O(MAX)
1– D
term accounts for the temperature
• R
2
DS(ON)
of the MOSFET, which is typically
and the R
)
• C
• D •
RSS
DS(ON)
• f
T
of the MOSFET
3782af
2
R