ltc3405a Linear Technology Corporation, ltc3405a Datasheet - Page 8

• Manufacturer Part Number: ltc3405a
• Description: 1.5mhz, 300ma Synchronous Step-down Regulator In Thinsot
• Manufacturer: Linear Technology Corporation

LTC3405A
APPLICATIO S I FOR ATIO
The basic LTC3405A application circuit is shown in Fig-
ure 1. External component selection is driven by the load
requirement and begins with the selection of L followed by
C
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 2.2µH 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
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
8
IN
∆ =
and C
I
L
OUT
( )( )
f L
1
.
V
OUT
U
⎛
⎜
⎝
1
IN
−
or V
U
V
V
OUT
IN
L
OUT
= 120mA (40% of 300mA).
⎞
⎟
⎠
also increases the ripple
W
L
) will cause this
U
(1)
Table 1. Representative Surface Mount Inductors
MANUFACTURER PART NUMBER
Taiyo Yuden
Panasonic
Murata
Taiyo Yuden
Panasonic
Panasonic
Sumida
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 LTC3405A requires to operate. Table 1 shows
some typical surface mount inductors that work well in
LTC3405A applications.
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.
RMS
IN
C
and C
IN
= I
required I
OUT
OUT
/2. This simple worst-case condition is com-
Selection
LB2016T2R2M
LB2012T2R2M
LB2016T3R3M
ELT5KT4R7M
LQH3C4R7M34
LB2016T4R7M
ELT5KT6R8M
ELT5KT100M
CMD4D116R8MC 6.8µH
RMS
≅
I
OMAX
VALUE CURRENT DCR HEIGHT
2.2µH
2.2µH
3.3µH
4.7µH
4.7µH
4.7µH
6.8µH
10µH
[
OUT
V
OUT
/V
MAX DC
315mA 0.13Ω 1.6mm
240mA 0.23Ω 1.25mm
280mA
950mA
450mA
210mA 0.25Ω 1.6mm
760mA
680mA 0.36Ω 1.2mm
620mA 0.23Ω 1.2mm
(
IN
IN
V
. To prevent large
IN
= 2V
V
−
IN
V
OUT
0.2Ω 1.6mm
0.2Ω 1.2mm
0.2Ω
0.3Ω 1.2mm
OUT
, where
)
]
1 2 /
2mm
3405afa