LTC3409 Datasheet, PDF (10/16 Pages) Linear Technology – 600mA Low Vin Buck Regulator in 3mm x 3mm DFN

LTC3409
APPLICATIO S I FOR ATIO
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 ∆I_L = 240mA (40% of
600mA).
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE DCR
MAX DC
SIZE
(µH) (Ω MAX) CURRENT (A) W × L × H (mm³)
Sumida
2.2
CDRH3D18/LD 3.3
0.041
0.054
0.85
3.2 × 3.2 × 2.0
0.75
Sumida
CDRH2D11
1.5 0.068
2.2 0.170
0.90
3.2 × 3.2 × 1.2
0.78
Sumida
CMD4D11
2.2 0.116
3.3 0.174
0.950
0.770
4.4 × 5.8 × 1.2
Murata
LQH32CN
1.0 0.060
2.2 0.097
1.00
2.5 × 3.2 × 2.0
0.79
Toko
D312F
2.2 0.060
3.3 0.260
1.08
2.5 × 3.2 × 2.0
0.92
Panasonic
ELT5KT
3.3
0.17
4.7
0.20
1.00
4.5 × 5.4 × 1.2
0.95
∆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 resis-
tance inductor. The inductor value also has an effect on
Burst Mode operation. The transition to low current opera-
tion begins when the inductor current peaks fall to ap-
proximately 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 LTC3409 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3409 applications.
C_IN and C_OUT Selection
In continuous mode, the source current of the top MOS-
FET 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 maxi-
mum RMS capacitor current is given by:
[ ^{( )}] _{CIN Required IRMS ≅ IOUT(MAX)}
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 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
⎞
⎠⎟
3409f
10