LTC3550_15 Datasheet, PDF (16/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550
APPLICATIO S I FOR ATIO
Buck Regulator 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 inductor ripple current. Large value inductors
lower ripple current and small value inductors result in
higher ripple currents. Higher V_CC 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
=
V_OUT
f_O • L
•
⎛
⎝⎜
1−
V_OUT
V_CC
⎞
⎠⎟
(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 best efficiency, choose a low DC-resistance
inductor.
The inductor value also has an effect on Burst Mode opera-
tion. The transition to low current operation begins 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 LTC3550 requires to operate. Table 2
shows some typical surface mount inductors that work
well in LTC3550 applications.
Table 2. Representative Surface Mount Inductors
PART
NUMBER
VALUE DCR
MAX DC
SIZE
(µH) (Ω MAX) CURRENT (A) W × L × H (mm)
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
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
ELT5KT
4.7
0.20
1.00
4.5 × 5.4 × 1.2
0.95
Murata
1.0
LQH32CN 2.2
4.7
0.060
0.097
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_CC. 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_OMAX
V_OUT (V_CC − V_OUT )
V_CC
(2)
This formula has a maximum at V_CC = 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 capaci-
tor, 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 ΔV_OUT is
determined by:
∆V_OUT
≅
∆I_L
⎛
⎝⎜ ^{E SR}
+
1
8 fC_OUT
⎞
⎠⎟
(3)
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