LTC3550 Linear Technology, LTC3550 Datasheet - Page 16

• Manufacturer Part Number: LTC3550
• Description: Dual Input USB/AC Adapter Li-Ion Battery Charger
• Manufacturer: Linear Technology

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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
the ripple current as shown in Equation 1. A reasonable
starting point for setting ripple current is ΔI
(40% of 600mA).
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 effi ciency, 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
at lower load currents, which can cause a dip in effi ciency
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 fi eld/EMI requirements
than on what the LTC3550 requires to operate. Table 2
shows some typical surface mount inductors that work
well in LTC3550 applications.
16
∆I
L
=
V
f
O
OUT
•
L
• 1
⎛
⎝ ⎜
U
−
V
V
OUT
CC
U
⎞
⎠ ⎟
CC
L
) will cause this to occur
or V
W
OUT
also increases
L
(1)
U
= 240mA
Table 2. Representative Surface Mount Inductors
C
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
This formula has a maximum at V
= I
used for design because even signifi cant 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
series resistance (ESR).
Typically, once the ESR requirement for C
met, the RMS current rating generally far exceeds the
I
determined by:
C
RIPPLE(P-P)
PART
NUMBER
Sumida
CDRH3D16
Sumida
CMD4D06
Panasonic
ELT5KT
Murata
LQH32CN
IN
IN
OUT
∆
and C
required I
V
/2. This simple worst-case condition is commonly
OUT
OUT
≅ ∆
VALUE
requirement. The output ripple ΔV
(µH)
1.5
2.2
3.3
4.7
2.2
3.3
4.7
3.3
4.7
1.0
2.2
4.7
Selection
I ESR
RMS
L
⎛
⎝ ⎜
OUT
(Ω MAX)
≅
0.043
0.075
0.110
0.162
0.116
0.174
0.216
0.060
0.097
0.150
DCR
0.17
0.20
I
is driven by the required effective
OMAX
+
8
fC
1
CURRENT (A)
OUT
OUT
MAX DC
V
0.950
0.770
0.750
OUT
1.55
1.20
1.10
0.90
1.00
0.95
1.00
0.79
0.65
CC
⎞
⎠ ⎟
/V
= 2V
CC
(
V
. To prevent large
V
CC
CC
OUT
W × L × H (mm)
−
OUT
3.8 × 3.8 × 1.8
3.5 × 4.3 × 0.8
4.5 × 5.4 × 1.2
2.5 × 3.2 × 2.0
, where I
V
OUT
SIZE
has been
OUT
)
3550fa
RMS
(3)
(2)
is