LTC3409_15 Datasheet, PDF (10/16 Pages) Linear Technology – 600mA Low VIN Buck Regulator in 3mm 3mm DFN

LTC3409
APPLICATIONS INFORMATION
The basic LTC3409 application circuit is shown on the first
page of this data sheet. External component selection is
driven by the load requirement and begins with the selec-
tion 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 10μH. 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 ΔI_L = 240mA
(40% of 600mA).

I_L
=
f
1
•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. The inductor value also has an effect on Burst
Mode operation. The transition to low current operation be-
gins when the inductor current peaks fall to approximately
200mA. Lower inductor values (higher ΔI_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 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 requirements
than on what the LTC3409 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3409 applications.
10
Table 1. Representative Surface Mount Inductors
PART
NUMBER
Sumida
CDRH2D18/LD
Sumida
CDRH2D11
Sumida
CMD4D11
Murata
LQH32CN
Toko
D312F
Panasonic
ELT5KT
VALUE
(μH)
2.2
3.3
1.5
2.2
2.2
3.3
1.0
2.2
2.2
3.3
3.3
4.7
DCR
(Ω MAX)
0.041
0.054
0.068
0.170
0.116
0.174
0.060
0.097
0.060
0.260
0.17
0.20
MAX DC
CURRENT (A)
0.85
0.75
0.90
0.78
0.950
0.770
1.00
0.79
1.08
0.92
1.00
0.95
SIZE
W × L × H (mm³)
3.2 × 3.2 × 2.0
3.2 × 3.2 × 1.2
4.4 × 5.8 × 1.2
2.5 × 3.2 × 2.0
2.5 × 3.2 × 2.0
4.5 × 5.4 × 1.2
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:
C_IN
Required
I_RMS

I_OUT(MAX)
⎡⎣ _VOUT
(V_IN – V_OUT )^{⎤⎦1/2}
V_IN
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS = I_OUT/2. This simple worst-case condition is common-
ly 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 manufacturer if there is any
question. The selection of C_OUT is driven by the required
effective series resistance (ESR). Typically, once the ESR
requirement for C_OUT has been met, the RMS current
rating generally far exceeds the I_RIPPLE(P-P) requirement.
The output ripple DV_OUT is determined by:

V_OUT
=

I_L

 ^ESR
+
8
•
f
1
• C_OUT


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