LTC3406AB Datasheet, PDF (8/16 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

LTC3406AB
APPLICATIONS INFORMATION
The basic LTC3406AB application circuit is shown on the
front page. External component selection is driven by the
load requirement and begins with the selection of L fol-
lowed by C_IN and C_OUT.
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 1μH to 4.7μH. Its value is chosen based on the
desired ripple current. Large value inductors lower ripple
current and small value inductors result in higher ripple
currents. Higher V_IN or V_OUT also increases the ripple cur-
rent as shown in equation 1. A reasonable starting point for
setting ripple current is ΔI_L = 240mA (40% of 600mA).
ΔI_L
=
1
(f)(L)
V_OUT
⎛
⎝⎜
1−
V_OUT
V_IN
⎞
⎠⎟
(1)
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 720mA
rated inductor should be enough for most applications
(600mA + 120mA). For better efficiency, choose a low
DC-resistance inductor.
Inductor Core Selection
Different core materials and shapes will change the
size/current and price/current relationship of an induc-
tor. Toroid or shielded pot cores in ferrite or permalloy
materials are small and don’t radiate much energy, but
generally cost more than powdered iron core inductors
with similar electrical characteristics. The choice of which
style inductor to use often depends more on the price
vs size requirements and any radiated field/EMI require-
ments than on what the LTC3406AB requires to operate.
Table 1 shows some typical surface mount inductors that
work well in LTC3406AB applications.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE DCR
MAX DC
SIZE
(μH) (Ω MAX) CURRENT (A) W × L × H (mm³₎
Sumida
1.5
0.043
1.55
3.8 × 3.8 × 1.8
CDRH3D16 2.2
0.075
1.20
3.3
0.110
1.10
4.7
0.162
0.90
Sumida
2.2
CMD4D06 3.3
4.7
0.116
0.174
0.216
0.950
0.770
0.750
3.5 × 4.3 × 0.8
Panasonic 3.3
0.17
1.00
4.5 × 5.4 × 1.2
ELT5KT
4.7
0.20
0.95
Murata
1.0
0.060
1.00
2.5 × 3.2 × 2.0
LQH32CN
2.2
0.097
0.79
4.7
0.150
0.65
C_IN and C_OUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V_OUT/V_IN. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
( ) _{CIN required IRMS ≅ IOMAX} ^{⎡⎣V}_OUT
V_IN − V_OUT
V_IN
⎤⎦^{1/ 2}
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS = I_OUT/2. This simple worst-case condition is
commonly used for design because even significant de-
viations do not offer much relief. Note that the capacitor
manufacturer’s ripple current ratings are often based on
2000 hours of life. This makes it advisable to further derate
the capacitor, or choose a capacitor rated at a higher tem-
perature than required. Always consult the manufacturer
if there is any question.
The selection of C_OUT is driven by the required effective
series resistance (ESR).
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