LTC3405 Linear Technology, LTC3405 Datasheet - Page 8

• Manufacturer Part Number: LTC3405
• Description: Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger
• Manufacturer: Linear Technology

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LTC3405
APPLICATIO S I FOR ATIO
The basic LTC3405 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
C
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 3.3 H to 10 H. 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
current as shown in equation 1. A reasonable starting point
for setting ripple current is I
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 360mA rated
inductor should be enough for most applications (300mA
+ 60mA). 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 begins
when the inductor current peaks fall to approximately
100mA. Lower inductor values (higher I
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. Tor-
oid 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 induc-
tor to use often depends more on the price vs size require-
ments and any radiated field/EMI requirements than on
what the LTC3405 requires to operate. Table 1 shows some
typical surface mount inductors that work well in LTC3405
applications.
8
OUT
I
.
L
f L
1
V
OUT
U
1
IN
U
or V
V
V
OUT
IN
L
OUT
= 120mA (40% of 300mA).
also increases the ripple
W
L
) will cause this
U
IN
and
(1)
Table 1. Representative Surface Mount Inductors
MANUFACTURER PART NUMBER
Taiyo Yuden
Panasonic
Murata
Taiyo Yuden
Panasonic
Panasonic
Sumida
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
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.
The selection of C
series resistance (ESR). An ESR in the range of 100m to
200m
LTC3405, the general rule for proper operation is:
ESR is a direct function of the volume of the capacitor; that
is, physically larger capacitors have lower ESR. Once the
ESR requirement for C
rating generally far exceeds the I
The output ripple V
RMS
IN
0.1
C
and C
IN
V
= I
OUT
required I
OUT
is necessary to provide a stable loop. For the
OUT
C
/2. This simple worst-case condition is com-
OUT
Selection
I ESR
LB2016T3R3M
ELT5KT4R7M
LQH3C4R7M34
LB2016T4R7M
ELT5KT6R8M
ELT5KT100M
CMD4D116R8MC 6.8 H
L
required ESR 0.6
RMS
OUT
OUT
OUT
is driven by the required effective
I
OMAX
8
is determined by:
has been met, the RMS current
fC
1
OUT
VALUE CURRENT DCR HEIGHT
3.3 H
4.7 H
4.7 H
4.7 H
6.8 H
10 H
OUT
V
RIPPLE(P-P)
OUT
/V
MAX DC
280mA
950mA
450mA
210mA
760mA
680mA
620mA
IN
IN
V
. To prevent large
IN
= 2V
V
IN
V
requirement.
0.25
0.36
0.23
OUT
0.2
0.2
0.2
0.3
OUT
sn3405 3405fs
, where
1.6mm
1.2mm
1.6mm
1.2mm
1.2mm
1.2mm
1 2 /
2mm