Datasheet

LT3742
15
3742fa
(Q
G
). A few simple guidelines will make the selection
process easier.
The maximum drain current must be higher than the
maximum rated current, I
RATED
, calculated on the previous
page. Note that the I
D
specification is largely temperature
dependent (lower I
D
at higher ambient temperatures), so
most data sheets provide a graph or table of I
D
versus
temperature to show this.
Ensure that the V
DS
breakdown voltage is greater than the
maximum input voltage and that the V
GS
breakdown volt-
age is 8V or greater. The peak-to-peak gate drive for each
MOSFET is ~7V, so also ensure that the device chosen will
be fully enhanced with a V
GS
of 7V. This may preclude the
use of some MOSFETs with a 20V V
GS
rating, as some have
too high of a threshold voltage. A good rule of thumb is that
the maximum threshold voltage should be V
GS(TH)(MAX)
≤
3V. 4.5V MOSFETs will work as well.
Power losses in the N-channel MOSFET come from two
main sources: the on-resistance, R
DS(ON)
, and the reverse
transfer capacitance, C
RSS
. The on-resistance causes
ohmic losses (I
2
R
DS(ON)
) which typically dominate at
input voltages below ~15V. The reverse transfer capaci-
tance results in transition losses which typically dominate
for input voltages above ~15V. At higher input voltages,
transition losses rapidly increase to the point that the use
of a higher R
DS(ON)
device with lower C
RSS
will actually
applicaTions inForMaTion
provide higher efficiency. The power loss in the MOSFET
can be approximated by:
P
LOSS
= ohmic loss
( )
+ transition loss
( )
P
LOSS
≈
V
OUT
+ V
D
V
IN
+ V
D
• I
OUT
2
R
DS(ON)
• ρ
T
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
2 • V
IN
2
• I
OUT
• C
RSS
• f
( )
where f is the switching frequency (500kHz) and ρ
T
is a
normalizing term to account for the on-resistance change
due to temperature. For a maximum ambient temperature
of 70°C, using ρ
T
≈ 1.3 is a reasonable choice.
The trade-off in R
DS(ON)
and C
RSS
can easily be seen in an
example using real MOSFET values. To generate a 3.3V,
3A (10W) output, consider two typical N-channel power
MOSFETs, both rated at V
DS
= 30V and both available in
the same SO-8 package, but having ~5x differences in
on-resistance and reverse transfer capacitance:
M1: I
D
= 11.5A, V
GS
= 12V, R
DS(ON)
= 10mΩ, C
RSS
= 230pF
M2: I
D
= 6.5A, V
GS
= 20V, R
DS(ON)
= 50mΩ, C
RSS
= 45pF
Power loss is calculated for both devices over a wide input
voltage range (4V ≤ V
IN
≤ 30V), and shown in Figure 6 (as
a percentage of the 10W total power). Note that while the
low R
DS(ON)
device power loss is 5× lower at low input
Figure 6a. Power Loss Example for M1 (10mΩ, 230pF)
Figure 6b. Power Loss Example for M2 (50m, 45pF)
INPUT VOLTAGE (V)
0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
15 25
3742 F06a
5 10
20 30
MOSFET POWER LOSS (W)
TOTAL =
OHMIC + TRANSITION
TRANSITION
OHMIC
INPUT VOLTAGE (V)
0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
15 25
3742 F06b
5 10
20 30
MOSFET POWER LOSS (W)
TOTAL =
OHMIC + TRANSITION
TRANSITION
OHMIC