Datasheet

LTC3783
16
3783fb
OPERATION
During the switch on-time, the IMAX comparator limits the
absolute maximum voltage drop across the power MOSFET
to a nominal 150mV, regardless of duty cycle. The peak
inductor current is therefore limited to 150mV/R
DS(ON)
.
The relationship between the maximum load current, duty
cycle, and the R
DS(ON)
of the power MOSFET is:
R
DS(ON)
< 150mV •
1–D
MAX
1+
c
2






•I
OUT(MAX)
• r
T
The r
T
term accounts for the temperature coefficient of
the R
DS(ON)
of the MOSFET, which is typically 0.4%/°C.
Figure 8 illustrates the variation of normalized R
DS(ON)
over temperature for a typical power MOSFET.
It is worth noting that the 1 - D
MAX
relationship between
I
O(MAX)
and R
DS(ON)
can cause boost converters with a
wide input range to experience a dramatic range of maxi-
mum input and output currents. This should be taken into
consideration in applications where it is important to limit
the maximum current drawn from the input supply, and
also to avoid triggering the 150mV IMAX comparator, as
this condition can result in excessive noise.
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
DS(ON)
. As a
result, some iterative calculation is normally required to
determine a reasonably accurate value. Since the controller
is using the MOSFET as both a switching and a sensing
element, 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
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 boost converter
is:
P
FET
=
I
OUT(MAX)
1–D
MAX






2
•R
DS(ON)
•D
MAX
• r
T
+
k • V
OUT
1.85
•
I
OUT(MAX)
1–D
MAX






• C
RSS
• f
The first 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.
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON RESISTANCE
1.0
1.5
150
3783 F08
0.5
0
0
50
100
2.0
Figure 8. Normalized R
DS(ON)
vs Temperature
Another method of choosing which power MOSFET to
use is to check what the maximum output current is for a
given R
DS(ON)
, since MOSFET on-resistances are available
in discrete values.
I
O(MAX)
= 150mV •
1– D
MAX
1+
c
2






• R
DS(ON)
• r
T