LTC3831-1 Datasheet, PDF (14/20 Pages) Linear Technology – High Power Synchronous Switching Regulator Controller for DDR Memory Termination

LTC3831-1
APPLICATIO S I FOR ATIO
efficiency circuit designed for 1.5V input and 0.75V at 5A
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:
(0.75V)(5A/0.9)(0.03) = 0.125W per FET
and a required R_DS(ON) of:
R_DS(ON)Q1
=
(1.5V) • (0.125W)
(0.75V)(5A)²
=
0.01Ω
R_DS(ON)Q2
=
(1.5V) • (0.125W)
(1.5V – 0.75V)(5A)²
=
0.01Ω
Note that while the required R_DS(ON) values suggest large
MOSFETs, the power dissipation numbers are only 0.125W
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 Semi-
conductor 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 LTC3831-1 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 LTC3831-1 applications.
Inductor Selection
The inductor is often the largest component in an
LTC3831-1 design and must be chosen carefully. Choose
the inductor value and type based on output slew rate re-
quirements. The maximum rate of rise of inductor current
is set by the inductor’s value, the input-to-output voltage
differential and the LTC3831-1’s maximum duty cycle. In
a typical 1.5V input 0.75V output application, the maximum
rise time will be:
DC_MAX • (V_IN – V_OUT) ₌ 0.713 A
L_O
L_O µs
where L_O is the inductor value in µH. With proper fre-
quency compensation, the combination of the inductor
and output capacitor values determine the transient recov-
ery time. In general, a smaller value inductor improves
transient response at the expense of ripple and inductor
core saturation rating. A 1µH inductor has a 0.713A/µs rise
Table 1. Recommended MOSFETs for LTC3831-1 Applications
PARTS
Siliconix SUD50N03-10
TO-252
R_DS(ON)
AT 25°C (mΩ)
19
RATED CURRENT (A)
15 at 25°C
10 at 100°C
TYPICAL INPUT
CAPACITANCE
C_ISS (pF)
3200
Siliconix Si4410DY
SO-8
20
10 at 25°C
2700
8 at 70°C
ON Semiconductor MTD20N03HDL
35
20 at 25°C
880
D PAK
16 at 100°C
Fairchild FDS6670A
S0-8
8
13 at 25°C
3200
Fairchild FDS6680
SO-8
10
11.5 at 25°C
2070
ON Semiconductor MTB75N03HDL
9
DD PAK
75 at 25°C
4025
59 at 100°C
IR IRL3103S
DD PAK
19
64 at 25°C
1600
45 at 100°C
IR IRLZ44
TO-220
28
50 at 25°C
3300
36 at 100°C
Fuji 2SK1388
TO-220
37
35 at 25°C
1750
Note: Please refer to the manufacturer’s data sheet for testing conditions and detailed information.
14
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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