Datasheet

14
LTC3830/LTC3830-1
3830fa
P
MAX
should be calculated based primarily on required
efficiency or allowable thermal dissipation. A typical high
efficiency circuit designed for 5V input and 3.3V at 10A
output might allow no more than 3% efficiency loss at full
load for each MOSFET. Assuming roughly 90% efficiency
at this current level, this gives a P
MAX
value of:
(3.3V)(10A/0.9)(0.03) = 1.1W per FET
and a required R
DS(ON)
of:
R
VW
VA
R
VW
VVA
DS ON Q
DS ON Q
()
()
()•(.)
( . )( )
.
()•(.)
(–.)( )
.
1
2
2
2
511
33 10
0 017
511
53310
0 032
==Ω
==Ω
Note that the required R
DS(ON)
for Q2 is roughly twice that
of Q1 in this example. This application might specify a
single 0.03Ω device for Q2 and parallel two more of the
same devices to form Q1. Note also that while the required
R
DS(ON)
values suggest large MOSFETs, the power dissi-
pation numbers are only 1.1W per device or less; large
TO-220 packages and heat sinks are not necessarily
required in high efficiency applications. Siliconix Si4410DY
or International Rectifier IRF7413 (both in SO-8) or Siliconix
SUD50N03-10 (TO-252) or ON Semiconductor
MTD20N03HDL (DPAK) are small footprint surface mount
devices with R
DS(ON)
values below 0.03Ω at 5V of V
GS
that
work well in LTC3830 circuits. Using a higher P
MAX
value
in the R
DS(ON)
calculations generally decreases the MOSFET
cost and the circuit efficiency and increases the MOSFET
heat sink requirements.
Table 1 highlights a variety of power MOSFETs for use in
LTC3830 applications.
Inductor Selection
The inductor is often the largest component in an LTC3830
design and must be chosen carefully. Choose the inductor
value and type based on output slew rate requirements. The
maximum rate of rise of inductor current is set by the
inductor’s value, the input-to-output voltage differential and
the LTC3830’s maximum duty cycle. In a typical 5V input,
3.3V output application, the maximum rise time will be:
DC V V
LL
A
s
MAX IN OUT
OO
•( – ) .
=
µ
1 615
APPLICATIO S I FOR ATIO
WUUU
Table 1. Recommended MOSFETs for LTC3830 Applications
TYPICAL INPUT
R
DS(ON)
CAPACITANCE
PARTS AT 25°C (mΩ) RATED CURRENT (A) C
ISS
(pF) θ
JC
(°C/W) T
JMAX
(°C)
Siliconix SUD50N03-10 19 15 at 25°C 3200 1.8 175
TO-252 10 at 100°C
Siliconix Si4410DY 20 10 at 25°C 2700 150
SO-8 8 at 70°C
ON Semiconductor MTD20N03HDL 35 20 at 25°C 880 1.67 150
DPAK 16 at 100°C
Fairchild FDS6670A 8 13 at 25°C 3200 25 150
S0-8
Fairchild FDS6680 10 11.5 at 25°C 2070 25 150
SO-8
ON Semiconductor MTB75N03HDL 9 75 at 25°C 4025 1 150
DD PAK 59 at 100°C
IR IRL3103S 19 64 at 25°C 1600 1.4 175
DD PAK 45 at 100°C
IR IRLZ44 28 50 at 25°C 3300 1 175
TO-220 36 at 100°C
Fuji 2SK1388 37 35 at 25°C 1750 2.08 150
TO-220
Note: Please refer to the manufacturer’s data sheet for testing conditions and detailed information.