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

LTC3831
APPLICATIO S I FOR ATIO
efficiency circuit designed for 2.5V input and 1.25V 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:
(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)Q2
=
(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 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 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 differential 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 – V_OUT) ₌ 1.138 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 2µH inductor has a 0.57A/µs rise
Table 1. Recommended MOSFETs for LTC3831 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
3831f