lt3742 Linear Technology Corporation, lt3742 Datasheet - Page 15

• Manufacturer Part Number: lt3742
• Description: Dual, 2-phase Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

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