LTC3831_15 Datasheet, PDF (14/20 Pages) Linear Technology – High Power Synchronous Switching Regulator Controller for DDR Memory Termination

LTC3831
APPLICATIONS INFORMATION
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:
(1.25V)(5A/0.9)(0.03) = 0.21W per FET
and a required R_DS(ON) of:
R_DS(ON)Q1
=
(2.5V) • (0.21W)
(1.25V)(5A)²
=
0.017Ω
R_{DS(ON)Q 2}
=
(2.5V) • (0.21W)
(2.5V – 1.25V)(5A)²
=
0.017Ω
Note that while the required R_DS(ON) values suggest large
MOSFETs, the power dissipation numbers are only 0.21W
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 LTC3831 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 that are
for use in LTC3831 applications.
Inductor Selection
The inductor is often the largest component in an LTC3831
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 differen-
tial and the LTC3831’s maximum duty cycle. In a typical
2.5V input 1.25V output application, the maximum rise
time will be:
DC_MAX
• (V_IN
L_O
–
V_OUT
)
=
1.138
L_O
A
μs
where L_O is the inductor value in μH. With proper frequency
compensation, the combination of the inductor and output
Table 1. Recommended MOSFETs for LTC3831 Applications
PARTS
Siliconix SUD50N03-10
T0-252
Siliconix Si4410DY
SO-8
ON Semiconductor MTD20N03DHL
D PAK
Fairchild FDS6670A
SO-8
Fairchild FDS6680
SO-8
ON Semiconductor MTB75N03HDL
DS PAK
IR IRL3103S
DD PAK
IR IRLZ44
TO-220
Fuji 2SK1388
TO-220
R_DS(ON)
AT 25ºC (mΩ)
19
20
35
8
10
9
19
28
37
RATED CURRENT (A)
15 at 25°C
10 at 100°C
10 at 25°C
8 at 70°C
20 at 25°C
16 at 100°C
13 at 25°C
11.5 at 25°C
75 at 25°C
59 at 100°C
64 at 25°C
45 at 100°C
50 at 25°C
36 at 100°C
35 at 25°C
TYPICAL INPUT
CAPACITANCE
C_ISS (pF)
3200
2700
880
3200
2070
4025
1600
3300
1750
Note: Please refer to the manufacturer’s data sheet for testing conditions and detailed information.
θ_JC (°C/W)
1.8
1.67
25
25
1
1.4
1
2.08
T_JMAX (°C)
175
150
150
150
150
150
175
175
150
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