Datasheet

LT3782A
11
3782afb
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 R
SENSE
is:
R≤ V
SENSE(MAX)
•
1–D
MAX
1.2 •
I
O(MAX)
2
Power MOSFET Selection
Important parameters for the power MOSFET include the
drain-to-source breakdown voltage (BV
DSS
), the threshold
voltage (V
GS(TH)
), the on-resistance (R
DS(ON)
) versus gate-
to-source voltage, the gate-to-source and gate-to-drain
charges (Q
GS
and Q
GD
, respectively), the maximum drain
current (I
D(MAX)
) and the MOSFET’s thermal resistances
(R
TH(JC)
and R
TH(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 BV
DSS
specifi cations 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 coeffi cient of its R
DS(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
specifi cations for V
SENSE(MAX)
and the R
DS(ON)
of the
MOSFET listed in the manufacturer’s data sheet.
The power dissipated by the MOSFET in a 2-phase boost
converter is:
P
FET
=
I
O(MAX)
2






1–D
( )
2
•R
DS(ON)
•D•
T
+k•V
O
2
•
I
O(MAX)
2






1–D
( )
•C
RSS
•f
The fi rst term in the equation above represents the I
2
R
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
T
term accounts for the temperature
coeffi cient of the R
DS(ON)
of the MOSFET, which is typically
0.4%/°C. Figure 4 illustrates the variation of normalized
R
DS(ON)
over temperature for a typical power MOSFET.
APPLICATIONS INFORMATION
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON RESISTANCE
1.0
1.5
150
3782A F04
0.5
0
0
50
100
2.0
Figure 4. Normalized R
DS(ON)
vs Temperature