LT3782 Linear Technology, LT3782 Datasheet - Page 10

• Manufacturer Part Number: LT3782
• Description: 2-Phase Step-Up DC/DC Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT3782
Power MOSFET Selection
Important parameters for the power MOSFET include the
drain-to-source breakdown voltage (BV
old voltage (V
gate-to-source voltage, the gate-to-source and gate-to-
drain charges (Q
drain current (I
tances (R
The gate drive voltage is set by the 10V GBIAS regulator.
Consequently, 10V rated MOSFETs are required in most
high voltage LT3782 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 lay-
out (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 R
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
specifications for V
MOSFET listed in the manufacturer’s data sheet.
10
TH(JC)
GS(TH)
D(MAX)
and R
GS
U
and Q
), the on-resistance (R
SENSE(MAX)
) and the MOSFET’s thermal resis-
TH(JA)
GD
U
).
, respectively), the maximum
DSS
and the R
specifications for the
W
DSS
DS(ON)
DS(ON)
), the thresh-
U
) versus
DS(ON)
of the
).
The power dissipated by the MOSFET in a 2-phase boost
converter is:
The first 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
temperature coefficient of the R
which is typically 0.4%/°C. Figure 4 illustrates the varia-
tion of normalized R
power MOSFET.
P
FET
+
=
Figure 4. Normalized R
k V
⎛
⎜
⎝
2.0
1.5
1.0
0.5
•
I
0
– 50
O MAX
(
1
(
O
–
2
2
D
•
)
)
JUNCTION TEMPERATURE (°C)
⎛
⎜
⎝
⎞
⎟
⎠
I
DS(ON)
0
(
O MAX
2
1
(
–
•
2
R
D
DS ON
)
)
over temperature for a typical
50
⎞
⎟
⎠
(
DS(ON)
•
C
)
T
RSS
• •
DS(ON)
term accounts for the
D
vs Temperature
100
•
ρ
f
T
3782 F06
of the MOSFET,
150
3782fa
2
R