LTC3406-1.8_15 Datasheet, PDF (8/16 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

LTC3406
LTC3406-1.5/LTC3406-1.8
APPLICATIO S I FOR ATIO
The basic LTC3406 application circuit is shown in Figure 1.
External component selection is driven by the load require-
ment and begins with the selection of L followed 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
current as shown in equation 1. A reasonable starting point
for setting ripple current is ∆I_L = 240mA (40% of 600mA).
∆I_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-resis-
tance inductor.
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
200mA. Lower inductor values (higher ∆I_L) will cause this
to occur at lower load currents, which can cause a dip in
efficiency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to increase.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy mate-
rials are small and don’t radiate much energy, but gener-
ally 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 requirements
than on what the LTC3406 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3406 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
CDRH3D16
2.2
3.3
4.7
0.043
0.075
0.110
0.162
1.55
3.8 × 3.8 × 1.8
1.20
1.10
0.90
Sumida
2.2
0.116
CMD4D06
3.3
0.174
4.7
0.216
0.950
0.770
0.750
3.5 × 4.3 × 0.8
Panasonic
3.3
0.17
ELT5KT
4.7
0.20
1.00
4.5 × 5.4 × 1.2
0.95
Murata
LQH32CN
1.0
0.060
2.2
0.097
4.7
0.150
1.00
2.5 × 3.2 × 2.0
0.79
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 com-
monly used for design because even significant deviations
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
temperature than required. Always consult the manufac-
turer if there is any question.
3406fa
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